Nanomaterials: Sources and Toxicity | Nanobiotechnology

Let us make an in-depth study of the nanomaterials. The below given article will help you to learn about the following things:- 1. Introduction to Nanomaterials 2. Nano Etymology 3. Nanoparticle Classification 4. Sources of Nanoparticles and their Health Effects 5. Nanotoxicology of Nanoparticles 6. Physicochemical Characteristic-Dependent Toxicity and 7. Conclusions and Future-Directions.

Introduction to Nanomaterials:

Every person has been exposed to nanometer sized foreign particles; we inhale them with every breath, and consume them with every drink. In truth, every organism on earth continuously encounters nanometer-sized entities. The vast majority causes little ill effect, and goes unnoticed, but occasionally an intruder will cause appreciable harm to the organism.

The most advanced of the toxic intruders are viruses, composed as they are of nucleic acid- based structures that allows them to not only interfere with biological systems, but also to parasitically exploit cellular processes to replicate themselves.

Among the more benign vi­ruses are the ones causing the familiar human symptoms of the common cold or flu, which are the evident manifestations of biochemical battles occurring between these foreign intrud­ers and our immune systems, whose nanome­ter sized constituents (chemicals, and proteins) usually destroy and remove the viral invaders.

A growing number of recent studies show, however, that Nano and microorganisms may play a role in many chronic diseases where in­fectious pathogens have not been suspected, diseases that were previously attributed only to genetic factors and lifestyle.

Among these diseases are; leukemia (caused by viruses from the Retrovirus and Herpes virus families), cer­vical cancer (Papilloma virus), liver cancer (Hepatitis virus), gastric ulcer (Helicobacter pylori), nasopharyngeal cancer (Epstein-Barr virus), kidney stones (Nano bacteria), severe acquired respiratory syndrome SARS (Coro­na virus), heart disease (Chlamydia pneumo­nia), juvenile diabetes (Coxsackie virus), Alzheimer’s disease (Chlamydia pneumonia), pediatric obsessive-compulsive disorder (Streptococcal bacteria), psychotic disorders (Borna virus), and prion diseases such as mad cow disease (proteins-prions).

One is tempted to think that nanoparticles (such as dust, or ash particles), while similar in size to viruses, would be more benign, as these materials lack the viruses’ ability to replicate. Nevertheless, while non-replicating bodily intruders do not directly take control of cellular processes, some have been shown to sufficiently interfere with cellular function to influence basic process of cells, such as pro­liferation, metabolism, and death.

Many dis­eases can be associated with dysfunction of these basic processes, the most notable being cancer (uncontrolled cells proliferation), and neurodegenerative diseases (premature cell death). In addition, several diseases with un­known cause, including autoimmune diseas­es, Crohn’s, Alzheimer’s, and Parkinson’s dis­eases appear to be correlated with nanoparti­cles exposure. Conversely, the toxic properties of some nanoparticles may be beneficial, as they are thereby able to fight disease at a cellu­lar level, and could be used as a medical treat­ment, by, for example, targeting and destroy­ing cancerous cells.

Very small particles, so called nanoparticles, have the ability to enter, translocate within, and damage living organ­isms. This ability results primarily from their small size, which allows them to penetrate physiological barriers, and travel within the circulatory systems of a host. While natural processes have produced nanoparticles for eons, modern science has recently learned how to synthesize a bewildering array of artificial materials with structure that is engineered at the atomic scale.

The smallest particles con­tain tens or hundreds of atoms, with dimen­sions at the scale of nanometers—hence nano­particles. They are comparable in size to vi­ruses, where the smallest have dimensions of tens of nanometers (for example, a human immunodeficiency virus, or HIV, particle is 100 nm in diameter), and which in the emerg­ing science of nanotechnology might be called ‘Nano-organisms’.

Like viruses, some nanopar­ticles can penetrate lung or dermal (skin) bar­riers and enter the circulatory and lymphatic systems of humans and animals, reaching most bodily tissues and organs, and potentially dis­rupting cellular processes and causing disease. The toxicity of each of these materials depends greatly, however, upon the particular arrange­ment of its many atoms.

Considering all the possible variations in shape and chemistry of even the smallest nano­particles, with only tens of atoms, yields a huge number of distinct materials with potentially very different physical and toxicological prop­erties. Asbestos is a good example of a toxic nanomaterial, causing lung cancer and other diseases. Asbestos exists in several forms, with slight variations in shape and chemistry yet significantly varying toxicity.

Nanometer sized particles are created in countless physical processes from erosion to combustion, with health risks ranging from lethal to benign. Industrial nanoparticle materials today constitute a tiny but significant pollution source that is, so far, literally buried beneath much larger natural sources and nano­particle pollution incidental to other human activities, particularly automobile exhaust soot.

The misapprehension of Nano toxicity may create a general fear that all Nano-materials are toxic. The online and printed media are inad­vertently making no distinction between nanostructured fixed structures, which are not likely to cause harm (such as computer processors), and detachable or free nanoparticles, which are likely to cause adverse health effects.

While uncontained nanoparticles clearly rep­resent a serious health threat, fixed Nano-structured materials, such as thin film coatings, microchip electronics, and many other exist­ing Nano engineered materials, are known to be virtually benign.

Many synthetic Nano-particulate materials produce positive health ef­fects, for example functionalized fullerene chemicals that act as antioxidants. The use of nanoparticles in medical diagnostics and treat­ment is driven by their safety, as well as utility. Here, we outline existing sources of nanoparticles, both natural and man-made, and the known effects of exposure to nanoparticles.

Nano Etymology:

The prefix ‘Nano’, derived from the Greek ‘Nanos’ signifying ‘dwarf is becoming increas­ingly common in scientific literature. ‘Nano’ is now a popular label for much of modern science and many ‘Nano’ words have recently appeared in dictionaries, including: nanome­ter, Nano scale, Nano science, nanotechnology, nanostructure, and nanotube, nanowire, and Nano robot. Many words that are not yet widely recognized are used in respected publications, such as Science and Nature.

These include Nano electronics, Nano crystal, Nano valve, Nano antenna, Nano cavity, Nano scaffolds, Nano-fibres, Nano magnet, Nano porous, Nano arrays, nanolithography, Nano patterning, Nano encapsulation, etc. Although the idea of nanotechnology: producing Nano scale objects and carrying out Nano scale manipulations, has been around for quite some time, the birth of the concept is usually linked to a speech by Richard Feynman at the December 1959 meet­ing of the American Physical Society where he asked, ‘What would happen if we could arrange the atoms one by one the way we want them?’

The nanometer is a metric unit of length, and denotes one billionth of a meter or 10^-9 m. popularly, ‘Nano’ is also used as an adjec­tive to describe objects, systems, or phenome­na with characteristics arising from nanometer scale structure. While ‘micro’ has come to mean anything small, ‘Nano’ emphasizes the atomic granularity that produces the unique phenomena observed in Nano science.

While there are some exceptional examples, most of the exciting properties of ‘Nano’ begin to be apparent in systems smaller than 1,000 nm, or 1 micro-meter, 1 mm. For the purpose in this article we will describe particles with any di­mension smaller than 1 micro-meter as ‘nano­particles’, and those somewhat larger as ‘micro particles’.

Nanostructured materials did not first come into existence with the recent emer­gence of the field of nanotechnology. Many existing materials are structured on the micro and nanometer scales, and many industrial processes that have been used for decades (e.g. polymer and steel manufacturing) exploit Nano scale phenomena.

The most advanced Nano technological fabrication process is mi­croelectronic fabrication, where thin film coat­ings and lithography are used to create micro and Nano sized features on computer chips. The natural world is replete with examples of sys­tems with Nano scale structures, such as milk (a Nano scale colloid), proteins, cells, bacteria, viruses etc.

Moreover, many materials that seems smooth to the naked eye have an intricate structure on the scale of nanometers (Fig. 14.1). Thus in many ways Nano-materials are not new. Recent advances in synthesis and characterization tools, however, have fuelled a boom in the study and industrial use of nano­structured materials.

A new vocabulary has emerged from this research, and its important terms and concepts are defined:

Nano-materials are materials that have struc­tural components smaller than 1 micro-meter in at least one dimension. While the atomic and molecular building blocks (~0.2 nm) of matter are considered nanomaterial’s, examples such as bulk crystals with lattice spacing of nanometers but macroscopic dimensions overall, are commonly excluded.

Nanoparticles are particles with at least one dimension smaller than 1 micron and potentially as small as atomic and molecular length scales (~0.2 nm). Nanoparticles can have amorphous or crystalline form and their sur­faces can act as carriers for liquid droplets or gases.

To some degree, Nano particulate mat­ter should be considered a distinct state of matter, in addition to the solid, liquid, gaseous, and plasma states, due to its distinct proper­ties (large surface area and quantum size ef­fects). Examples of materials in crystalline nanoparticle form are fullerenes and carbon nanotubes, while traditional crystalline solid forms are graphite and diamond. Many authors limit the size of Nano-materials to 50 nm or 100 nm, the choice of this upper limit being justi­fied by the fact that some physical properties of nanoparticles approach those of bulk when their size reaches these values.

However, this size threshold varies with material type and cannot be the basis for such a classification. A legitimate definition extends this upper size limit to 1 micron, the sub-micron range being classified as Nano. Nano particulate matter refers to a collec­tion of nanoparticles, emphasizing their col­lective behaviour. Nanotechnology can be defined as the de­sign, synthesis, and application of materials and devices whose size and shape have been engineered at the Nano scale. It exploits unique chemical, physical, electrical, and mechanical properties that emerge when matter is struc­tured at the Nano scale.

Nano toxicology was proposed as a new branch of toxicology to address the adverse health effects caused by nanoparticles. Despite suggestions that Nano toxicology should only address the toxic effects of engineered nano­particles and structures we recommend that Nano toxicology should also encompass the toxic effects of atmospheric particles, as well as the fundamentals of virology and bacteri­ology.

While significant differences exist be­tween the health effects of non-biological par­ticles and viruses and bacteria, there are sig­nificant common aspects of intrusion and translocation. The new terminology of ‘Nano’ has united previously seemingly disparate fields, and a lexicon is needed to find and appreciate the great wealth of existing Nano research, not con­veniently labeled with the Nano keyword.

Health Sciences Epidemiology Terminolo­gy:

In existing medical and toxicological ter­minology, nanoparticles having a diameter smaller than 100 nm are often called ultrafine particles (UFP) or ultrafine particulate mat­ter. Ultrafine particles are labeled as a func­tion of their size. For example, particulate matter with constituents having diameters smaller than 10 microns is abbreviated PM₁₀. Particulate matter having a size smaller than 100 nm is labeled as PM_0.1.

Environmental Sciences Terminology:

Ambient particulate matter is categorized in three size distributions: ultrafine particles less than 0.1 mm in diameter (mainly resulting from combustion), accumulation mode par­ticles between 0.1 and 2.5 mm in diameter (re­sulting from aggregation of ultrafine particles and vapors), and coarse-mode particles larger than 2.5 mm (mostly mechanically generated).

Proposed Terminology:

It is important, and timely, to unify the terminology used for de­scribing particle size in nanotechnology, health and environmental sciences. The materials under discussion can be clas­sified as particles, regardless of their sources. The size of these particles varies between 1 nm to several microns, and they can therefore be classified as either nanoparticles NP (any di­mension smaller than 1 micron) or micro particles MP (all dimensions larger than one mi­cron).

To further specify particle size, we pro­pose a modification of the health sciences epidemiology terminology, labelling particles by their largest dimension; for example 10 nm in diameter are labeled ‘NP₁₀‘, while 10 mm micro particles are labeled ‘MP₁₀‘. Given that micro particles and nanoparticles vary in their conception by only their size, it can be difficult to fully appreciate the differ­ences between them. To illuminate the effect of the size difference, the sizes of several natu­ral micro and nanostructures are shown in (Fig. 14.2), as measured from scanning and trans­mission microscope images.

Generally, the siz­es of Nano-materials are comparable to those of viruses, DNA, and proteins, while micro- particles are comparable to cells, organelles, and larger physiological structures (Fig. 14.2). A red blood cell is approximately 7 mm wide, a hair 60 mm, while lung alveoli are approximately 400 mm.

Main Differences between Nano-Materials and Bulk Material:

Two primary factors cause Nano-materials to behave significantly differently than bulk materials: surface effects (causing smooth prop­erties scaling due to the fraction of atoms at the surface) and quantum effects (showing discontinuous behaviour due to quantum con­finement effects in materials with delocalized electrons). These factors affect the chemical reactivity of materials, as well as their mechan­ical, optical, electric, and magnetic properties.

The fraction of the atoms at the surface in nanoparticles is increased compared to micro- particles or bulk. Compared to micro particles, nanoparticles have a very large surface area and high particle number per unit mass. For illus­tration, one carbon micro particle with a diameter of 60 mm has a mass of 0.3 mg and a surface area of 0.01 mm².

The same mass of carbon in Nano particulate form, with each particle having a diameter of 60 nm, has a sur­face area of 11.3 mm² and consists of 1 billion nanoparticles. The ratio of surface area to vol­ume (or mass) for a particle with a diameter of 60 nm is 1,000 times larger than a particle with a diameter of 60 mm. As the material in Nano particulate form presents a much larger surface area for chemical reactions, reactivity is enhanced roughly 1,000 fold.

While chemical reactivity generally increases with decreas­ing particle size, surface coatings and other modifications can have complicating effects, even reducing reactivity with decreasing par­ticle size in some instances.

The atoms situat­ed at the surface have less neighbours than bulk atoms, resulting in lower binding energy per atom with decreasing particle size. A conse­quence of reduced binding energy per atom is a melting point reduction with particle radi­us, following the Gibbs-Thomson equation. For example, the melting temperature of 3 nm gold nanoparticles is more than 300 de­grees lower than the melting temperature of bulk gold.

An example of a class of materials that clearly exploits quantum dots synthesized nanostructures with sizes as small as a few nanometers. The electronic behaviour of quantum dots is similar to that of individual atoms or small molecules, and quantum effects are quantum dots are regarded as akin to artifi­cial atoms.

Notably, the confinement of the electrons in quantum dots in all three spatial directions results in a quantized energy spec­trum. Another result of quantum condiment effect is the appearance of magnetic moments in nanoparticles of materials that are non­magnetic in bulk, such as gold, platinum, or palladium. Magnetic moments result from sev­eral unpaired electron spins in nanoparticles formed of several hundred atoms.

Quantum confinement also results in quantified chang­es in the ability to accept or donate electrical charge (or electron affinity), also reflected in the catalytic ability. For example, the reactivi­ty of cationic platinum clusters in the decom­position of N₂O is dictated by the number of atoms in the cluster, namely 6-9, 11, 12, 15, 20 atom containing clusters are very reactive, while clusters with 10, 13, 14, 19 atoms have low reactivity.

Nano-Materials and Nano Toxicology Publications Statistics:

The number of publications on the topic of Nano-materials has increased at an almost exponential rate since the early 1990s, reaching about 40,000 in 2005 and 65,000 in 2009, as indicated by a search on ISI Web of knowl­edge database. There is also a notable rise in the number of publications discussing their toxicity, particularly in the past two years.

The total number of papers on toxicity, however, remains low compared to the total number of publications on Nano-materials, with only around 500 publications in 2005. The large number of publications on Nano-materials can be explained by the fact that Nano-Science and nanotechnology encompass a wide range of fields, including chemistry, physics, materials engineering, biology, medicine, and electron­ics.

There are several reviews addressing Nano toxicology aspects; however they are intend­ed for a narrow, specialized audience. Several are comparatively, while others address select­ed aspects of nanoparticles toxicology, such as: health effects of air pollution; epidemiological reviews of exposure to particles; epidemiolog­ical studies of cardiovascular effects of air­borne particles; occupational aspects of Nano- particle; particle inhalation, retention, and clearance; pulmonary effects of inhaled parti­cles; inhalation and lung cancer; toxicity of combustion derived particles inhalation; en­vironmental factors in neurodegenerative dis­eases; oxidative mechanisms; gastro-intestinal uptake of particles; targeted drug delivery; particle characterizations methods; screening strategies and future directions of research; and regulation of nanomaterial’s.

Existing reviews are either written in jargon comprehensive only to specialists in a particular field, or are, if more accessible, very succinct. Most nano­technology reviews written to date focus on a specific sub-field, disregarding the vast amount of existent knowledge on the general theme of Nano. Here, we attempt to bring to­gether a broader audience by unifying the lan­guage and experience of scientists working within these diverse fields.

Introduction to Nanoparticles Toxicity:

Human skin, lungs, and the gastro-intestinal tract are in constant contact with the environ­ment. While the skin is generally an effective barrier to foreign substances, the lungs and gastro-intestinal tract are more vulnerable. These three ways are the most likely points of entry for natural or anthropogenic nanoparti­cles. Injections and implants are other possi­ble routes of exposure, primarily limited to engineered materials.

Due to their small size, nanoparticles can translocate from these entry portals into the circulatory and lymphatic systems, and ulti­mately to body tissues and organs. Some Nano- particles— depending on their composition and size—can produce irreversible damage to cells by oxidative stress or/and organelle inju­ry. In addition, the toxicity of any nanoparticle to an organism is determined by the indi­vidual’s genetic complement, which provides the biochemical toolbox by which it can adapt to and fight toxic substances.

Diseases associ­ated with inhaled nanoparticles are asthma, bronchitis, emphysema, lung cancer, and neu­rodegenerative diseases, such as Parkinson’s and Alzheimer’s diseases. Nanoparticles in the gastro-intestinal tract have been linked to Crohn’s disease and colon cancer. Nanoparti­cles that enter the circulatory system are relat­ed to occurrence of arteriosclerosis, and blood clots, arrhythmia, heart diseases, and ultimate­ly cardiac death.

Translocation to other organs, such as liver, spleen, etc., may lead to diseases of these organs as well. Exposure to some nanoparticles is associated to the occurrence of autoimmune diseases, such as: systemic lu­pus erythematosus, scleroderma, and rheumatoid arthritis.

Nanoparticle Classification:

Nanoparticles are generally classified based on their dimensionality, morphology, composition, uniformity, and agglomeration. An im­portant additional distinction should be made between nanostructured thin films or other fixed nanometer-scale objects (such as the cir­cuits within computer microprocessors) and free nanoparticles.

The motion of free nano­particles is not constrained, and they can eas­ily be released into the environment leading to human exposure that may pose a serious health risk. In contrast are the many objects containing nanostructured elements that are firmly attached to a larger object, where the fixed nanoparticles should pose no health risk when properly handled.

An example of this important distinction is the material asbestos, which is perfectly safe in its primary state (ba­sically a type of solid rock), but is a significant health hazard when mined or worked in such a way as to produce the carcinogenic nanom­eter-scale fibrous particles that become air­borne (aerosol) and are therefore readily ab­sorbed in the lungs.

It is also very important to recognize that not all nanoparticles are toxic; toxicity depends on at least chemical composition and shape in addition to simply size and particle ageing. In fact, many types of nanoparticles seem to be non-toxic; others can be rendered non-toxic, while others appear to have beneficial health effects.

An important lesson we are in the pro­cess of learning from Nano science is that sim­ple classifications of physical behaviour (and therefore toxicity) are overly limiting and that we must study toxicology of each material and each morphology, in addition to particle age­ing, to obtain accurate information to inform policy and regulatory processes.

Dimensionality:

As shape, or morphology—dimensionality— of nanoparticles plays an important role in their toxicity, it is useful to classify them based on their number of dimensions.

This is a gen­eralization of the concept of aspect ratio:

ID Nano-Materials:

Materials with one di­mension in the nanometer scale are typically thin films or surface coatings, and include the circuitry of computer chips and the antireflection and hard coatings on eyeglasses. Thin films have been developed and used for de­cades in various fields, such as electronics, chemistry, and engineering. Thin films can be deposited by various methods and can be grown controllably to be only one atom thick, a so-called monolayer.

2D Nano-Materials:

Two-dimensional Nano-materials have two dimensions in the nanom­eter scale. These include 2D nanostructured films, with nanostructures firmly attached to a substrate, or Nano pore filters used for small particle separation and filtration, Free parti­cles with a large aspect ratio, with dimensions in the Nano scale range, are also considered 2D Nano-materials. Asbestos fibers are an exam­ple of 2D nanoparticles.

3D Nano-Materials:

Materials that are Nano scale in all three dimensions are considered 3D Nano-materials. These include thin films depos­ited under conditions that generate atomic- scale porosity, colloids, and free nanoparticles with various morphologies.

Nanoparticle Morphology:

Morphological characteristics to be taken into account are: flatness, sphericity, and aspect ratio. A general classification exists between high and low aspect ratio particles. High as­pect ratio nanoparticles include nanotubes and nanowires, with various shapes, such as heli­ces, zigzags, belts, or perhaps nanowires with diameter that varies with length. Small-aspect ratio morphologies include spherical, oval, cubic, prism, helical, or pillar. Collections of many particles exist as powders, suspension, or colloids.

Nanoparticle Composition:

Nanoparticles can be composed of a single constituent material or be a composite of several materials. The nanoparticles found in na­ture are often agglomerations of materials with various compositions, while pure single-composition materials can be easily synthesized today by a variety of methods.

Nanoparticle Uniformity and Agglomeration:

Based on their chemistry and electro-magnetic properties, nanoparticles can exist as dispersed aerosols, as suspensions/colloids, or in an ag­glomerate state. For example, magnetic nano­particles tend to cluster, forming an agglom­erate state, unless their surfaces are coated with a non-magnetic material.

In an agglomerate state, nanoparticles may behave as larger par­ticles, depending on the size of the agglomer­ate. Hence, it is evident that nanoparticle ag­glomeration, size and surface reactivity, along with shape and size, must be taken into ac­count when deciding considering health and environmental regulation of new materials.

Sources of Nanoparticles and their Health Effects:

Natural Sources of Nanoparticles:

Nanoparticles are abundant in nature, as they are produced in many natural processes, including photochemical reactions, volcanic eruptions, forest fires and simple erosion, and by plants and animals, e.g. shed skin and hair. Though we usually associate air pollution with human activities —cars, industry, and charcoal burning, natural events such as dust storms, volcanic eruptions and forest fires can produce such vast quantities of Nano particulate matter that they profoundly affect air quality world­wide.

The aerosols generated by human activ­ities are estimated to be only about 10% of the total, the remaining 90% having a natural ori­gin. These large-scale phenomena are visible from satellites and produce particulate matter and airborne particles of dust and soot rang­ing from the micro to Nano scales.

Small parti­cles suspended in the atmosphere, often known as aerosols, affect the entire planet’s energy balance because they both absorb ra­diation from the sun and scatter it back to space. It has been estimated that the most sig­nificant components of total global atmosphere aerosols are, in decreasing mass abundance: mineral aerosols primarily from soil deflation (wind erosion) with a minor com­ponent (<1%) from volcanoes (16.8 Tg), sea salt (3.6 Tg), natural and anthropogenic sul­fates (3.3 Tg), products of biomass burning excluding soot (1.8 Tg), and of industrial sources including soot (1.4 Tg), natural and anthropogenic non-methane hydrocarbons (1.3 Tg), natural and anthropogenic nitrates (0.6 Tg), and biological debris (0.5 Tg) (note: ‘Tg’ here denotes terragram, equal to 1012 grams).

Dust Storms and Health Effects: Ter­restrial Dust Storms:

Dust storms appear to be the largest single source of environmental nanoparticles. Long range migration of both mineral dust and anthropogenic pollutants from the major continents has recently been the subject of intense investigation. Approxi­mately 50% of troposphere atmospheric aero­sol particles are minerals originating from the deserts. The size of particles produced during a dust storm varies from 100 nm to several microns, with one third to a half of the dust mass being smaller than 2.5 microns. Particles in the range 100-200 nm can reach concen­trations of 1,500 particles/cm³.

Meteorological Observations and Modelling have Identified Ten Main Sources of Global Dust Events:

1. The Salton Sea,

2. Patagonia,

3. The Altiplano,

4. The Sahel region,

5. The Sahara Desert,

6. The Namibian desert lands,

7. The Indus Valley,

8. The Taklimakan Desert,

9. The Gobi Desert, and

10. The Lake Eyre Basin.

Sat­ellite imagery has revealed the dynamics of large-scale dust migration across continents, and demonstrated that nanoparticles generat­ed by major environmental events in one part of the world can affect regions thousands of kilometers away. For example, dust storms occurring every spring in Gobi desert strong­ly affect air quality in Asia and North Ameri­ca.

The dust route across the Pacific can be seen in satellite images by the yellow colour of the dust itself. During this event, the dust cloud reached the west coast of North America with­in 5-6 days after emission, with the region af­fected experiencing an intense haze and ele­vated particles concentrations, with an aver­age excess 20-50 mg/m³ with local peaks > 100 mg/m³.

Extra-terrestrial Dust:

Nanoparticles exist widely in extra-terrestrial space. Examples of dust are collected from space, from the moon, and on Mars. The extra-terrestrial dust poses major environmental problems for astronauts as well as for equipment. Lunar dust is very fine grained compared to typical terrestrial dust, with more than 50% of particles found to be in the micron range or smaller.

The lu­nar dust contains a considerable amount of magnetic nanoparticles, clinging to electrostat­ically charged surfaces such as the astronauts’ space suits, rendering it nearly impossible to remove. On Mars, dust accumulating on the solar panels of the exploration robots has lim­ited the power available to them for locomo­tion, sensing, and communication. Aiming to mitigate the environmental effects of extra-ter­restrial dust on humans and machines, vari­ous research projects are directed towards the fabrication of filters or thin film coatings that repel dust.

Health Effects:

Terrestrial airborne dust par­ticles can lead to a number of health problems, especially in subjects with asthma and emphy­sema. The composition of dusts is important, as iron or other metals rich-dust can generate reactive oxygen species on the lung surface that can scar lung tissues.

In addition, viruses, bac­teria, fungi, or chemical contaminants hitch­hiking dust particles may adversely affect health and the environment. In this regard it is important to note that two hundred types of viable bacteria and fungi have been found to survive ultraviolet light exposure during intercontinental journeys from Africa to America.

Extra-terrestrial dust brought inside the Lunar Module became airborne and irri­tated lungs and eyes of Apollo astronauts. On longer missions to the moon or Mars, pro­longed exposure could increase the risk of res­piratory diseases in the astronauts, and me­chanical failures of spacesuits and airlocks. Studies on rats have found that intra-tracheal administration of small amounts of lunar ma­terial resulted in pneumoconiosis with fibro­sis formation (lung disease and abnormal tis­sue growth).

Forest Fires and Health Effects:

For­est fires and grass fires have long been a part of Earth’s natural history, and are primarily caused by lightning strikes or by human ac­tivity. Major fires can spread ash and smoke over thousands of square miles and lead to an increase of particulate matter (including nano­particles) exceeding ambient air quality stan­dards.

Satellite maps show a unique picture of global fire activity. Using daily global fire de­tection provided by MODIS on NASA’s Terra satellite, the fire activity for the entire surface of the Earth has been mapped every day since February 2000. As noticed in this figure, nu­merous fires occur throughout the world in the savannas of Africa, Australia, and Brazil, in North America, Europe and Asia.

Health Effects:

Epidemiological studies showed that during the weeks of forest fires, medical visits increase more than 50% in the affected regions. Patients with pre-existing cardiopulmonary conditions reported worsen­ing symptoms during smoke episodes.

The usage of air cleaners was associated with less adverse health effects on the lower respiratory tract. Around 75% of fire-related deaths are due to respiratory problems related to smoke inhalation and not necessarily burns. The treatment for smoke in an emergency room is usually oxygen. Due to the fact that the symp­toms may be delayed until 24-36 hours after inhalation, the patient must be kept under observation for several days.

Volcanoes and Health Effects:

When a volcano erupts, ash and gases containing particulate matter ranging from the Nano scale to microns are propelled high into the atmo­sphere, sometimes reaching heights over 18,000 meters. The quantity of particles re­leased into the atmosphere is enormous; a sin­gle volcanic eruption can eject up to 30 mil­lion tons of ash.

Volcanic ash that reaches the upper troposphere and the stratosphere (the two lowest layers of the atmosphere) can spread worldwide and affect all areas of the Earth for years. A primary effect of upper at­mospheric particulate debris is the blocking and scattering of radiation from the sun.

One particularly harmful volcanic product is par­ticles composed of heavy metals, as these are known to be toxic to humans. While some ef­fects are seen worldwide, the highest levels of particulate matter are found in areas within tens of km from the volcano.

Health Effects:

Short-term effects of ash of health include: respiratory effects (nose and throat irritation, bronchitis symptoms), and eye and skin irritation. To assess the impact of long-term exposure to volcanic particulate pollution, we can look to the barefoot agricul­tural populations living in parts of the world containing volcanic soils, such as Africa, Mediterranean, and Central America.

A large per­centage of this population is affected by dis­eases of lympho-endothelial origin. The dis­ease includes podoconiosis and Kaposi’s sarco­ma. Podoconiosis is a non-communicable disease producing lymphedema (localized fluid re­tention) of the lower limbs.

The cause of this disease is believed to be the absorption through the skin of the feet (podos) of Nano and micro particles from the soil (konia). Lymphedema occurs when the lymphatic system fails to properly collect and drain the intersti­tial fluid of the body, resulting in the long-term swelling of a limb or limbs.

The lymphatic system is a secondary circulatory system in the body that collects fluid from several sources, primarily that lost from the circulatory system (blood), for example from damaged blood ves­sels in an area of inflammation (e.g. after a burn, or other injury). The lymphatic system lacks a central pump, i.e. the equivalent to the heart in the circulatory system, so it relies on a network of vessels and nodes that pumps during usual (muscle) motion of the body. If the accumulation of the interstitial fluid is fast­er than the pumping, then the tissue swells.

In podoconiosis the effect is irreversible, and af­fects about 10% of the populations in volcanic tropics. Soil particles with size ranging from 400 nm up to 25 microns, were found in the dermis of the foot of individuals with podoco­niosis. These particles were found in the mac­rophages, the cytoplasm of other cells, as well as in lymph node biopsies, as indicated by scanning electron microscopy.

Energy disper­sive x-ray analysis techniques showed compo­sitions consistent with the elements present in black lava soil and red clay soil. It is hypothe­sized that large quantities of small particles and chronic exposure overwhelm the normal func­tion of the lymphatic drainage system in the patients with podoconiosis, blocking drainage of both particles and lymph fluid. Kaposi’s sar­coma is a form of cancer affecting the blood and lymph vessels, and is also related to hu­man herpes virus infection. Endemic Kaposi’s sarcoma is characteristic to parts of the world containing volcanic soils.

It was found that iron particles from the iron rich volcanic soils may be one of the co-factors involved in the etiolo­gy (set of causes) of Kaposi’s sarcoma. In chronic exposure to iron volcanic clays, ferro­magnetic nanoparticles penetrate the skin of barefoot agricultural workers leading to im­paired lymphatic drainage and local immuni­ty, leaving the organism prone to infections (such as herpes virus).

Treatment: The treatment of podoconiosis in early stages involves elevation and elastic stockings, while in more advanced stages the only treatment is surgical. Treatment of Kaposi’s sarcoma involves iron withdrawal, and iron chelators. Both these diseases, podoconi­osis and Kaposi’s Sarcoma, could be prevent­ed by wearing shoes or boots (not sandals or shoes with open spaces) starting from early childhood.

Ocean and Water Evaporation and Health Effects:

A large amount of sea salt aero­sols are emitted from seas and oceans around the world. These aerosols are formed by water evapo­ration and when wave-produced water drops are ejected into the atmosphere. Their size ranges from 100 nm to several microns. Nano­particles can also form in bodies of water through precipitation, as a result of tempera­ture changes and evaporation. An example of this phenomenon is Lake Michigan that rests in a limestone basin, the water containing high levels of calcium carbonate.

During most of the year the calcium carbonate remains dis­solved in the cold water, but at the end of sum­mer the water temperature increases, lower­ing the solubility of calcium carbonate. As a result, the calcium carbonate may precipitate out of the water, forming clouds of nanome­ter-scale particles that appear as bright swirls when viewed from above.

Health Effects:

No adverse health effects have been associated to sea salt aerosols. On the contrary, beneficial health effects have been suggested from the use of salt aerosols in the restoration of the mucociliary clearance in patients with respiratory diseases. The unique microclimate of salt mines is a popular way to treat asthma, particularly in Eastern Europe. However, sea salt aerosols may transport pol­lutants and microorganisms those themselves may cause adverse health effects.

Organisms and Health Effects:

Many organisms are smaller than a few microns, in­cluding viruses (10 nm – 400 nm), and some bacteria (30 nm – 700 nm). However, we should make a clear distinction between what we call ‘particles’ (micro particle or nanoparti­cle) and Nano-organisms or their components (including bacteria, viruses, cells, and their organelles). Cells, bacteria, and viruses are self-organizing, self-replicating, dissipative struc­tures, with a shorter-lived structure than in­organic solids. Nano-organisms generally dis­sipate when their supply of energy is exhaust­ed.

In contrast, nanoparticles are typically in­organic solids that require no supply of ener­gy to remain in a stable form. They interact, dissipate, or transform via chemical reactions with their environment. Many organisms, both uni- and multicellular, produce Nano particulate inorganic materials through intracellular and extracellular processes. For example, mag­netic nanoparticles are synthesized by magneto tactic bacteria and used for navigation relative to the earth’s magnetic field, siliceous materials are produced by diatoms, or calci­um carbonate layers are produced by S-layer bacteria.

Magneto tactic bacteria orient and migrate along the geomagnetic field towards favourable habitats using nanometer-size mag­netic particles inside the cell. These bacteria are aquatic microorganisms inhabiting fresh­water and marine environments. Diatoms are unicellular algae with cell walls made of silica. They are abundant in plankton communities and sediments in marine and freshwater eco­systems, where they are an important food source for other marine organisms.

Some may even be found in moist soils. Diatoms are used in forensic science to confirm drowning as a cause of death and localize the site of drown­ing, based upon the observation of diatoms in lungs, blood, bone marrow, and organs.

Nano- bacterium is a Nano-organism that synthesiz­es a shell of calcium phosphate to cover itself, and resembles an inorganic particle. The shell ranges in size between 20 to 300 nm, and due to its porous nature it allows the flow of a slimy substance. This slime (presumably together with electrical charge) promotes the adhesion to biological tissues and the formation of col­onies. Nano bacteria are very resilient, being temperature and gamma radiation resistant.

Health Effects and Treatment:

Among these biological nanoparticles, diatoms might pose a health risk to workers of diatomaceous earth mining and processing; biogenic magnetite is associated with neurodegenerative diseases, and Nano bacteria shells were found in humans and animals.

Nano bacteria are ubiquitous within living organisms, humans and animals, being identified in blood, serum, and organs. These very small bacteria are suspected of be­ing the cause (at least in part) for many dis­eases involving calcifications, such as: artery plaque, aortic aneurysm, heart valves, renal stone formation, chronic prostatitis, ovarian and breast tumors.

They may also be the cause of rapid kidney stone formation in astronauts on space travels, according to a NASA study, probably due to the fact that their multiplica­tion rate in a microgravity environment in­creases fourfold compared to the rate under normal condition of gravity (of only about 3 days for doubling rate).

Definitive mechanisms relating Nano bacteria to these above men­tioned diseases are unknown, however, there are speculations that Nano bacteria colonies may act as nucleation sites for plaque or stone formation. Specific therapies, such as laser ir­radiation, or antibiotics, have shown reduced plaque formation, and even the regression of plaques.

Anthropogenic Nano-Materials:

Humans have created Nano-materials for mil­lennia, as they are by-products of simple combustion (with sizes down to several nm) and food cooking, and more recently, chemical manufacturing welding, ore refining and smelting, combustion in vehicle and airplane engines, combustion of treated pulverized sew­age sludge, and combustion of coal and fuel oil for power generation.

While engineered nanoparticles have been on the market for some time and are commonly used in cosmet­ics, sporting goods, tires, stain-resistant cloth­ing, sunscreens, toothpaste, food additives, etc., these nanoparticles—and new more de­liberately fabricated nanoparticles, such as car­bon nanotubes constitute a small minority of environmental Nano-materials. The quantity of man-made nanoparticles ranges from well-established multi-ton per year production of carbon black (for car tires) to microgram quantities of fluorescent quantum dots (mark­ers in biological imaging).

Diesel and Engine Exhaust Nanopar­ticles and Health Effects:

Diesel and automo­bile exhaust are the primary source of atmo­spheric Nano and micro particles in urban ar­eas. Most particles from vehicle exhaust are in the size range of 20-130 nm for diesel engines and 20-60 nm for gasoline engines and are typ­ically approximately spherical in shape. Car­bon nanotubes and fibers, already a focus of on-going toxicological studies, were recently found to be present in engine exhaust as a by-product of diesel combustion and also in the environment near gas-combustion sources.

The aspect ratio of these fibers is comparable to those of lung-retained asbestos, suggesting that strong carcinogens may exist in exhaust. Prior to the releases of these findings they were thought not to exist in the environment and their existence was attributed exclusively to engineering by material scientists. Nanopar­ticles constitute 20% of the particles mass but more than 90% of the number of diesel gener­ated particles.

Due to recent health concern, particle size distribution and number concen­trations studies were conducted in various cit­ies along different continents. A high number concentration of nanoparticles can be located near freeways on scales of hundreds of meters, showing that vehicular pollution is a major source of local contaminant particulate mat­ter that includes nanoparticles.

The daily pro­file of nanoparticles matches that of local ve­hicles usage. High pollution episodes or proximity to high-traffic roads can increase the mass concentration of nanoparticles by sever­al times from typically low background levels of approximately 0.5-2 µg/m³.

Health Effects:

Research has shown some heterogeneity in the magnitude of adverse health effects of engine exhaust in different cities, probably related to the complexity and composition of particles mixtures. Generally, diesel exhaust is known to be toxic as it con­tains high levels of poly-nuclear aromatic hy­drocarbons (PAHs) including the known car­cinogen benzo-a pyrene (BaP).

Atmospheric particle pollution from automobile exhaust seems to have a major influence on mortality, with a strong association between increased cardiopulmonary mortality and living near major roads. The findings of this epidemio­logical study are in concordance with measure­ments of nanoparticle concentration near highways, the concentration decreasing expo­nentially over several hundred meters from the traffic.

Childhood cancers were also found to be strongly determined by prenatal or early postnatal exposure to oil-bases combustion gases, primarily engine exhaust. Professional drivers show elevated rates of myocardial inf­arction (heart attack). Studies done in non­-smoking, healthy, young patrol officers have shown that nanoparticles from vehicular traf­fic may activate one or more signaling path­ways that cause pro-inflammatory, pro-thrombotic and hemolytic (breakdown of red blood cells) responses. It was noted that heart rate variability was significantly associated with measures of pollution.

Epidemiological studies conducted on diesel locomotive driv­ers showed a correlation between occupational exposure to diesel engine exhaust and inci­dence of lung cancer in the workers. These findings suggest that pollutants emitted by vehicles harm the health of many people, and that professional drivers, frequent drivers, pas­sengers, and peoples living near major roads are at elevated risk. Results seen in these stud­ies suggest that exposure to exhaust nanopar­ticles leads to increased risk of cardiovascular events over the long term.

Indoor Pollution and Health Effects:

Indoor air can be ten times more polluted than outdoor air, according to the Environmental Protection Agency (EPA) of US. Humans and their activities generate considerable amounts of particulate matter indoors. Nanoparticles are generated through common indoor activities, such as: cooking, smoking, cleaning, and combustion (e.g. candles, fireplaces). Exam­ples of indoor nanoparticles are: textile fibers, skin particles, spores, dust mite’s droppings, chemicals, and smoke from candles, cooking, and cigarettes. A quantitative determination of nanoparticle emissions from selected indoor sources is given in (Table 14.1).

Particles have also been shown to enter buildings from out­doors through ventilation systems. As humans generally spend much of their time indoors (more than 80%), indoor pollution directly affects our health.

Health Effects:

Long-term exposure to in­door cooking emissions may pose adverse health effects due to particulate matter inha­lation. During cooking, the level of particu­late matter increases more than ten-fold com­pared to non-cooking hours. In many regions of the world, death caused from indoor smoke from solid fuels inconsiderable, especially in Africa and Asia.

Poorly ventilated stoves us­ing biomass fuels (wood, crop residue, dung, coal) are the main responsible for the death of an estimated 1.6 million peoples annually, from which more than a half are children un­der the age of five.

World Health Organiza­tion estimates more than 50% of the world population uses solid fuels for cooking and heating, including biomass fuels. Wood burn­ing is often disregarded as a source of nano­particles and assumed to be benign to the en­vironment simply because wood is a renew­able source.

Cigarette Smoke and Health Effects:

As a combustion product, tobacco smoke is composed of nanoparticles with size ranging from around 10 nm up to 700 nm, with a max­imum located around 150 nm. The environmental tobacco smoke has a very complex composition, with more than 100,000 chemi­cal components and compounds.

Health Effects:

Environmental tobacco smoke is known to be toxic, both due to some of its gas phases as well as nanoparticles. A plethora of studies have investigated the ad­verse health effects of environmental cigarette smoke. Substantial evidence shows that, in adults, first or second hand cigarette smoke is associated with an increased risk of chronic res­piratory illness, including lung cancer, nasal cancer, and cardiovascular disease, as well as other malignant tumors, such as pancreatic cancer and genetic alterations.

Children ex­posed to cigarette smoke show an increased risk of sudden infant death syndrome, middle ear disease, lower respiratory tract illnesses, and exacerbated asthma. Cigarette smokers are more likely than non-smokers to develop many conditions including cancers and vascular dis­eases. It was noted that the risk of myocardial infarction decreases substantially within two years after smoking cessation, proving a revers­ibility of inhaled nanoparticles induced vulnerability.

Buildings Demolition and Health Effects:

Particulate matter concentrations can raise to very high levels when large buildings are demolished, especially the reparable ones with diameter smaller than 10 microns. Older buildings are very likely to have been con­structed with parts containing known toxins. Consequently, respirable asbestos fibers, lead, glass, wood, paper, and other toxic particles are often found at the site of demolition. In addition, the dust cloud can travel tens of kilometers and affect the neighboring region of the collapsed building site.

Health effects of exposure to demolition par­ticles and soot are not entirely known. Early clinical and epidemiological assess­ments of fire-fighters present at the site of the environmental disaster generated by the attack on the World Trade Center on September 11, 2001, indicated exposure-related health effects, with prevalence of respiratory symptoms, es­pecially increased cough and bronchial hyper­activity. Long-term effects, however, remain to be seen.

Cosmetics, Other Consumer Prod­ucts and Health Effects:

Cosmetics:

The use of Nano-materials in cosmetics is not new. Black soot and mineral powders have been used as cosmetics since thousands of years ago in an­cient Egypt, and some of them continue to be used today.

Due to the recent development of nanotechnology, engineered Nano-materials have been embraced by the cosmetics indus­try for several reasons:

(a) Because of their ability to penetrate deeper into the protective layers of skin than any cosmetic before, they are used as delivery agents for skin nutrients, such as synthetic peptides that instruct cells to regenerate,

(b) Some nanoparticles have antioxidant properties, feature that helps main­tain a youthful appearance of the skin. For ex­ample, functionalized fullerenes are now in­corporated into cosmetic products, such as creams, claiming radical scavenging proper­ties.

(c) Due to their small size and specific optical properties, they are thought to conceal wrinkles and small creases. For example, alu­mina Nano powder is used for optical reduc­tion of fine lines. Many cosmetic and person­al care products incorporate Nano-materials. For a Compilation of websites and product in­formation see reference. They include: person­al care products (deodorants, soap, tooth-paste, shampoo, and hair conditioner), sunscreen, cos­metics (cream, foundation, face powder, lip­stick, blush, eye shadow, nail polish, perfume and after-shave lotion).

There are two trends regarding the use of engineered nanoparticles in cosmetics.

First, a swift application of nan­otechnology advances in the cosmetic indus­try, in addition to relabeling of the products that already contain nanoparticles, so that they are more appealing to the consumers.

Second, targeting of cosmetic companies that use nano­particles. For the general public and unin­formed journalists there is not much of a dif­ference between the various types of nanopar­ticles currently used in cosmetics, such as lip­id based nanoparticles, fullerenes, silicon, etc. Everything labeled ‘nanoparticle’ is considered dangerous to some.

These trends result at least in part from the lack of regulations for testing of cosmetic products before they are sold to the public, unlike pharmaceutical products that are required to undergo several years of research before being considered safe. Despite the fact that many of the cosmetic companies claim safety related research, their results are not always disclosed to the public.

Other Consumer Products:

Many consum­er products incorporate Nano or micro particles. Titanium dioxide (TiO₂) particles with diameter larger than 100 nm are considered biologically inert in both humans and animals. Based on this understanding, titanium diox­ide nanoparticles have been widely used in many products, such as white pigment, food colorant, sunscreens and cosmetic creams.

However, adverse effects of titanium dioxide nanoparticles have recently been uncovered. New research is exploring the potential use of nanostructured titanium dioxide photo catalyst materials for sterilizing equipment of en­vironmental microorganisms in the health care facility.

Silver nanoparticles are used as anti-bacterial/antifungal agents in a diverse range of applications: air sanitizer sprays, socks, pil­lows, slippers, face masks, wet wipes, detergent, soap, shampoo, toothpaste, air filters, coatings of refrigerators, vacuum cleaners, washing machines, food storage containers, cellular phones, and even in liquid condoms. Coatings of nanoparticles are widely used for modify­ing fabrics to create stain and wrinkle free properties. In additions, one can find clothes with built-in sunscreen and moisture manage­ment technology.

Fabric containing bamboo- charcoal nanoparticles claims antibacterial antifungal properties. They are intended for use as face cloth masks, shoes insoles. Nano- coatings are applied to wetsuits for higher per­formance of athletes, or self-cleaning surfac­es. Textiles with 30 nm embedded nanoparti­cles help prevent pollen from entering gaps in the fabric.

Nanoparticles or Nano fibers are starting to be used in water-repellent, stain resistant plush toys, stain repellent mattress­es. Nano-sealant sprays for fabrics or leather, and hydrophobic nanoparticle solutions ad­hering to concrete, wood, glass, cloth, etc., al­low the surfaces to deflect water. The most peculiar applications of Nano fibers and nano­particles discovered in our literature review are: Nano fibers that hide hair loss, and liquid condoms.

Health Effects:

All the health effects of the gamut of nanoparticles used in consumer products are not yet known, though Nano toxicology has revealed adverse health effects of materials previously considered safe. For ex­ample, silver, widely used as an antibacterial agent, proves to be toxic to humans or animal cells when in nanoparticle form, its cytotoxic­ity being higher than that of asbestos.

Inhala­tion of silver nanoparticles leads to their mi­gration to the olfactory bulb, where they lo­cate in mitochondria, as well as translocation to circulatory system, liver, kidneys, and heart. Silver nanoparticles have been found in the blood of patients with blood diseases and in the colon of patients with colon cancer. A controversial subject is the association between the uptake of aluminium and Alzheimer’s dis­ease. Epidemiological studies researching the connection between aluminium in antiperspirants, antacids, or drinking water and Alzhe­imer’s disease are conflicting, some finding positive associations and others none.

Due to their latent evolving nature and multi-part eti­ology, these neurological diseases are difficult to associate with specific factors. For exam­ple, the exposure takes place much earlier than the disease occurrence; hence the subjects may not recall possible exposure, their memory being already affected by the disease.

More­over, subjects that suffer from advanced neu­rodegenerative diseases are not likely to par­ticipate in epidemiological studies due to their reduced ability to communicate and remem­ber. In addition, multiple factors are known to contribute to Alzheimer’s disease, such as: ge­netics, increasing age, endocrine conditions, oxidative stress, inflammation, smoking, in­fections, pesticides, electromagnetic fields.

In general, several questions arise related to the safety of nanoparticles as consumer products. Are they biocompatible?

Do the nanoparticles enter the lymphatic and circulatory systems?

If not, do they accumulate in the skin and what are the long-term effects of accumulation?

Do they produce inflammation?

If they enter the lymphatic and circulatory system, is the amount significant? What are the long-term effects of this uptake?

Related to the benefi­cial antioxidant properties of some Nano-materials, long-term effect need to be studied, in addition to the short-term antioxidant effect. What is the long-term fate of these nanoparti­cles?

Are they stored in the skin? Do they en­ter circulation?

What happens when the nano­particles undergo chemical reactions and lose their antioxidant properties?

The answers to some of these questions are known, and will be presented in this article dedicate to nano­particles toxicity; however most of the remain­ing questions still remain unanswered.

Engineered Nano-Materials and Health Effects:

The fabrication of Nano-materials is a broad and evolving field. Nano-materials can be synthesized by many methods in­cluding: gas phase processes (flame pyrolysis, high temperature evaporation, and plasma synthesis); vapour deposition synthesis (elec­tron, thermal, laser beam evaporation); colloi­dal, or liquid phase methods in which chemi­cal reactions in solvents lead to the formation of colloids; and mechanical processes includ­ing grinding, milling and alloying.

A review of nanomaterial fabrication processes is given in. A critical fact to consider with engineered Nano-materials is that they can be synthesized in almost any shape and size by materials sci­entists. Several examples are given in (Figs. 14.3, and 14.4). Nanostructure materials shown in (Figure 14.3) are firmly attached to a substrate and do not pose a health risk as long as they do not detach from the substrate. (Figure 14.4) shows nanostructured materials where nano­structures are free and can become airborne, consequently posing a potential health risk.

Environmental and Occupational Exposure to Toxic Substances:

Metals and Other Dusts:

Small quan­tities of many metals, including copper, mag­nesium, sodium, potassium, calcium and iron are essential for proper functioning of biolog­ical systems. At higher doses, however, metals can have toxic effects and exposure to high lev­els of environmental metals causes diseases in humans. The metals listed below in this para­graph are known to be toxic upon inhalation, ingestion or dermal exposure. Nanoparticles manufactured from these metals will have health effects not necessarily easily predicted from previous studies of non-Nano particulate quantities of the same metals.

As it could eas­ily expose workers to these toxic materials, manufacturing of metal nanoparticles should be considered a serious occupational hazard. The inhalation of metallic or other dusts is known to have negative health effects. The type of lung disease caused by dust inhalation de­pends on the nature of the material, exposure duration, and dose.

The inhalation of some metal fumes (e.g. zinc, copper) may lead to metal fume fever, an influenza-like reaction. Several metal dusts (e.g. platinum, nickel, chro­mium, cobalt) can lead to asthma, while inha­lation of other metallic dusts can cause pul­monary fibrosis, and ultimately lung cancer. The percentage of lung cancers attributable to occupational hazards is about 15%, with exposure to metals being a major cause.

Beryllium:

Beryllium alloys are used for making electrical and electronic parts, and molds for plastic. Inhalation can cause lung damage leading to a pneumonia-like syn­drome called acute beryllium disease. Berylli­um exposure can also lead to hypersensitivity, and allergic reaction characterized by an in­flammatory immune response to even tiny amounts of beryllium.

Hypersensitivity can lead to chronic beryllium disease, where white blood cells accumulate around absorbed be­ryllium particles and form granulomas lead­ing to anorexia, weight loss, cyanosis of the extremities, and heart enlargement. Long- term exposure to beryllium causes cancer in animals and increased risk of lung cancer in humans.

Lead:

Exposure to lead occurs through the air, household dust, food, and drinking water. Airborne lead may be present in industrial emission, such as those from smelters and re­fineries. Exposure to high levels of lead and its compounds can cause serious disability.

At highest risk are workers involved in the man­ufacture of batteries, metals, and paints; the printing industry; or chronically exposed to lead dust (e.g. through sanding of surfaces coated with lead) or insecticides. Inhaled or ingested lead circulates in the blood and is deposited in bone and other tissues.

Follow­ing inhalation, about 50-70% of lead is ab­sorbed into the blood, allowing it to circulate to most organs. Manifestations of lead intoxi­cation include impairment of mental func­tions, visual motor performance, memory, and attention span, as well as anemia, fatigue, lack of appetite, abdominal pain, and kidney dis­ease, among others.

Cobalt:

Diseases associated with exposure to cobalt are—asthma, acute illness (fever, an­orexia, malaise, and difficulty breathing, re­sembling a viral illness), and interstitial pneu­monitis.

Cadmium:

Cadmium is used in batteries, pigments, metal coatings, plastics, and is a by­-product of the burning of fossil fuels and cig­arettes. As a result of industrial and consumer waste, cadmium accumulates in soil at a rate increase by 1% per year. Plants and food-crops growing in contaminated soil take up cadmi­um, leading to contamination of vegetables and animals. High dose inhalation exposure leads to severe lung irritation, nausea, and vomiting.

Long-term low dosage exposure in humans causes lung emphysema, impairment of the immune system, central nervous system and liver damage. Occupational exposure to cadmium has been linked to lung cancer in humans, some studies associating cadmium exposure with cancer of the liver, bladder and stomach, and possibly of pancreas.

Aluminum:

Exposure to aluminum occurs through consumption of food and water, as well as usage of many products containing alu­minum, including antacids and antiperspirants. The use of antiperspirants combined with under arm shaving is associated with an earlier age of breast cancer diagnosis. Alumi­num excess can lead to anemia, bone disease, and dementia.

Exposure to high levels of alu­minum (and other metals, such as iron) is re­lated to neurological disorders, such as dialy­sis encephalopathy, Parkinson dementia, and especially Alzheimer’s disease. Studies of brain plaques associated with Alzheimer’s disease show abnormally high aluminum, but have not shown if this is a cause or effect of the disease.

However, one can hypothesize that a critical mass of metabolically errors is important in pro­ducing Alzheimer’s disease. If aluminum can reach the brain via the olfactory bulb by pass­ing the blood brain barrier, or via the circula­tory system, then brain metabolically errors re­sulting from accumulations of this metal in parts of the brain could contribute to the on­set of Alzheimer’s disease.

Rats that received subcutaneous injection of aluminum glutamate show pathological signs similar to those observed in human Alzheimer’s disease. They show a significant increase of aluminum con­tent in the brain (hippocampus, occipito-parietal cortex, cerebellum, striatum), and symp­toms that include trembling, equilibrium in­stabilities, and convulsions, followed by death one hour after the injection.

Nickel and Chromium:

Nickel is used for the production of stainless steel and other nickel alloys with numerous applications. Occupa­tional exposure to nickel via inhalation of dust and fumes is associated with cancers of lung and sinus. Chromium derived from smelting has also been found to cause cancer.

Manganese:

Manganese is both an essential nutrient and is known to have neurotoxic ef­fects. At high levels, manganese exposure to contaminated water or through inhalation re­sults in neurological impairment. Occupation­al exposure generally occurs only to those in­volved in mining and welding. There is a clear association between manganese and neurolog­ical disease in miners exposed to MnO₂ dust.

The neurological disorder linked most closely to manganese is Parkinson’s disease. Some welders develop Parkinson’s disease much ear­lier in their life, usually in their mid-forties, compared to the sixties in the general popula­tion. Of concern for public health is the risk of neurological diseases emerging after long la­tencies in regions with only mildly elevated environmental manganese levels.

Iron:

Iron is incorporated into numerous enzymes involved in cell division, DNA repli­cation, and cellular metabolism, and it is es­sential for oxygen transport and has exchange. As with manganese, low doses of iron are vital for survival. Several observations have been made linking cellular iron content to the de­velopment of cancers. In studies of animals administered excessive amounts of iron—oral­ly and by injection—an increased risk of adenocarcinomas, colorectal tumors, hematomas, mammary tumors, mesothelioma, renal tubular cell carcinomas, and sarcomas was observed.

In humans, injection of iron com­pounds has been shown to cause sarcomas at the sites of deposition. Patients with hemo­chromatosis (genetic disease characterized by increased iron absorption) have an enhanced susceptibility to liver cancer.

The accumula­tion of iron in brain regions with decreased function and cell loss has been observed in many neurological diseases, such as Parkin­son’s disease, Alzheimer’s disease, etc. Inhala­tion of iron dust causes a respiratory disease called pneumoconiosis.

Organic Dust:

Organic dusts originate from animals and/or plants and contain fragments and fibers from wood, bone, fur, skin, leather, brooms, flour, grains, tobacco, carpets, paper, etc. Organic dust from these various sources irritates the upper respiratory system, eyes, and skin, causing bronchitis, allergic reactions, asthma, conjunctivitis, and dermatitis.

Silica:

Exposure to silica, or silicon dioxide (SiO₂), the main constituent of sand and gran­ite, produces silicosis, a disabling pulmonary fibrosis. A controversial subject in occupation­al medicine is the association of silicosis with lung cancer. In addition, exposure to silica is associated to autoimmune disease including: scleroderma, rheumatoid arthritis, and systemic lupus erythematous.

Coal and Coal Ash:

Coal dust produces pneu­moconiosis in coal miners, their lungs retain­ing a considerable amount of dust, of up to 30 g (roughly two tablespoons of dust). Epidemi­ological study on more than 500 chimney sweeps showed an increased number of deaths due to heart and respiratory diseases, lung, esophageal, and liver cancer.

Asbestos:

Asbestos is a naturally occurring fibrous material consisting of very long chain of silicon and oxygen (polysilicate or long chain silicate). Asbestos fibers have high ten­sile strength, flexibility and have flame retardant and insulating properties. In ancient times, asbestos was woven and used in fab­rics such as Egyptian burial cloths and Char­lemagne’s tablecloth, which according to leg­end he threw in a fire to clean.’ (Wikipedia, the Free Encyclopedia).

Due to its desirable properties it was once used extensively in con­struction materials (cement, floors, roofing, pipe insulation, and fire-proofing) and in ma­terials industry (brake pads). Asbestos expo­sure occurs when its handling produces small fibers, nanoparticles, which are easily carried as a suspension in both air and water where they are absorbed by inhalation and ingestion.

Studies of occupational health show that ex­posure can cause lung cancer and mesothelioma (a rare cancer of the membranes lining the abdominal cavity and surrounding internal organs). Recent studies in a community with occupational and environmental exposure to asbestos showed increased risk of autoimmune diseases, such as: systemic lupus erythemato­sus, scleroderma, rheumatoid arthritis. These diseases affect connective tissues, skin, and organs.

Polymer Fumes:

Humans exposed to poly-tetrafluoroethylene (or Teflon, PTFE) and oth­er polymer fumes develop an influenza-like syndrome (polymer fume fever). The symp­toms occur several hours after exposure, and include chest pain, fever, chills, sweating, nau­sea and headache. Severe toxic effects, like pulmonary edema, pneumonitis and death, are also possible.

Carcinogens and Poorly Soluble (Du­rable) Particles:

It is clear that some types of particles cause cancer, but it is not known which characteristics of the particles are re­sponsible for their carcinogenicity. Some par­ticles are inherently toxic, such as metal dust, welding fume, and quartz dust, while other particles have a much lower toxicity, but still cause toxic effects under some circumstances. The latter category includes poorly soluble particles, bio-durable particles without known specific toxicity that include: diesel exhaust particles, carbon black, coal-mine dust and ti­tanium dioxide.

Poorly soluble particles have been shown to cause cancer in rodents; how­ever, epidemiologic studies do not clearly in­dicate increased cancer rates in humans ex­posed to these particles. The latest research on nanoparticles shows that they can exhibit more pronounced toxicity than larger micro particles, suggesting that environmental and health regulating agencies must take more consider­ation of particle size distribution, shape, and agglomeration when establishing regulatory exposure guidelines.

Aerosol Pollution, Monitoring, and Health Effects:

Aerosol Size and Composition:

Aero­sol pollution is a combination of particulate matter and gaseous and liquid phases from natural and anthropogenic sources. Ambient particulate matter is generally classified ac­cording to three size distributions: nanoparti­cles smaller than 100 nm in diameter (mainly resulting from combustion), accumulation mode particles between 100 nm and 2.5 mm in diameter (from aggregation of smaller par­ticles and vapors), and coarse-mode particles larger than 2.5 mm (mostly mechanically gen­erated), These three particle categories have distinct chemical compositions, sources, and lifetime in the atmosphere.

The larger parti­cles, which settle faster due to gravity, are re­moved fastest from the atmosphere. Smaller particles are transported over greater distanc­es and have longer lifetimes in the atmosphere. Nanoparticles usually form atmospheric frac­tal-like dendritic aggregates. The poly-dispersity (variation in particle sizes) varies with the source, for example, primary particles in die­sel aggregates ranging from 10 to 40 nm. At­mospheric measurements show that nanopar­ticles make up a small portion of the particu­late matter mass concentration compared to micro-particles.

However, the number concen­tration of nanoparticles is significantly larger than the number of micro-particles. Combus­tion-derived carbon particles, with traces of transition metals, make up about 50% of the mass of typical urban particulate matter, while the remaining 50% includes salts, geological dust, and organic matter.

In general, environ­mental pollution particles differ in their quan­tities of nitrates, sulfates, crustal materials, and carbon, with blown soil a major source in ru­ral areas. Due to the high chemical reactivity of atmospheric nanoparticles (resulting from their high surface area), they are very likely to interact with water or other chemicals in the atmosphere to form new species.

This dynamic nature of aerosol nanoparticles means that their environmental impact will be long and complex, as reactions create a cascade of prod­ucts with varying effects—while some parti­cles will be long-lived, or persistent, others may experience transformations to more or less damaging states.

Aerosol Concentration: Air Quality Index:

Nanoparticles with size smaller than 100 nm are present in large numbers in typi­cal ambient air with a level ranging between 5000-10000 particles per ml, increasing dur­ing pollution episodes to 3,000,000 particles/ ml. Their concentration varies from region to region, as well as from season to season. Nano­-particles smaller than 100 nm make up about 70% of the total number of ambient aerosols in urban areas, while their mass contribution is only about 1%.

In certain parts of the world the peak concentration or airborne nanopar­ticles was found to increase over time. For ex­ample, in California, the peak concentration of nanoparticles in January 1999 (1.45 1011 particles/m³) was found to be three times high­er than previously measured peaks.

At the oth­er extreme are modern cleanroom facilities where air particles are almost eliminated through careful design of airflow and filter­ing, and meticulous elimination of potential particle sources. A typical cleanroom, with Class 10 or ISO 3 particle levels has only sev­eral hundred 100 nm particles per cubic meter. Increased awareness of the influence of parti­cle size and shape on health impact has led the Environment Protection Agency to propose new ambient standards on fine particles small­er than 2.5 microns.

The Air Quality Index (AQI) is a standard measure used by the Envi­ronmental Protection Agency for monitoring daily air quality. It quantifies air pollution and predicts health effects of concern that may be experienced within a few hours or days of ex­posure to polluted air.

The calculation of the AQI includes five major pollutants: particu­late matter, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide, all of which are regulated under the Clean Air Act. The AQI has not been standardized internationally, and other countries use different systems for de­scribing air quality.

Satellite Monitoring of Aerosol Con­centration and Size:

Aerosols play an impor­tant role in the global atmosphere, directly in­fluencing global climate and human health. Dust, smoke, and haze locally impair visibili­ty and health in both urban and rural regions. Anthropogenic aerosol nanoparticles are es­pecially abundant in the atmosphere, and they constitute a significant uncertainty factor in estimating the climatic change resulting from human pollution.

Satellite images clearly show particulate matter from both anthropogenic and natural sources in industrialized and heavily populated parts of the world. Atmo­spheric aerosols are monitored worldwide via satellites, and several years’ worth of measured global aerosol maps is available from NASA’s MISR.

Global aerosol data is measured by im­aging sequential columns through the atmo­sphere below the satellite as it orbits the earth, in each of 4 wavelengths (blue, green, red, and near-infrared). These measures also give some indication of particle size and shape, from the variation on scene brightness over several dif­ferent view angles and wavelengths.

The MISR results distinguish desert dust from pollution and forest fire particles: desert dust particles and sea salt are usually larger than aerosols originating from the processes of combustion e.g. forest fires and burning of fossil fuels.

MISR can help to determine ground-level pol­lution concentrations necessary in understanding and assessing links between pollu­tion exposure and human health. A full assess­ment of the impact of pollution aerosol expo­sure will require records of aerosol mapping for several decades—the typical timescale of pollution linked disease appearance.

Health Effects Associated to Air Pol­lution:

Human exposure to inhaled ambient particles can have adverse health effects. Pul­monary and cardiovascular diseases result when inhaled particles interfere with the nor­mal function of bodily systems. The health consequences of particle inhalation vary great­ly with particle composition, concentration, etc., from benign candle wax to carcinogenic asbestos, or tobacco smoke.

As our under­standing of nanoparticles has grown, so has our knowledge of disease resulting from their exposure. Until recently it was believed that particles 10 microns or smaller were respon­sible for disease resulting for particle pollution.

But further study has shown that most of these diseases are caused by particles smaller than 100 nm, similar in size to viruses. Nanoparti­cles seem to be generally more toxic than micro particles, primarily due to their ability to penetrate living cells, translocate within the body, and affect function of major organs.

Cardiovascular Diseases:

The correlation between ambient particles exposure and heart disease was accepted in the mid-nineties, when it was observed that hospital admission for cardiovascular illness increased on days with high concentrations of particles.

Atmospher­ic particle pollution from automobile exhaust seems to have a major influence on mortality, with a strong association between increased cardiopulmonary mortality and living near major roads. The risk of myocardial infarction onset increases with elevated concentrations of particulate matter smaller than 2.5 mm in the day before onset and with volume of ve­hicular traffic.

Cardiovascular diseases and effects associated with particulate pollution include: ischemic heart disease, hypertensive heart disease, arrhythmia, heart failure, arte­riosclerosis, brachial artery vasoconstriction, and increased blood pressure in subjects with lung disease.

Respiratory Illnesses:

Pneumonia, bronchi­al asthma, chronic bronchitis, emphysema, lung cancer, acute deterioration of lung func­tion, and hospital admissions for respiratory illnesses were all found to increase with high­er levels of pollution.

Malignant Tumors:

An epidemiological study researching the effects of chronic expo­sure to particulate matter smaller than 10 mm in non-smoking subjects revealed a high inci­dence of lung cancer. This study also showed an 8% increase in risk of lung cancer for each 10 mg/m³ increase in particulate matter small­er than 2.5 mm.

To some surprise, levels of par­ticulate matter smaller than 2.5 mm pollution were also found to correlate significantly with cancers of the breast, endometrium and ova­ry, an effect that might be explained by recent studies of nanoparticles translocation to or­gans. Childhood cancers were also found to be strongly determined by prenatal or early postnatal exposure to oil-based combustion gases, primarily engine exhaust.

Mortality and Morbidity:

There is compel­ling evidence of correlation between particle pollution levels on a given day, and overall mortality the following day. Epidemiological studies have shown that the increased morbid­ity and mortality, correlated with increased particle pollution, are frequently the result of respiratory problems, but primarily due to car­diovascular diseases.

In 1998 it was estimated that around 4000 deaths were related to atmo­spheric pollution in Canada. These deaths oc­cur mainly in heavily industrialized urban Centre’s. Analysis of mortality statistics for approx­imately 500,000 adults residing in the United States of America covering a 16 year period of chronic exposure to air pollutants shows that cardiovascular deaths increased by 0.69% for each 10 mg/m³ increase in particulate matter.

The study found a strong correlation between a cause of death of either cardiopulmonary disease or lung cancer, and levels of particu­late matter smaller than 2.5 mm. It has been suggested that a high concentration of aerosol nanoparticles would promote particle aggregation. Aggregation of nanoparticles at high particle concentrations reduces toxicity by decreasing the reactive surface area and pos­sibly limiting the translocation of the particles.

Post-Neonatal Infant Mortality and Birth Defects:

Positive associations between expo­sure to particles and selected birth defects (such as atrial septal defects) were reported in studies in various countries. It was found that outdoor air pollution above a reference level of 12.0 mg/m³ of particulate matter smaller than 10 mm contributes substantially to post neonatal infant mortality in infants born with a normal birth weight.

Exacerbation of pre-existing diseases and other risks:

Certain segments of the popula­tion appear to be at greater risk to the toxic effects of particulate pollution. Patients suffer­ing of various diseases, such as: diabetes, chronic pulmonary diseases, heart diseases, or with previous myocardial infarction is likely to suffer an increase in the severity of symp­toms on days with high levels of pollutants. In addition, the presence of inflammation may enhance the translocation of nanoparticles into circulation, or via blood-brain-barrier.

Cumulative Exposure:

In addition to imme­diate effects, time-series studies have shown cumulative effects over weeks, associated with elevated particle concentrations. Further stud­ies are needed to assess the health effects of chronic exposure to nanoparticles.

Treatment:

Ambient particles induce oxida­tive stress in biological systems, either direct­ly by introducing oxidant substances, or more indirectly by supplying soluble metals, includ­ing transition metals, that shift the redox bal­ance of cells toward oxidation. Oxidative stress is believed to be the primary mechanism by which nanoparticles generate disease. Conse­quently, dietary nutrients that play a protec­tive role in the oxidative process are suggested as potential mitigators of the toxic effects of nanoparticle pollution.

Antioxidant vitamins (such as Vitamin C) have a protective effect against lung diseases, and a high intake of fresh fruit and some vegetables appears to have a beneficial effect on overall lung health perhaps due to reducing the toxic effects of environ­mental nanoparticles. Treatment of underly­ing health conditions also reduces the impact of air pollution.

Nanotoxicology of Nanoparticles:

Respiratory Tract Uptake and Clearance:

Particle Size Dependent Inhalation:

After inhalation, nanoparticles deposit throughout the entire respiratory tract, start­ing from nose and pharynx, down to the lungs. Lungs consist of airways, that transport air in and out, and alveoli, which are gas exchange surfaces.

Human lungs have an internal sur­faces area between 75-140 m² and about 300 million alveoli. Due to their large surface area, the lung is the primary entry portal for inhaled particles. Spherically-shaped solid material with particle diameters smaller than 10 mi­crons can reach the gas exchange surfaces. Larger diameter particles tend to be deposited further up in the respiratory tract as a result of gravitational settling, impaction, and intercep­tion.

Many larger-diameter fibers are deposit­ed at ‘saddle points’ in the branching respira­tory tree. Smaller diameter particles are more affected by diffusion and these can collect in the smaller airways and alveoli. Fibers having a small diameter may penetrate deep into the lung, though very long aspect ratio fibers will remain in the upper airways. The nasopharyn­geal region captures mainly micro particles and nanoparticles smaller than 10 nm, while the lungs will receive mainly nanoparticles with diameter between 10-20 nm.

Upper Airway Clearance-Mucociliary Escalator:

Pulmonary retention and clear­ance of particles has been under study for many years. The nineteen-fifties were marked by a great interest in pneumoconiosis and studies of the effects of inhalation of radioac­tive particles, while in the nineties studies of occupational and environmental particles gen­erated a considerable amount of knowledge regarding the adverse health effects of Nano and micro particles in the respiratory tract.

The clearance of deposited particles in the respira­tory tract is by physical translocation to other sites, and chemical clearance. Chemical dis­solution in the upper or lower respiratory tract occurs for bio-soluble particles in the intra-cellular or extra-cellular fluids, and will not make the subject of further discussions in this re­view.

Non-soluble particles will undergo a dif­ferent much slower clearance mechanism that we will discuss further in detail later. For rela­tively insoluble particles, the elimination pro­cess is very slow in comparison to soluble nanoparticles. In the upper airways, particle clearance is performed mainly by the muco­ciliary escalator.

The first contact of inhaled nanoparticles in the respiratory tract is with the lining fluid, composed of phospholipids and proteins, their contact leads to particle wetting and displacement towards the epithe­lium by surface forces from the liquid-air in­terface. When in contact with esophageal epi­thelial cells, nanoparticles uptake by these cells is possible in the presence of pre-existent in­flammation.

The cilia of the bronchial epithe­lial cells move the covering mucous layer, in­cluding particles, away from the lungs and into the pharynx, a process generally requiring up to several hours. The nanoparticles that are cleared from the lung via the mucociliary es­calator enter the gastro-intestinal tract.

Lower Airways Clearance-Phagocy­tosis and Passive Uptake:

Phagocytosis:

Par­ticles smaller than 10 microns can reach the lower airways. Particle clearance from the lungs alveoli occurs primarily through mac­rophage phagocytosis. Macrophages are cells that act as vehicles for the physical removal of particles from alveoli to the mucociliary esca­lator or across the alveolar epithelium to the lymph nodes in the lung or to those closely associated with the lungs.

When the lung is subject to prolonged exposure, white blood cells from the circulatory system (neutrophils) are recruited to help. Phagocytes engulf and break down pathogenic microorganisms, dam­aged or apoptotic cells, and inert particles.

In addition to the ‘professional cleaners’, phago­cytes (neutrophils and monocyte/macrophag­es), most cells also have some phagocytic abil­ity. The main difference between the phago­cytic ability of professional and non-profes­sional phagocytes is related to the presence of dedicated receptors able to recognize mole­cules pertaining to pathogens, molecules very different from those found in the human body.

Phagocytosis is a very complex mechanism due to the diversity of receptors, its understanding requiring thorough knowledge of chemical processes at molecular level. Many phagocytic receptors serve a dual function, adhesion and particle internalization.

The ph­agocytosis of particles is more effective if the particles are labeled with special molecules (such as antibodies or complement molecules) able to speed-up phagocytosis, a labelling pro­cess called opsonisation.

Opsonins are present in the lung-lining fluid. Hydrophobic particles will be readily coated by opsonins and subse­quently available for phagocytosis. Coating of particles with hydrophilic polymers, such as polyethylene glycol, diminishes the opsonisa­tion of particles, consequently decreasing the probability of being phagocytized. However, un-opsonized particles are nevertheless even­tually phagocytized by macrophages.

Phagocytosis takes up to Several Hours and Involves Several Steps:

1. First, specific receptors on the phagocyte membrane bind with specific molecules (ligands) localized on the surface, of particle. Older studies suggest that the opsonisation with complement protein 5a may be responsi­ble for the chemotactic (pertaining to the movement of a cell in a direction correspond­ing to a concentration gradient of a chemical substance) signal of nanoparticles, while newer studies propose the electric charge may play a role in activating the scavenger-type receptors for certain type of nanoparticles (such as: tita­nium dioxide, iron oxide, quartz). For un­charged nanoparticles, such as carbon based (diesel exhaust), some authors suggest that toll like receptors are responsible for the recogni­tion of these nanoparticles (as well as bacte­ria, virus, and fungi).

2. After the binding of the phagocyte recep­tor with a ligand, the cytoskeleton (a network of protein filaments) of the phagocyte rear­ranges, resulting in pseudopod formation, and ultimately leading to internalization of the particle with the formation of a phagocytic vesicle (phagosome).

3. The phagosome fuses with a lysosome (an organelle containing digesting enzymes), forming a phagolysosome. The fusion process can take from 30 minutes up to several hours, depending on the chemical interaction be­tween the surface of the particle and the phagosome membrane.

Lysosomes release pro­tease (which break down proteins) and NAD- PH oxidase (oxygen radicals). This process assists in the chemical dissolution of the par­ticle. Depending on the type of receptor used in the detection of the particle, macrophages may also release intercellular chemical mes­sengers alerting the immune system that an infection is present.

4. If the particle is digested by lysosome en­zymes, the residues are removed by exocytosis (release of chemical substances into the environment). If not, phagocytosis is followed by gradual movement of macrophages with internalized particles towards the mucociliary escalator, a process that can last up to 700 days in humans.

If the macrophage is unable to di­gest the particle and the particle produces damage to phagosomal membrane due to per­oxidation, the oxidative compounds will like­ly interact with macrophages cytoskeleton, and lead to reduced cell mobility, impaired phago­cytosis, macrophage death, and ultimately re­duced clearance of particles from the lung.

Macrophage death can lead to release of oxi­dative lysosome compounds outside the cells. If particles cannot be cleared they can kill suc­cessive macrophages attempting to clear them, and create a source of oxidative compounds, and inflammation with macrophage debris accumulation (pus). Oxidative stress is asso­ciated to various diseases, such as cancer, neu­rodegenerative, and cardiovascular diseases.

This mechanism of alveolar clearance is not perfect, as it allows smaller nanoparticles to penetrate the alveolar epithelium and reach the interstitial space. From the interstitial space nanoparticles may enter the circulatory and lymphatic systems and reach other sites throughout the body.

Phagocytosis occurs in different areas of the body, phagocytes present in lungs, spleen, liver, etc., having different names, according to their location, such as alveolar macrophages, splenic macrophages, Kupfer cells, respectively

Nanoparticle Size Dependent Phago­cytosis:

Human alveolar macrophages mea­sure between 14 to 21 mm, while rat alveolar macrophages measure between 10 to 13 mm. Macrophages can engulf particles of a size comparable to their own dimensions, but are significantly less effective with particles that are much larger or smaller.

Experimental data show that, compared with larger particles, nanoparticles smaller than 100-200 nm are more capable of evading alve­olar macrophages phagocytosis, entering pulmonary interstitial sites, and interacting with epithelial cells to get access to the circulatory and lymphatic systems.

There are contradictory reports related to the phagocytosis of nanoparticles smaller than 100 nm. In vitro studies show that nanoparticles activate, and are phagocytized by alveolar macrophages. However, macrophage lavage recovery studies show that nanoparticles smaller than 100 nm are not efficiently phago­cytized in comparison with particles between 1 -3 mm.

A twelve week inhalation study in rats showed that 20 nm nanoparticles of titanium dioxide are characterized by longer retention time in the lungs and increased translocation to interstitial sites than larger nanoparticles (250 nm) of the same material.

Small nano­particles that evade the alveolar macrophages penetrate the alveolar epithelium, resulting in a slower clearance rate from the lung and pos­sibly later translocation to the circulatory and lymphatic system.

Concentration Dependent Phagocy­tosis:

At high concentrations, nanoparticles tend to cluster, forming aggregates often larg­er than 100 nm. Larger nanoparticles (>100 nm) can be readily phagocytized by alveolar macrophages. Results of studies involving in­halation or intra-tracheal instillation of high concentrations of nanoparticle (silver, iron, India ink, or titanium dioxide) smaller than 100 nm, which aggregate in larger particles, suggest that most nanoparticles are indeed stopped by alveolar macrophages.

Rat studies based on inhalation of low concentrations of 15 nm diameter silver nanoparticles showed that soon after inhalation (30 minutes), nano­particles are distributed in the blood and brain, and subsequently to organs, such as heart, kid­ney, while the lungs rapidly cleared of the nanoparticles.

Hence, minute concentrations of nanoparticles with size smaller than 100 nm can have a higher probability of trans locating to circulatory system and organs (and produce damage) than high concentrations of the same particles, which are likely to form aggregates, and which will be stopped from translocation by macrophage phagocytosis.

Lung Burden:

Insoluble particle burden in the lung can induce a range of toxicological responses differing from those due to soluble particles. Particles that are soluble or partly soluble (for example, cement) will dis­solve in the aqueous fluid lining the epitheli­um (and pass into the circulatory and lymphat­ic systems), while the insoluble ones (such as carbon black) must be removed through oth­er mechanisms such as the mucociliary esca­lator.

Particles that are not soluble or degradable in the lung will rapidly accumulate upon continued exposure. If the macrophage clear­ance capacity is exceeded, then the lung defense mechanisms are overwhelmed, resulting in injury to the lung tissue.

The adverse effect of inhaled nanoparticles on the lungs depends on the lung burden (determined by the rate of particle deposition and clearance) and on the residence time of the nanoparticles in the lung. For example, carbon nanotubes are not elimi­nated from the lungs or very slowly eliminat­ed (81% found in rat lungs after 60 days). The persistent presence within the alveoli of in­haled particles, especially those with mutagen­ic potential, increases the risk of lung cancer.

Translocation and Clearance of In­haled Nanoparticles:

Inhaled nanoparticles are shown to reach the nervous system via the olfactory nerves, and/or blood-brain-barrier. Nanoparticles that reach the lung are predom­inantly cleared via: mucociliary escalator into the gastrointestinal tract (and then eliminat­ed in the feces), lymphatic system, and circu­latory systems. From the lymphatic and circu­latory systems, nanoparticles may be distrib­uted to organs, including kidneys from where partial or total clearance may occur.

Adverse Health Effects in the Respi­ratory Tract:

Adverse health effects: Recent research has led to changes in terminology and brought about the realization that no par­ticles are completely inert, and that even low concentrations of particles can have negative health effects.

The adverse health effects of nanoparticles depend on the residence time in the respiratory tract. Smaller particles have a higher toxicity than larger particles of the same composition and crystalline structure, and they generate a consistently higher inflamma­tory reaction in the lungs, smaller nanoparti­cles are correlated with adverse reactions such as: impaired macrophage clearance, inflammation, accumulation of particles, and epithe­lial cell proliferation, followed by fibrosis, em­physema, and the appearance of tumors. Par­ticle uptake and potential health effects may be dependent on genetic susceptibility and health status.

Recent research has demonstrated that nano­particles inhalation can affect the immune system defense ability to combat infections. Nanoparticles of various compositions are able to modulate the intrinsic defensive function of macrophages, affecting their reactivity to infections.

It was found that several types of nanoparticles (such as ZrO₂) enhance the ex­pression of some viral receptors, making mac­rophages exposed to nanoparticles hyper-re­active to viral infections and leading to exces­sive inflammation. On the other hand, expo­sure to other nanoparticles (SiO₂, TiO₂) leads to a decrease in the expression of some other viral and bacterial receptors, leading to lower resistance to some viruses or bacteria.

Adaptability:

Organisms are capable of adapting to specific environmental stresses. Recent studies suggest that pre-exposure to low concentrations of nanoparticles stimulates the phagocytic activity of cells, while high con­centration of nanoparticles impairs this activ­ity. At the same time, genotype is an impor­tant factor in adaptability.

Treatment:

Treatments for nanoparticles inhalation include those that act to enhance mucociliary clearance, and those that reduce the effects of oxidation and inflammation. Mucociliary clearance can be enhanced two fold by inhalation of increasing concentrations of saline solutions. The saline solution acts as an osmotic agent increasing the volume of air­way surface liquid.

Anti-inflammatory medi­cine (sodium cromoglycate) was found to strongly reduce airway inflammation caused by diesel exhaust nanoparticles. Sodium cro­moglycate works by reducing allergic respons­es (inhibits the release of mediators from mast cells—cells responsible for the symptoms of allergy).

Antioxidant vitamins (particularly vitamin C), rosmarinic acid, and a high intake of fresh fruit and some vegetables have a pro­tective effect against lung diseases. In order to better understand the adverse health effects and possible treatment of inhaled nanoparticles, the next section is the biologi­cal interaction of nanoparticles at a cellular level.

Cellular Interaction with Nanoparticles:

Cellular Uptake:

Like Nano organisms (viruses), nanoparticles are able to enter cells and interact with subcellular structures. Cel­lular uptake, subcellular localization, and abil­ity to catalyse oxidative products depend on nanoparticle chemistry, size, and shape.

The mechanism by which nanoparticles penetrate cells without specific receptors on their outer surface is assumed to be a passive uptake or adhesive interaction. This uptake may be ini­tiated by van der Waals’ forces, electrostatic charges, steric interactions, or interfacial ten­sion effects, and does not result in the forma­tion of vesicles. (Steric interactions occur when nanoparticles have molecules with size, geom­etries, bonding, and charges optimized for the interaction with the receptors).

After this type of uptake, the nanoparticles are not necessari­ly located within a phagosome (which offers some protection to the rest of the cellular or­ganelles from the chemical interaction with the nanoparticle). For example C60 molecules enter cells and can be found along the nuclear, membrane, and within the nucleus. This type of uptake and free movement within the cell makes them very dangerous by having direct access to cytoplasm proteins and organelles.

Upon non-phagocytic uptake, nanoparticles can be found in various locations inside cells, such as the outer-cell membrane, cytoplasm, mitochondria, lipid vesicles, along the nucle­ar membrane, or within the nucleus.

Depend­ing on their localization inside the cell, the nanoparticles can damage organelles or DNA, or ultimately cause cell death. Nanoparticles are internalized not only by professional ph­agocytes such as alveolar macrophages, but by various types of cells, including endothelial cells, pulmonary epithelium, gastrointestinal epithelium, red blood cells, platelets and nerve cells.

Particle internalization location depends on nanoparticle size. For example, environmen­tal particles with size between 2.5-10 mm were found to collect in large cytoplasmic vacuoles, while smaller nanoparticles (<100 nm) local­ize in organelles, such as mitochondria, lead­ing to disruption of mitochondrial architec­ture.

Very small nanoparticles, such as C60 molecules with a diameter of 0.7 nm, are able to penetrate cells via a different mechanism than phagocytosis, probably through ion chan­nels or via pores in the cell membrane. Up­take location is likely to depend on material type; however current research does not pro­vide sufficient information to drawing conclu­sions on this subject.

Oxidative Stress, Inflammation, and Geno-Toxicity:

While the exact mechanism whereby nanoparticles induce pro-inflamma­tory effects is not known, it has been suggest­ed that they create reactive oxygen species (ROS), and thereby modulate intracellular cal­cium concentrations, activate transcription factors, and induce cytokine production. Be­low we outline in a very simplified and sche­matic depiction the current understanding of these very complex cellular mechanisms.

Oxidative stress generation:

Both in vivo and in vitro studies have shown that nanopar­ticles of various compositions (fullerenes, car­bon nanotubes, quantum dots, and automo­bile exhaust) create reactive Oxygen species.

Reactive oxygen species have been shown to damage cells by per-oxidizing lipids, altering proteins, disrupting DNA, interfering with sig­naling functions, and modulating gene transcription. Oxidative stress is a response to cell injury, and can also occur as an effect of cell respiration, metabolism, ischemia/reperfusion, inflammation, and metabolism of foreign compounds.

The Oxidative Stress Induced by Nanoparticles may have Several Sources:

(i) Reactive oxygen species can be generat­ed directly from the surface of particles when both oxidants and free radicals are present on the surface of the particles. Many compounds hitch-hiking on the surface of nanoparticles (usually present in ambient air) are capable of inducing oxidative damage, including ozone (O₃) and NO₂.

(ii) Transition metals (iron, copper, chromi­um, vanadium, etc.) nanoparticle can gener­ate reactive oxygen species acting as catalysts in Fenton type reactions. For example, the reduction of hydrogen peroxide (H₂O₂) with fer­rous iron (Fe₂)

O₂^– + H₂O₂ (Fe)→ OH + OH^– + O₂

results in the formation of hydroxyl radical (‘OH) that is extremely reactive, attacking biological molecules situated within diffusion range.

(iii) Altered functions of mitochondrion. As shown in several studies, small nanoparticles are able to enter mitochondria and produce physical damage, contributing to oxidative stress.

(iv) Activation of inflammatory cells, such as alveolar macrophages and neutrophils, which can be induced by phagocytosis of nanoparticles, can lead to generation of reac­tive oxygen species and reactive nitrogen spe­cies. Alveolar macrophages participate in the initiation of inflammation in the lung. Nanoparticles have been shown to generate more free radicals and reactive oxygen species than larger particles, likely due to higher sur­face area.

Inflammation:

Inflammation is the normal response of the body to injury. When generat­ed in moderation, inflammation stimulates the regeneration of healthy tissue, however when in excess, it can lead to disease. In vitro and in vivo experiments demonstrate that exposure to small nanoparticles is associated with in­flammation, with particle size and composi­tion being the most important factors. Inflam­mation is controlled by a complex series of intracellular and extracellular events.

The ox­idative stress results in the release of pro-inflammatory mediators or cytokines-intercellular chemical messengers alerting the im­mune system when an infection is present. Some nanoparticles can produce cell death via mitochondrial damage without inflammation.

Antioxidants:

The oxidative stress also re­sults in the release of antioxidants-proteins that act to remove the oxidative stress. In ad­dition to the antioxidants released as a re­sponse to the oxidative process, nanoparticles may interact with metal-sequestering proteins and antioxidants (from body fluids and intracellular), that will likely modify the surface properties of the nanoparticle to some extent, rendering them less toxic.

DNA Image:

Generation of reactive oxygen species to the point that they overwhelm the antioxidant defense system (shifting the redox balance of the cell) can result in oxidation, and therefore destruction, of cellular biomolecules, such as DNA, leading to heritable mutations. For example, the chemical modification of hi- stones (or binding proteins that support the supercoiled structure of DNA) opens the coiled DNA and allows its alteration.

Epide­miological, in vitro and in vivo studies show that nanoparticles of various materials (die­sel, carbon black, welding fumes, transition metals) are geno-toxic in humans or rats. A general schematic, of the molecular events by which nanoparticles exert their toxic effects at the cellular level, is given in (Fig. 14.5).

In sum­mary, nanoparticles can directly generate re­active oxygen species on their surfaces or by activation of macrophages. Overall, the gen­eration of oxidative species leads to increased inflammation and increased antioxidant pro­duction.

The activation of macrophages leads to modulation in intracellular calcium concen­tration that in turn activates further the reac­tive oxygen species production, which in turn enhances further calcium signaling by oxida­tion of calcium pumps in the endoplasmic reticulum, leading to calcium depletion.

Intra­cellular calcium modulation results in imparted motility and reduced macrophage phago­cytosis. Non-phagocytized nanoparticles are likely to access and interact with epithelial cells, thus enhancing inflammation. Ultimately, the interaction of nanoparticles with cells may lead to DNA modifications, cell injury, and disease.

Adverse Health Effects and Treat­ment:

Nanoparticles, due to their small size, can influence basic cellular processes, such as proliferation, metabolism, and death. Many diseases can be associated with dysfunction of these basic processes. For example, cancer results from uncontrolled cell proliferation, while neurodegenerative diseases are caused in part by premature cell death.

Oxidative stress has been implicated in many diseases, including cardiovascular and neurological dis­ease, pancreatitis, and cancer. Severe inflam­mation is assumed to be the initiating step in the appearance of autoimmune diseases (sys­temic lupus erythematosus, scleroderma, and rheumatoid arthritis) associated with expo­sure to some nanoparticles, such as silica and asbestos.

Regarding the treatment of adverse health effects caused by nanoparticles cytotoxi­city, antioxidants, anti-flammatory drugs, and metal chelators show promising effects. It has been reported that rats that underwent instil­lation of nanoparticles into the lungs togeth­er with an antioxidant (nacystelin) showed inflammation reduced by up to 60% in com­parison to those exposed to nanoparticles alone.

Antioxidant therapy has been found to protect against the development of hyper­tension, arteriosclerosis, cardiomyopathies, and coronary heart disease, providing fur­ther evidence of the link between the oxida­tive stress response and cardiovascular ef­fects. The adverse health effects of transition metals can be diminished by metal chelators.

‘Non-Invasive’ Terminology to be Questioned:

The process of nanoparticle up­take by cells is clinically used today in target­ed drug delivery and cell imaging. The safety of these techniques, however, depends on cel­lular uptake of nanoparticles without affect­ing normal cellular function.

Cellular imag­ing techniques are currently named ‘non-in­vasive’ techniques, which means non-penetrating, however they should perhaps be re­labeled as ‘minimally-invasive’, given that the nanoparticles enter the cells and are likely to affect cellular functions.

Iron oxide and oth­er magnetic nanoparticles have been used for many years as magnetic resonance imaging (MRI) contrast agents. Depending on their size and coating, MRI nanoparticles can lo­calize in liver, spleen, lymph nodes, etc.

Some nanoparticles were found to be teratogenic (causing birth defects) in rats and rabbits. Minor side effects of contrasting agents are nausea, vomiting, hives, and headache. More serious adverse reactions involving life- threatening cardiovascular and respiratory reactions are possible in patients with respi­ratory disorders.

Nervous System Uptake of Nanoparticles:

The nervous system is composed of the brain, spinal cord, and nerves that connect the brain and spinal cord to the rest of the body. In ad­dition to nanoparticle uptake due to inhala­tion, nervous system uptake may occur via other pathways (such as dermal). Uptake via olfactory nerves and the blood-brain-barrier are the most studied pathways.

Neuronal Uptake via Olfactory Nerves:

Neuronal uptake of inhaled nanopar­ticles may take place via the olfactory nerves or/and blood-brain-barrier. The nasal and tracheobronchial regions have many sensory nerve endings. As demon­strated several decades ago with polio viruses (30 nm) and silver coated gold nanoparticles (50 nm) in monkeys, intra-nasally instilled vi­ruses and particles migrate to the olfactory nerves and bulb with an axonal transport ve­locity of about 2.5 mm/hr.

The silver coated gold nanoparticles that reached the olfactory bulb were preferentially located in mitochon­dria, raising a major concern of their toxicity. More recent studies confirm the uptake of in­haled nanoparticles from olfactory mucosa via the olfactory nerves in the olfactory bulb. For example, rat inhalation studies with 30 nm magnesium oxide and 20-30 nm carbon nano­particles indicate that nanoparticles translo­cate to the olfactory bulb. If inhalation oc­curred via one nostril only, the accumulation was observed only in the side of the open nos­tril.

Experiments show that micro particles with diameter larger than a micron do not cross the olfactory nerve, as expected from the geometrical restrictions imposed by the diam­eter of the olfactory axons of only 100-200 nm. Translocation of nanoparticles into deeper brain structures may be possible, as suggest­ed by the movement of viruses through neu­rons.

Neuronal Uptake via Blood-Brain- Barrier:

The passage of nanoparticle to the nervous system is also possible via the blood- brain-barrier. The blood-brain-barrier is a physical barrier with negative electrostatic charge between the blood vessels and brain, selectively restricting the access of certain substances.

This anionic barrier is believed to stop most anionic molecules, while the cationic molecules increase the permeability of the blood-brain-barrier by charge neutral­ization. This route has been extensively stud­ied for the purpose of drug delivery to the brain.

Regarding the passage of nanoparti­cles, the blood-brain-barrier permeability is dependent upon the charge of nanoparticles. It allows a larger number of cationic nano­particles to pass compared to neutral or an­ionic particles, due to the disruption of its integrity.

As shown by magnetic resonance imaging (MRI) with magnetic nanoparticles, the blood-brain-barrier in healthy subjects stops some proteins and viruses present in the brain vascular system from trans-locating to the brain.

However, subjects with spe­cific circulatory diseases (like hypertension), brain inflammation, respiratory tract inflam­mation (increased levels of cytokines that cross blood-brain-barrier and induce in­flammation) may have increased blood- brain-barrier permeability, which will allow nanoparticles access to the nervous system.

Adverse Health Effects of Neuronal Nanoparticles Uptake and Treatment:

Exper­imental evidence suggests that the initiation and promotion of neurodegenerative diseas­es, such as Alzheimer’s disease, Parkinson’s disease, Pick’s disease, are associated with ox­idative stress and accumulation of high con­centrations of metals (like copper, aluminum, zinc, but especially iron) in brain regions as­sociated with function loss and cell damage.

Iron is necessary in many cellular functions, especially in the brain, where it participates in many neuronal processes. In excess, how­ever, iron is toxic to cells. The brain continu­ously accumulates iron, resulting in in­creased stored iron amounts with the age. In order to prevent its toxicity, organisms de­veloped a way to store excess iron in pro­teins called ferittin (Ft).

Dysfunction of ferittin resulting from excessive accumulation of iron may lead to oxidative stress and myelin (the electrically insulating coatings of axons) breakdown. Metal homeostasis imbalance and neuronal loss are both present in neuro­degenerative diseases. (Homeostasis is a dy­namic equilibrium balancing act necessary for a proper function of a living system.)

It is not known if the presence of metals in brain of subjects with neurodegenerative diseases is due to nanoparticles themselves trans-locating to the brain or their soluble compounds. Despite the fact that the etiology of neurodegenerative diseases is unknown, en­vironmental factors are believed to play a crucial factor in their progress, being able to trigger pro-inflammatory responses in the brain tissue.

Recent studies on DNA damage in nasal and brain tissues of canines exposed to air pollutants shows evidence of chronic brain inflammation, neuronal dysfunction, and similar pathological findings with those of early stages of Alzheimer’s disease.

Autop­sy reports on humans suggest similar results. Significant oxidative damage was found in the brain of largemouth bass after exposure to C60. Rat inhalation studies with stainless steel welding fumes showed that manganese accumulates in blood, liver and brain.

Epidemiological studies show a clear asso­ciation between inhalation of dust contain­ing manganese and neurological diseases in miners and welders. Some welders develop Parkinson’s disease much earlier in their life, usually in their mid-forties, compared to the sixties in the general population.

Brain inflam­mation appears to be a cumulative process, and the long-term health effects may not be ob­served for decades. Currently there are 1.5 million peoples suffering from Alzheimer’s in the United States of America and an estimated 18 million worldwide.

Treatment:

Antioxidants and metal chela­tors are treatment options for the adverse health effects caused by the neuronal uptake of nanoparticles. In the therapy of neurode­generative diseases, metals chelators trans­ported across the blood-brain-barrier seem to be a very promising approach.

Functionalized fullerenes and nanoparticles made of compounds holding oxygen vacancies show great antioxidant properties. Fullerols, or poly-hydroxylated fullerenes, are excellent antioxidants with high solubility and ability to across the blood-brain-barrier, showing promising results as neuro-protective agents. CeO₂, and Y₂O₃ nanoparticles have strong antioxidant properties on rodent nervous system cells. Cerium oxide tends to be nonstoichiometric, Ce atoms having a dual oxi­dation state, +3 or +4, leading to oxygen vacancies.

Dual oxidation state confers CeO₂ and probably Y₂O₃ nanoparticles antioxi­dant properties that promote cell survival under conditions of oxidative stress. It ap­pears that the antioxidant properties depend upon the structure of the particle but they are independent of its size within 6-1,000 nm.

Nanoparticle Translocation to the Lymphatic Systems:

Translocation of nanoparticles to lymph nodes is a topic of intense investigation today for drug delivery and tumor imaging. Progression of many cancers (lung, esophageal, mesothelio­ma, etc.) is seen in the spread of tumor cells to local lymph nodes. The detection and target­ed drug delivery to these sites are the steps involved in the therapeutic treatment of cancer. Several studies show that interstitially injected particles pass preferentially through the lym­phatic system and not the circulatory sys­tem, probably due to permeability differenc­es. After entering the lymphatic system, they locate in the lymph nodes.

The free nanopar­ticles reaching the lymph nodes are ingested by resident macrophages. Nanoparticles that are able to enter the circulatory system can also gain access to the interstitium and from there are drained through the lymphatic sys­tem to the lymph nodes as free nanoparticles and/or inside macrophages.

The adverse health effects of nanoparticle uptake by lymphatic system are not sufficiency explored. However, one can hypothesize that oxidative stress created by certain types of nanoparticles could lead to damage of lympho­cytes (type of white blood cell), lymph nodes, and/or spleen.

Nanoparticles Translocation to the Circulatory System:

Inhalation or instillation studies in healthy animals show that metallic nanoparticles with size smaller than 30 nm pass rapidly into the circulatory system, while non-metallic nano­particles with size between 4 and 200 nm pass very little; or not at all. In contrast, subjects suffering from respiratory and circulatory dis­eases have higher capillary permeability, allow­ing fast translocation of metallic or non-metallic nanoparticle into circulation.

Long-Term Translocation:

Nanopar­ticles, unlike larger particles, are able to trans­locate across the respiratory epithelium after being deposited in the lungs. Once they have crossed the respiratory epithelium, they may persist in the interstitial for years, or they may enter the lymphatic system and circulatory system. From the circulatory system long-term translocation to organs (such as the liver, heart, spleen, bladder, kidney, and bone marrow) is pos­sible, depending on the duration of exposure.

Smaller particles (20 nm) are cleared faster from the lung than larger particles (100 nm), probably because small nanoparticles are not efficiently phagocytized by macrophages and are able to enter more rapid the circula­tory and/or lymphatic systems.

Short-Term Translocation of Met­als:

Evidence of rapid translocation of metal nanoparticles from lungs into the circulation and to organs has been provided by animal studies. These results show the location of nanoparticles with diameters of 30 nm (Au), 22 nm (TiO₂) in pulmonary capillaries; 15 nm (Ag), and welding fumes in blood, liver, kid­ney, spleen, brain, and heart.

Animal studies on rats with inhalation of titanium dioxide nanoparticles (22 nm diameter) show that they can translocate to the heart and can be found in the heart connective tissue (fibro­blasts). Within 30 minutes post-exposure, large quantities of intratracheally instilled gold nanoparticles (30 nm) have been found in platelets inside of pulmonary capillaries of rats, motivating the hypothesis that nano­particles may induce aggregation of platelets, leading to the formation of blood clots.

Short-Term Translocation of Non- Metals:

There is no conclusive evidence show­ing fast translocation of carbon-based Nano-materials into systemic circulation. Short-term translocation of radiolabeled nanoparticles from lungs to the organs is currently the sub­ject of debate as a significant fraction of ra­dioactive labels detaches from their labeled nanoparticles, so radioactivity observed throughout the body may not indicate the ac­tual translocation of nanoparticles, but of radiolabels.

Technetium’s short lived isotope ^99mTc, with an atomic diameter of about 0.37 nm, is used in labelling nanoparticles that are subsequently injected or inhaled by subjects. In many cases the radiolabel can separate from the nanoparticles and follow a different translocation route.

In the presence of oxygen, the radioactive label can transform into pertechnetate (^99m,TcO^4-) having a slightly larger diame­ter of roughly 0.5 nm. Most studies show very little or no translocation of radiolabeled poly­styrene nanoparticles with diameters of 56 nm and 200 nm, or carbon nanoparticles with diameters of 5 nm, 4-20 nm, 35 nm, 100 nm, while others show a rapid and substantial translocation into circulation for particles sized 5-10 nm, 20-30 nm.

While the short- term extra-pulmonary translocation into cir­culation in healthy subjects is still under de­bate, there seems to be agreement on the fact that nanoparticle fast translocation into circu­lation may be enhanced by pulmonary inflammatory and increased micro vascular permeability. Subjects suffering from respiratory or blood diseases may have an increased sus­ceptibility of nanoparticles translocation from lungs to circulation and organs.

Nanoparticles’ Interaction with and Uptake by Blood Cells:

There are three main types of cells in the blood: red cells in charge of oxygen transport; white cells responsible for fighting infections; and platelets that help pre­vent bleeding by forming blood clots. The up­take of nanoparticles by each type of blood cells is essentially different. Nanoparticle uptake by red blood cells (that do not have phagocytic abilities, due to the lack of phagocytic recep­tors) is entirely dictated by size, while the nanoparticle charge or material type plays lit­tle importance.

In contrast, nanoparticle charge plays an essential role in their uptake by platelets and their influence on blood clot formation. Uncharged polystyrene particles do not have an effect on blood clots formation. Negatively charged nanoparticles significant­ly inhibit thrombosis formation, while positively charge nanoparticles enhance platelet aggre­gation and thrombosis.

The interaction be­tween platelets and positively charged parti­cles aggregation and thrombosis. The inter­action between platelets and positively charged particles seems to be due to the net negative charge that platelets carry on their surface.

The positively charged nanoparticles interact with negatively charged platelets and reduce their surface charge, making them more prone to aggregation. Until now it was thought that blood clots can be formed due to three main causes: when the blood flow is obstructed or slowed down, when the vascu­lar endothelial cells are damaged, or due to the blood chemistry.

However, it seems pos­sible, in the view of recent findings that nano­particles may act as nucleating centers for blood clots. It is important to note that pulmo­nary instillation of large nanoparticles (400 nm) caused pulmonary inflammation of similar intensity to that caused by 60 nm particles, but did not lead to peripheral thrombosis.

The fact that the larger particles failed to produce a thrombotic effect suggests that pulmonary inflammation itself is insufficient to cause peripheral thrombosis, and that throm­bi formation occur via direct activation of platelets.

Microscopic and energy dispersive spectrometry (EDS) analysis of blood clots from patients with blood disorders revealed the presence of foreign nanoparticles. Most notably, patients with the same type of blood disorder show fibrous tissue clots embedding nanoparticle with different composition: gold, silver, cobalt, titanium, antimony, tungsten, nickel, zinc, mercury, barium, iron, chromi­um, nickel, silicon, glass, talc, stainless steel. The common denominator of the particles is their size, ranging from tens of nanometers to few microns.

Adverse Health Effects of Circula­tory System Uptake:

Thrombosis:

Translo­cation of nanoparticles into the circulatory system was correlated with the appearance of thrombi (or blood clots). The time frame of this process is very short thrombosis occurring during the first hour after exposure.

Hamster studies of tracheally or intrave­nously instilled nanoparticles of charged polystyrene (60 nm) and diesel exhaust par­ticles (20-50 nm) significantly increased ar­terial or venous thrombus formation during the first hour after administration.

There is a clear dose-dependent response correlating the quantity of pollutant administered and the observed thrombus sizes. Pro-thrombotic effects persisted 24 h after instillation. If inhaled nanoparticles were to be found in red blood cells located in pulmonary capillaries, one would expect adverse health effects as blood-related diseases, such as anemia, due to reduced oxygen transport capacity of the red blood cells.

Cardiovascular Malfunction:

It is clear from clinical and experimental evidence that inhalation of Nano and micro particles can cause cardiovascular effects. Despite the fact that there is an intuitive relationship between inhaled nanoparticles and adverse respira­tory effects, the causal link between particles in the lung and cardiovascular effects is not entirely understood.

It was thought that the pulmonary inflammation caused by the par­ticles triggers a systemic release of cytokines, resulting in adverse cardiovascular effects. However, recent studies on animals and hu­mans have shown that nanoparticles diffuse from lungs into the systemic circulation, and then are transported to the organs, demon­strating that cardiovascular effects of in­stilled or inhaled nanoparticles can arise di­rectly from the presence of nanoparticles within the organism. Proposed mechanisms of cardiovascular effects are summarized in (Fig. 14.6).

Liver, Spleen, Kidneys Uptake of Nanoparticles:

Organs’ Nanoparticles Uptake:

En­dothelial cells (cells that line the vascular sys­tem) form a physical barrier for particles, hav­ing very tight junctions, typically smaller than 2 nm. Nevertheless larger values, from 50 nm up to 100 nm have been reported, depending on the organ or tissue. A very tight endothe­lial junction is present in the brain, often called the blood-brain-barrier.

However, experi­ments performed on rats injected with ferritin macromolecules (with size around 10 nm) into the cerebrospinal fluid, demonstrated passage of ferritin into deep brain tissue. In certain organs, such as liver, the endothelium is fenes­trated with pores of up to 100 nm, allowing easier passage of larger particles. In the pres­ence of inflammation the permeability of the endothelium is increased, allowing a larger passage of particle.

Micro and nanoparticle debris was detect­ed by scanning electron microscopy in organs and blood of patients with: orthopedic im­plants, drug addiction, worn dental prosthe­ses, blood diseases, colon cancer, Crohn’s dis­ease, ulcerative colitis, and with diseases of unknown etiology. Coal workers autopsies re­veal an increased amount of particles in the liver and spleen compared to non-coal work­ers. The workers with pronounced lung dis­eases have more nanoparticles in their organs than healthier ones.

The pathway of exposure most likely involves the translocation from lungs to circulation of the inhaled nanoparti­cles, followed by uptake by the organs. Rat in­halation studies with stainless steel welding fumes showed that manganese accumulates in blood and liver. Rat inhalation studies with 4-10 nm silver nanoparticles show that with­in 30 minutes the nanoparticles enter the circu­latory system, and after a day can be found in the liver, kidney and heart, until subsequently cleared from these organs after a week.

Clear­ance from the liver can occur via biliary se­cretion into the small intestine. A case study shows that the wear of dental bridges leads to the accumulation of wear nanoparticles in liver and kidneys. The most probable absorp­tion pathway was assumed to be via intestinal absorption.

Scanning electron microscopy and energy-dispersive micro analytical tech­niques identified the chemical compositions of particles in the liver and kidney biopsies, as well in stool, as the same as the porcelain from dental prostheses. The maximum size of par­ticles found in the liver (20 microns) was larg­er than in the kidneys (below 6 microns), sug­gesting that particles are absorbed by intesti­nal mucosa, translocate to liver before reach­ing the circulatory system and kidneys. After the removal of dental bridges, particles in stool are no longer observed.

Adverse Health Effects of Liver and Kidney Uptake:

Up to now there is little knowl­edge (or discussion) on the effect of nanopar­ticles on organs such as liver, kidneys, spleen, etc. However, one can speculate that as long as there is translocation to and accumulation of nanoparticles in these organs, potentially ad­verse reactions and cytotoxicity may lead to disease. Diseases with unknown origins have been correlated with the presence of micro and nanoparticles in kidneys and liver. For com­parison, the liver and kidneys of healthy sub­jects did not show any debris. Particles debris has been found also in the liver of patients with worn orthopedic prosthesis.

Dental prosthesis debris internalized by in­testinal absorption can lead to severe health conditions, including fever, enlarged spleen and liver, suppression of bile flow, and acute renal failure. These symptoms appeared about a year after the application of dental porcelain bridges. After the removal of den­tal bridges, and subsequent treatment with steroids, the clinical symptoms declined.

Gastro-Intestinal Tract Uptake and Clearance of Nanoparticles:

Exposure Sources:

Endogenous sources of nanoparticles in the gastro-intesti­nal tract are derived from intestinal calcium and phosphate secretion. Exogenous sources are particles from food (such as colorants— titanium oxide), pharmaceuticals, water, or cosmetics (toothpaste, lipstick), dental pros­thesis debris, and inhaled particles.

The dietary consumption of nanoparticles in developed countries is estimated around 1,012 particles/ person per day. They consist mainly of TiO₂ and mixed silicates. The use of specific prod­ucts, such as salad dressing containing a nano­particle TiO₂ whitening agent, can lead to an increase by more than 40-fold of the daily av­erage intake.

These nanoparticles do not de­grade in time and accumulate in macrophag­es. A portion of the particles cleared by the mucociliary escalator can be subsequently in­gested into the gastro-intestinal tract. Also, a small fraction of inhaled nanoparticles was found to pass into the gastro-intestinal tract.

Size and Charge Dependent Uptake:

The gastro-intestinal tract is a complex bar­rier-exchange system, and is the most impor­tant route for macromolecule to enter the body. The epithelium of the small and large intestines is in close contact with ingested ma­terial, which is absorbed by the villi. The uptake of Nano and micro-particles have been the focus of many investigations, the ear­liest dating from mid seventeen century, while more recently entire issues of scientific journals have been devoted to the subject.

The extent of particles absorption in the gastro-intestinal tract is affected by size, surface chemistry and charge, length of administration, and dose. The absorption of particles in the gastro-intestinal tract depends on their size, the uptake dimin­ishing for larger particles.

A study of polysty­rene particles with size between 50 nm and 3 mm indicated that the uptake decreases with increasing particle size from 6.6% for 50 nm, 5.8% for 100 nm nanoparticles, 0.8% for 1 mm, to 0% for 3 mm particles. The time required for nanoparticles to cross the colonic mucus layer depends on the particle size, with small­er particles crossing faster than larger ones: 14 nm diameter latex nanoparticles cross with­in 2 minutes, 415 nm within 30 minutes, and 1000 nm particles do not pass this barrier.

Particles that penetrate the mucus reach the enterocytes and are able to translocate further. Enterocytes are a type of epithelial cell of the superficial layer of the small and large intestine tissue, which aid in the absorption of nutrients. When in contact with the sub-mucosal tissue, nanoparticles can enter the lymphatic system and capillaries, and then are able to reach various organs.

Diseases, such as diabetes, may lead to higher absorp­tion of particles in the gastro-intestinal tract. For example, a rat with experimentally in­duced diabetes has a 100-fold increase in ab­sorption of 2 mm polystyrene particles rela­tive to non-diabetic rats. Also inflammation may lead to the uptake and translocation of larger particles of up to 20 mm.

The kinetics of particles in the gastro-intestinal tract de­pend strongly on the charge of the particles, positively charged latex particles are trapped in the negatively charged mucus while nega­tively charged latex nanoparticles diffused across the mucus layer and became available for interaction with epithelial cells.

Translocation:

Varying the charac­teristics of nanoparticles, such as size, sur­face charge, attachment of ligands, or surfac­tant coatings, offers the possibility for site-specific targeting of different regions of the gastrointestinal tract. The fast transit of ma­terial through the intestinal tract (on the or­der of hours), together with the continuous renewal of epithelium, led to the hypothesis that Nano-materials will not remain there for indefinite periods.

Most of the studies of in­gested nanoparticles have shown that they are eliminated rapidly: 98% in the feces with­in 48 hours and most of the remainder via urine. However, other studies indicate that certain nanoparticles can translocate to blood, spleen, liver, bone marrow, lymph nodes, kidneys, lungs, and brain, and can also be found in the stomach and small intestine.

Oral uptake of polystyrene spheres of vari­ous sizes (50 nm-3 mm) by rats resulted in a systemic distribution to liver, spleen, blood, and bone marrow. Particles larger than 100 nm did not reach the bone marrow, while those larger than 300 nm were absent from blood. In the study no particles were detect­ed in heart or lung tissue.

Studies using irid­ium did not show significant uptake, while titanium oxide nanoparticles were found in the blood and liver. For several days follow­ing oral inoculation of mice with a relatively biologically inert nanometer-sized plant vi­rus (cowpea mosaic virus), the virus was found in a wide variety of tissues throughout the body, including the spleen, kidney, liver, lung, stomach, small intestine, lymph nodes, brain, and bone marrow.

The exact order of translocation from the gastro-intestinal tract to organs and blood is not known, however, a case study of dental prosthesis porcelain debris internalized by intestinal absorption suggests that intestinal absorption of parti­cles is followed by liver clearance before they reach the general circulation and the kidneys.

Adverse Health Effects of Gastro-In­testinal Tract Uptake:

Reaction reduced tox­icity: In the intestinal tract there is a complex mix of compounds, enzymes, food, bacteria, etc. that can interact with ingested particles and sometimes reduce their toxicity. It was reported that particles in vitro are less cyto­toxic in a medium with high protein content.

Crohn’s Disease, Ulcerative Colitis, Cancer:

Nanoparticles have been constantly found in colon tissue of subjects affected by cancer, Crohn’s disease, and ulcerative colitis, while in healthy subjects nanoparticles were absent. The nanoparticles present in diseased subjects had various chemical compositions and are not considered toxic in bulk form. Microscopic and energy dispersive spectroscopy analysis of colon mucosa indicated the presence of car­bon, ceramic filo-silicates, gypsum, sulphur, calcium, silicon, stainless steel, silver, and zir­conium.

The size of debris varied from 50 nm to 100 microns, the smaller the particle the further is able to penetrate. The particles were found at the interface between healthy and cancerous tissue. Based on these findings it was suggested that the gastro-intestinal barrier is not efficient for particles smaller than 20 mi­crons.

Crohn’s disease affects primarily peo­ple in developed countries, and occurs in both the native population and in immigrants from under-developed countries. It affects 1 in 1000 people. Crohn’s disease is believed to be caused by genetic predisposition together with envi­ronmental factors.

Recently it was suggested that there is an association between high lev­els of dietary nanoparticles (100 nm-1 mm) and Crohn’s disease. Exogenous nanoparticles were found in macrophages accumulated in lymphoid tissue of the human gut, the lym­phoid aggregates being the earliest sign of le­sions in Crohn’s disease.

Microscopy studies showed that macrophages located in lymphoid tissue uptake nanoparticles of: spherical anatase (TiO₂) with size ranging between 100-200 nm from food additives; flaky-like aluminum- silicates 100-400 nm typical of natural clay; and environmental silicates 100-700 nm with various morphologies. A diet low in exogenous particles seems to alleviate the symptoms of Crohn’s disease.

This analysis is still controversial, with some proposing that an abnormal response to dietary nanoparticles may be the cause of this disease, and not an excess intake. More precisely, some members of the population may have a genetic predisposition where they are more affected by the intake of nanoparticles, and therefore develop Crohn’s disease.

Some evidence suggests that dietary nanoparticles may exacerbate inflammation in Crohn’s dis­ease. These studies measured the intake of di­etary particle, but did not analyse the levels of outdoor and indoor nanoparticle pollution at the subjects’ residences. As was described pre­viously, significant quantities of nanoparticles are cleared by the mucociliary escalator and subsequently swallowed, ultimately reaching the gastro-intestinal tract.

Treatment:

The diseases associated with gastro-intestinal uptake of nanoparticles (such as Crohn’s disease and ulcerative coli­tis) have no cure and often requires surgical intervention. Treatments aim to keep the dis­ease in remission and consist of anti-inflam­matory drugs and specially formulated liq­uid meals. If dietary nanoparticles are con­clusively shown to cause these chronic dis­eases, their use in foods should be avoided or strictly regulated.

Dermal Uptake of Nanoparticles:

Penetration Sites:

The skin is com­posed of three layers—epidermis, dermis and subcutaneous. The outer portion of the epidemic, called stratum corneum is a 10 mm thick keratinized layer of dead cells and is dif­ficult to pass for ionic compounds and water soluble molecules. The surface of epidermis is highly micro-structured having a scaly ap­pearance as well as pores for sweat, seba­ceous glands, and hair follicle sites.

As with many subjects involving nanopar­ticles, dermal penenetration is still contro­versial. Several studies show that nanoparti­cles are able to penetrate the stratum corne­um. Nanoparticle penetration through the skin typically occurs at hair follicles, and flexed and broken skin.

Intracellular nano­particles penetration is also possible, as dem­onstrated by cell culture experiments. MWCNTs are internalized by human epider­mal keratinocytes (the major cell type of the epidermis) in cytoplasmic vacuoles and in­duce the release of pro-inflammatory media­tors.

Spherical particles with diameter be­tween 750 nm and 6 microns selectively pen­etrate the skin at hair follicles with a maxi­mum penetration depth of more than 2,400 microns (2.4 mm). Broken skin facilitates the entry of a wide range of larger particles (500 nm-7 mm).

While stationary unbroken skin has been shown to be impervious to penetra­tion, nanoparticles have been observed to penetrate when the skin is flexed. Thus me­chanical deformation is capable of transport­ing particles through the stratum corneum and into the epidermis and dermis.

A current area under discussion is whether or not nanoparticles of TiO₂ found in commercially available sunscreens penetrate the skin. For example, the application of a sunscreen containing 8% nanoparticles (10-15 nm) onto the skin of humans showed no penetration, while oil-in-water emulsions showed penetra­tion, higher penetration being present in hairy skin at the hair follicles site or pores. The quan­tity of nanoparticles that penetrate is very small, with less than 1% of the total amount in the applied sunscreen being found in a given hair follicle.

Translocation:

The dermis has a rich supply of blood and macrophages, lymph vessels, dendritic cells, and nerve endings. Therefore, the particles that cross through the stratum corneum and into the epidermis and dermis are potentially available for rec­ognition by the immune system. Transloca­tion of nanoparticles through skin into the lymphatic system is demonstrated by soil particles found in lymph nodes of patients with podoconiosis. Neuronal transport of small nanoparticles along sensory skin nerves may be possible, in a similar way to the proven path for herpes virus.

Adverse Health Effects of Dermal Uptake:

Many manufacturing processes pose an occupational health hazard by exposing workers to nanoparticles and small fibers, as suggested from the intracellular uptake of MWCNTs by human epidermal kerati­nocytes. This can explain beryllium sensiti­zation in workers wearing inhalation protec­tive equipment exposed to Nano particulate beryllium. Also, this may be relevant for la­tex sensitivity and other materials that pro­voke dermatologic responses.

Titanium Dioxide:

Currently, a controver­sial subject is the toxicity of titanium dioxide from cosmetics. There are concerns about the toxicity of titanium dioxide—commonly used as a physical sunscreen since it reflects and scatters UV B (190-320 nm) and UV A (320-400 nm) light rays—the skin-damaging portion of the solar spectrum. TiO₂ also ab­sorbs a substantial amount of UV radiation, however, which in aqueous media leads to the production of reactive oxygen species, includ­ing superoxide anion radicals, hydrogen per­oxide, free hydroxyl radicals, and singlet oxygen’s.

These reactive oxygen species can cause substantial damage to DNA. Titanium dioxide particles under UV light irradiation have been shown to suppress tumor growth in cultured human bladder cancer cells via reactive oxygen species. Sun-illuminated ti­tanium dioxide particles in sunscreen were observed to catalyse DNA damage both in vitro and in vivo.

Reports regarding the tox­icity of titanium dioxide nanoparticles in the absence of UV radiation are contradictory. Nanoparticles were seen to have no inflam­matory effect or genotoxicity in rats (when introduced by instillation). However, several other studies reported that titanium dioxide caused chronic pulmonary inflammation in rats (again by instillation), and in vitro had a pro-inflammatory effect in cultured human endothelial cells.

Silver:

It is known that silver has a benefi­cial antibacterial effect when used as a wound dressing, reducing inflammation and facili­tating healing in the early phases. However, there are contradictory studies on silver nanoparticles and ions cytotoxicity from lab­oratories around the world.

Silver is known to have a lethal effect on bacteria, but the same property that makes it antibacterial may render it toxic to human cells. Concentra­tions of silver that are lethal for bacteria are also lethal for both keratinocytes and fibro­blasts.

Nanoparticle Uptake Via Injection:

Injection is the administration of a fluid into the subcutaneous tissue, muscle, blood ves­sels, or body cavities. Injection of nanoparti­cles has been studied in drug delivery. The translocation of nanoparticles following in­jection depends on the site of injection: in­travenously injected nanoparticles quickly spread throughout the circulatory system, with subsequent translocation to organs; in­tradermal injection leads to lymph nodes uptake; while intramuscular injection is fol­lowed by neuronal and lymphatic system uptake. For example, the injection of mag­netic nanoparticles smaller than 100 nm into the tongues and facial muscles of mice result­ed in synaptic uptake.

Nanoparticles inject­ed intravenously are retained longer in the body than ingested ones. For example, 90% of injected functionalized fullerenes are re­tained after one week of exposure. Intrave­nously injected nanoparticles (quantum dots, fullerenes, polystyrene, plant virus) with size ranging from 10-240 nm show localiza­tion in different organs, such as liver, spleen, bone marrow, lymph nodes, small intestine, brain, lungs.

Talc particles introduced by in­jection are found in the liver of intravenous drug users. The distribution of particles in the body is a function of their surface characteris­tics and their size. Coating nanoparticles with various types and concentrations of surfac­tants before injection significantly affects their distribution in the body. For example, coating with polyethylene glycol or other substances almost completely prevents hepatic and splen­ic localization.

Another example is the modi­fication of nanoparticles surface with cationic compounds that facilitate arterial uptake by up to 10 fold. The adverse health effects of in­jected nanoparticles are a function of particle chemistry and charge. A common side effect of injecting nanoparticles intravenously is hy­persensitivity, a reaction that occurs in a large number of recipients and is probably due to the complement activation.

Nanoparticle Generation by Implants:

Nanoparticle debris produced by wear and corrosion of implants is transported to re­gion beyond the implant. Implants release metal ions and wear particles and, after sev­eral years of wear, in some cases the concen­tration of metals in blood exceeds the bio­logical exposure indices recommended for occupational exposure.

Materials considered chemically inert in bulk form (like ceramic porcelain and alumina), or in other terms biocompatible, are used for implants and prostheses. However, nanoparticles with the same composition have been observed in liv­er and kidneys of diseased patients with im­plants and prostheses. It was suggested that the concept of biocompatibility should be revised in view of these findings.

Patients with orthopedic implants have a statistically significant rise in the incidence of autoimmune diseases, perhaps due to the particulate wear debris generated by the implant, which is associated with electrochem­ical processes that may activate the immune system.

Immunological responses and aseptic inflammation in patients with total hip replace­ment are a response to wear particles. Expo­sure to orthopedic wear-debris leads to inflam­matory initiated bone resorption, implant fail­ure, dermatitis, urticaria, and vasculitis.

Positive Effects of Nanoparticles:

Nanoparticles as Antioxidants:

Fullerene derivatives and nanoparticles made of compounds holding oxygen vacancies (CeO₂, and Y₂O₃) have demonstrated neuro-­protective properties and anti-apoptotic activity. Fullerene derivatives have been shown to prevent apoptosis in hepatic, kidney, and neuronal cells, a fact attributed to their anti­oxidant properties.

The decrease of apoptotic cell death is related to the neutralization of reactive oxygen species both in vitro and in vivo. Neurodegenerative disorders, such as Parkinson’s and Alzheimer’s diseases present hyper-production of oxygen and nitric oxide radical species.

As described previously, oxi­dative stress by oxygen radical induces cellu­lar instability by a cascade of events, leading to cell death. The use of fullerenes as radical sponges (or scavengers) has been shown to decrease neuronal death.

Functionalized fullerenes can react with oxygen species that attack lipids, proteins, and DNA, conferring neuro-protective properties. In particular, poly-hydroxylated fullerenes (fullerols) [C₆₀(OH_n) are excellent antioxidants and of­fer exceptional neuro-protective properties, having high solubility and ability to cross the blood-brain barrier.

Anti-Microbial Activity:

Several types of nanoparticle are known to have an antimicrobial effect, such as—silver, titani­um dioxide, fullerenes, zinc oxide, and mag­nesium oxide. Antimicrobial activity of fullerenes was observed on various bacteria, such as E. Coli, Salmonella, Streptococcus spp..

The bactericide action is probably due to inhibition of energy metabolism once the bacteria have internalized the nanoparticles. Zinc oxide nanoparticles are bactericidal, dis­rupting membrane permeability and being internalized by Escherichia coli bacteria.

Sil­ver nanoparticles and ions are broad spec­trum antimicrobial agents. Their antibacte­rial action results from destabilization of the outer membrane of bacteria, and depletion of the levels of adenosine triphosphate, a molecule that is the principal form of energy immediately usable by the cell.

Fullerenes have also been shown to have an anti-HIV activity, probably due to a good geometrical fit of a C₆₀ sphere into the active site (diameter of about 1 nm) on the fundamental enzyme (HIV protease) necessary for HIV virus sur­vival, leading to strong van der Waals’ inter­actions between the enzyme and fullerene.

It has been demonstrated that silver nanoparti­cles undergo a size dependent interaction with HIV-1 virus, with nanoparticles exclusively in the range of 1 -10 nm attached to the virus. Due to this interaction, silver nanoparticles inhib­it the virus from binding to host cells, as dem­onstrated in vitro.

Physicochemical Characteristic-Dependent Toxicity:

From previous knowledge of toxicological properties of fibrous particles (such as as­bestos), it is believed that the most impor­tant parameters in determining the adverse health effects of nanoparticles are dose, di­mension, and durability (the three D).

How­ever, recent studies show different correla­tions between various physicochemical properties of nanoparticles and the associ­ated health effects, raising some uncertain­ties as to which are the most important pa­rameters in deciding their toxicity—mass, number, size, bulk or surface chemistry, ag­gregation, or altogether. In the following we will emphasize what we believe are the most important nanoparticle characteristics asso­ciated with their toxicity.

Dose-Dependent Toxicity:

Dose is defined as the amount or quantity of substance that will reach a biological system. The dose is directly related to exposure or the concentration of substance in the rele­vant medium (air, food, water) multiplied by the duration of contact.

Generally, the negative health effects of nanoparticles do not correlate with nanopar­ticle mass dose. Comparing the health effects of inhaled TiO₂ nanoparticles with different sizes, it is remarkable that the low dose (10 mg/m³) exposure to 20 nm diameter parti­cles resulted in a greater lung tumor incidence than the high dose (250 mg/m³) exposure of 300 nm diameter particles. The measure that correlates with the effects is the surface area and not the mass dose.

Size-Dependent Toxicity:

In the last decade, toxicological studies have demonstrated that small nanoparticles (<100 nm) cause adverse respiratory health effects, typically causing more inflammation than larger particles made from the same. Rat inhala­tion and instillation of titanium oxide parti­cles with two sizes, 20 nm and 250 nm diam­eter, having the same crystalline structure show that smaller particles led to a persistently high inflammatory reaction in the lungs compared to larger size particles.

In the post-exposure period (up to 1 year) it was observed that the smaller particle has:

1. A significantly prolonged retention,

2. In­creased translocation to the pulmonary interstitium and pulmonary persistence of nanoparticles,

3. Greater epithelial effects (such as type II cell proliferation),

4. Impair­ment of alveolar macrophages function.

Surface Area-Dependent Toxicity:

For the same mass of particles with the same chemical composition and crystalline structure, a greater toxicity was found from nano­particles than from their larger counterparts. This led to the conclusion that the inflam­matory effect may be dependent on the sur­face area of nanoparticles, suggesting a need for changes in definitions and regulations related to dose and exposure limits.

Indeed, smaller nanoparticles have higher surface area and particle number per unit mass com­pared to larger particles. The body will react differently to the same mass dose consisting of billions of nanoparticles compared to sev­eral micro particles.

Larger surface area leads to increased reactivity and is an increased source of reactive oxygen species, as demon­strated by in vitro experiments. Intra-trache­al instillation studies on mice with titanium dioxide anatase show that small nanoparti­cles (20 nm) induce a much greater inflam­matory response than larger nanoparticles (250 nm) for the same mass dose. If instilled at the same surface area dose, they generated similar toxicity, fitting the same curve.

The higher surface area of nanoparticles causes a dose dependent increase in oxidation and DNA damage, much higher than larger particles with the same mass dose. Giving an example for the dose, high levels of oxidative DNA have been observed in cell culture experiments at 25 mg per well, with surface area of wells of 9.6 cm². In a simplified calculation, for a total surface area of the human lung al­veolar region of 75 m², from which 3% are Type II epithelial cells (target for cancer develop­ment), this dose is equivalent to about 4 years of exposure at the highest ambient particle concentration.

However, mathematical modelling of particle deposition in the airways indicates that some cells may receive 100 fold more particles depending on their orientation geometry. Other studies suggested a thresh­old of 20 cm² surface areas of instilled nano­particles below which there is no significant inflammatory response in mice.

Extrapolating these findings to humans and environmental pollution, the critical surface area of nanopar­ticles becomes 30,000 cm². In a busy urban area with nanoparticles concentrations of up to 10 mg/m³, with specific surface area of 110 m²/g, deposition efficiency of 70%, the lung bur­den results in 150 cm²/day.

If deposited par­ticles accumulate in the lungs, the surface threshold for significant inflammatory effects is reached in about half a year. However, sub­jects with respiratory or cardiovascular diseas­es may have a lower threshold. In addition, cardiovascular consequences may appear at a lower pollution threshold.

We must emphasize that epidemiological studies do not indicate the existence of a threshold below which there are no adverse health effects. Attempts have been made to contradict surface-area-depen­dent toxicity. One study claims that they test­ed toxicity of smaller nanoparticles against larger nanoparticles of similar composition and their findings show that they generate sim­ilar cytotoxicity or inflammatory reaction within the lung.

However, they used two dif­ferent forms of titanium dioxide: rutile and anatase, which seems to have different toxici­ty levels regarding generation of oxidative compounds. Similar composition does not necessary implies similar chemistry, chemical bonds. The best example is carbon, whose allotropes are—graphite, diamond, carbon nan­otubes, and fullerenes, each with distinct phys­ical and biological characteristics.

Concentration-Dependent Toxicity:

There are many contradictory results relat­ed to the toxic effects of nanoparticles at different concentrations. Some studies show that certain materials are not as toxic as was observed by other studies. When comparing the results of different studies one must take into account that there are differences in the aggregation properties of nanoparticles in air and water, resulting in inherent discrepan­cies between inhalation studies and instilla­tion or in vitro experiments.

The aggregation may depend on surface charge, material type, and size, among others. One must stress the fact that aggregation of nanoparticles is essen­tial in determining their toxicity, due to a more effective macrophage clearance for larger par­ticles compared to smaller ones (that seem to evade easier this defense mechanism), lead­ing to reduced toxicity of nanoparticle ag­gregates larger than 100-200 nm.

It has been demonstrated that a high concentration of nanoparticles would promote particle aggre­gation, and, therefore, reduce toxic effects compared to lower concentrations. Most ag­gregates are observed to be larger than 100 nm, a size that seems to be a threshold for many of the adverse health effects of small particles. Therefore, experiments performed with high concentrations of nanoparticles will lead to the formation of nanoparticle aggregates that may not be as toxic as lower concentrations of the same nanoparticles.

Particle Chemistry and Crystalline Structure Dependent Toxicity:

Although there have been suggestions that size may be more important than chemical composition in deciding nanoparticles toxicity, one cannot generally extrapolate the results of studies showing similar extent of inflamma­tion for different nanoparticles chemistries.

Particle chemistry is critical in determining nanoparticles toxicity. Particle chemistry is especially relevant from the point of view of cell molecular chemistry and oxidative stress. Namely, depending on their chemistry, nano­particles can show different cellular uptake, subcellular localization, and ability to catalyse the production of reactive oxygen spe­cies.

One must make the distinction between composition and chemistry. Though parti­cles may have the same composition, they may have different chemical or crystalline struc­ture. The toxicity of a material depends on its type of crystalline form.

Let’s take for ex­ample rutile and anatase (fig. 14.7a and 14.7b) both allotropes of titanium dioxide, i.e. poly­morphs with the same chemical composition, but different crystalline structure, and hence chemical and physical properties. Rutile nanoparticles (200 nm) were found to induce oxidative DNA damage in the absence of light, but anatase nanoparticles of the same size did not.

Nanoparticles can change crystal structure after interaction with water or liquids. For example, it is reported that zinc sulphide ZnS nanoparticles (3 nm across containing around 700 atoms) rearrange their crystal structure in the presence of water and become more ordered, closer to the structure of a bulk piece of solid ZnS.

Nanoparticles often exhibit unex­pected crystal structures due to surface effects [Fig.14.7(c)] The collection of gold Nano and micro particles was made evaporating gold by heating it with an electron beam, and allow­ing the vaporized atoms sufficient time and density to condensate into clusters before col­lection on a substrate.

Condensation dynam­ics dictate that gold under these conditions will form these crystalline particles, which form equilibrium-seeking quasi-spheres as the con­densing atoms jostle each other in random walks on the surface towards final resting plac­es within the crystal.

The effects of crystallinity on condensation are clearly observed in the faceting, and fine (Nano) structure of the crys­tal faces. Incidentally interesting is the dendrit­ic patterns on the faces where the condensa­tion forms a classic diffusion-limited aggre­gation structure.

These nanoparticles are sim­ilar to the engineered nanoparticles produced in many industrial processes—they are engi­neered or designed by developing unique rec­ipes that yield materials with beneficial char­acteristics.

Finally, note the size of the largest gold particle in (Fig. 14.7c), and that of the two progressively smaller particles stacked one upon the other. The largest is 2.5 mm in diameter with approximately 1011 atoms, the middle is 450 nm with 109 atoms, and the smallest on top is 80 nm with 107 atoms.

The smallest nanoparticle in the image, just be­low the ‘x’ arrow, is only 25 nm in diameter, and contains roughly half a million atoms. Unique behaviour emerges from these and other Nano-materials when small clusters of atoms form and manifest quantum effects.

Aspect Ratio-Dependent Toxicity:

It was found that the higher the aspect ratio, the more toxic the particle is. More exactly, lung cancer was associated with the presence of asbestos fibers longer than 10 microns in the lungs, mesothelioma with fibers longer than 5 microns, asbestosis with fibers longer than 2 microns.

All of these fibers had a min­imum thickness of about 150 nm. Long fibers (longer than 20 microns for humans) will not be effectively cleared from the respiratory tract due to the inability of macrophages to phagocytize them.

Alveolar macrophages were measured to have average diameters of 14-21 mm. The bio-persistence of these long aspect ratio fibers leads to long-term carcinogenic effects. The toxicity of long aspect fibers is closely re­lated to their bio-durability.

The bio-durabili­ty of a fiber depends on its dissolution and mechanical properties (breaking). Longer fi­bers that break perpendicular to their long axis become shorter and can be removed by mac­rophages. Asbestos fibers break longitudinal­ly, resulting in more fibers with smaller diam­eter, being harder to clear.

If the lung clear­ance is slow, the longer the time these fibers will stay in the lung and the higher the proba­bility of an adverse response. Fibers that are sufficiently soluble in the lung fluid can dis­appear in a matter of months, while the insol­uble fibers are likely to remain in the lungs indefinitely.

Even short insoluble fibers that are efficiently phagocytized by alveolar macroph­ages may induce biochemical reactions (release of cytokines, reactive oxygen species, and other mediators). Long aspect ratios engineered nanoparticles—such as carbon nanotubes (CNTs)—are new materials of emerging tech­nological relevance and have recently attract­ed a lot of attention due to their possible neg­ative health effects, as suggested by their morphological similarities with asbestos.

Howev­er, there is no consensus in the characteriza­tion of CNTs toxicity. The contradictory re­ports on CNTs toxicity could be associated with the multitude of morphologies, size, and chemical functionalization of their surface or ends.

Carbon nanotubes can be single walled (SWCNTs) or multiple walled (MWCNTs), with varying diameter and length, with closed capped sections or open ends. In addition to the many forms of nanotubes, they can also be chemically modified. The diameter of CNTs varies between 0.4 nm and 100 nm. Their lengths can range between several nanometers to centimeters.

Due to their hydrophobicity and tendency to aggregate, they are harmful to living cells in culture. For many applications, CNTs are oxidized to create hydroxyl and carboxyl groups, especially in their ends, which make them more readily dispersed in aque­ous solutions.

The conclusions of research on carbon nanotube cytotoxicity are that in gen­eral CNTs are very toxic, inducing cell deaths at sufficiently high doses of400 mg/ml on hu­man T cells, and 3.06 mg/cm² on alveolar mac­rophages. Cell cultures with added SWCNTs at much lower doses of 3.8 mg/ml did not show cytotoxicity.

However, dose related inflamma­tion or cell death is not in agreement between various studies. It was found that cells actively respond to SWCNTs by secreting proteins to aggregate and wrap them. At the same time, SWCNTs induce up-regulation of apoptosis- associated genes.

Long-aspect ratio particles (SWCNTs) were reported to produce signifi­cant pulmonary toxicity compared to spheri­cal particles (amorphous carbon black). Pha­ryngeal introduction of SWCNTs resulted in acute inflammation with onset of progressive fibrosis and granulomas in rats.

For compari­son, equal doses of carbon black or silica nano­particles did not induce granulomas, alveolar wall thickening, causing only a weak inflam­mation and limited damage. The enhanced toxicity was attributed to physicochemical properties and fibrous nature. Carbon nano­tubes are not eliminated from the lungs or very slowly eliminated, 81% are found in rat lungs 60 days after exposure.

Surface Coating and Functionalization:

Due to the possibility of chemical interactions, the combined effects of inhalation, ingestion, or dermal application of nanoparticles with other nanoparticles, chemicals, and gases are largely unknown. The estimated risk of two or more pollutants is not a simple additive pro­cess. Particle surface plays a critical role in toxicity as it makes contact with cells and bio­logical material.

Surfactants can drastically change the physicochemical properties of nanoparticles, such as magnetic, electric, optical properties and chemical reactivity, affecting their cytotoxici­ty. Surface coatings can render noxious parti­cles non-toxic while less harmful particles can be made highly toxic.

The presence of oxygen, ozone, oxygen radicals, and transition metals on nanoparticle surfaces leads to the creation of reactive oxygen species and the induction of inflammation. For example, specific cyto­toxicity of silica is strongly associated with the occurrence of surface radicals and reactive ox­ygen species.

Experiments performed on ham­sters showed that the formation of blood clots is more prominent when the surface of poly­styrene nanoparticles is aminated. Diesel ex­haust particles interacting with ozone cause increased inflammation in the lungs of rats compared to diesel particles alone.

Nickel ferrite particles, with and without surface oleic acid, show different cytotoxicity. The cyto­toxicity of C₆₀ molecules systematically cor­relates with their chemical functionality in human (skin and liver) carcinoma cells with cell death occurring due to lipid oxidation caused by the generation of oxygen radicals.

Spherical gold nanoparticles with various surface coatings are not toxic to human cells, despite the fact that they are internalized. Quantum dots of CdSe can be rendered non­toxic when appropriately coated.

Adaptability to Nano-Materials Inhalation:

Recent studies suggest that pre-exposure to lower concentrations of nanoparticles or shorter exposure times stimulate the phagocytic activity of cells, while high concentration of nanoparticles impairs this activity. As a result, pulmonary inflammation is drastically re­duced by several previous shorter exposure times to the same Nano-materials. The severe pulmonary inflammatory response observed after only 15 minutes of rats exposed to 50 mg/m³ Teflon fume particles (with diameter of about 16 nm) can be prevented by three pre­ceding daily 5 minutes exposure to the fumes.

During the three days of adaptation, the ani­mals did not show clinical symptoms of respiratory effects, in contrast to the non-adapted group rats that were severely affected, show­ing difficulty breathing starting 1 h after ex­posure. The number of alveolar macrophages was significantly lower in non-adapted group.

Comparison Studies:

In order to assess the toxicity of various Nano-materials one must compare their toxic effects with those of known toxic particles. Several studies have pioneered this. However, the da­tabase of studied materials is limited. The conclusions of these studies indicate that CNTs are extremely toxic, producing more damage to the lungs than carbon black or silica. Varieties of CNT aggregates, and some carbon blacks, were shown to be as cytotoxic as asbestos. Silver nanoparticle aggregates were found to be more toxic than asbestos, while titanium oxide, alumina, iron oxide, zirconium oxide were found to be less toxic.

Conclusions and Future-Directions:

Human exposure to nanoparticles from nat­ural and anthropogenic sources has occurred since ancient times. Following the invention of combustion engines and the development of industry, however, significant levels of nano­particle pollution have arisen in most major cities and even across large regions of our plan­et, with climatic and environmental effects that are generally unknown.

There is heightened concern today that the development of nano­technology will negatively impact public health, and it is indisputable that engineered Nano-materials are a source of nanoparticle pollution when not safely manufactured, han­dled, and disposed of or recycled.

A large body of research exists regarding nanoparticle tox­icity, comprising epidemiological, animal, hu­man, and cell culture studies. Compelling ev­idence that relates levels of particulate pollu­tion to respiratory, cardiovascular disease, and mortality, has shifted attention to particles with smaller and smaller sizes (nanometer scale).

Research on humans and animals indicates that some nanoparticles are able to enter the body, and rapidly migrate to the organs via the circulatory and lymphatic systems. Subjects with pre-existing diseases (such as asthma, diabetes, among others) may be more prone to the toxic effects of nanoparticles. Genetic factors may also play an important role in the response of an organism to nanoparti­cles exposure.

It is clear that work­ers in nanotechnology related industries may be potentially exposed to uniquely engineered Nano-materials with new sizes, shapes and physicochemical properties. Exposure moni­toring and control strategies are necessary.

In­deed, there is a need for new discipline-Nano toxicology—that would evaluate the health threats posed by nanoparticles, and would enable safe development of the emerging nanotechnology industry. We emphasize that this field of study should include not only newly engineered Nano-materials, but also those generated by nature and pollution.

The abil­ity of nanoparticles to enter cells and affect their biochemical function makes them im­portant tools at the molecular level. The tox­ic properties of nanoparticles can in some instances be harnessed to improve human health through targeting cancer cells or harm­ful bacteria and viruses.

These very proper­ties that might be exploited as beneficial may also have secondary negative effects on health and the environment. For example, nanopar­ticles used to destroy cancer cells may cause harmful effects elsewhere in the body, or nanoparticles used for soil remediation may have an adverse impact upon entering the food chain via microorganisms, such as bac­teria and protozoa.

In the following we high­light important questions and research di­rections that should be addressed in the near future by the scientific community involved in the study of nanoparticles sciences and by government agencies responsible for regula­tions and funding.

Advanced analysis of the physical and chemical characteristics of nano­particles will continue to be essential in re­vealing the relationship between their size, composition, crystallinity, and morphology and their electromagnetic response proper­ties, reactivity, aggregation, and kinetics.

It is important to note that fundamental proper­ties of nanoparticles are still being discovered, such as magnetism in nanoparticles made of materials that are non-magnetic in bulk form. A systematic scientific approach to the study of nanoparticle toxicity requires correlation of the physical and chemical char­acteristics of nanoparticles with their toxici­ty.

Existing research on Nano toxicity has con­centrated on empirical evaluation of the tox­icity of various nanoparticles, with less re­gard given to the relationship between nano­particle properties (such as exact composi­tion, crystallinity, size, size dispersion, aggre­gation, ageing) and toxicity. This approach gives very limited information, and should not be considered adequate for developing predictions of toxicity of seemingly similar nanoparticle materials.

Further studies on kinetics and biochemi­cal interactions of nanoparticles within organ­isms are imperative. These studies must in­clude, at least, research on nanoparticles trans­location pathways, accumulation, short and long term toxicity, their interactions with cells, the receptors and signaling pathways involved, cytotoxicity, and their surface functionalization for an effective phagocytosis.

Existent knowledge on the effects of nanoparticle ex­posure on the lymphatic and immune systems, as well as various organs, is sparse. For exam­ple it is known that nanoparticle exposure is able to modulate the response of the immune system to different diseases, however much research is needed in order to better under­stand to what extent this occurs and the full implications of risk groups (age, genotype).

In order to clarify the possible role of nano­particles in diseases recently associated with them (such as Crohn’s disease, neurodegen­erative diseases, autoimmune diseases, and cancer), Nano scale characterization tech­niques should be used to a larger extent to identify nanoparticles at disease sites in af­fected organs or tissues, and to establish pertinent interaction mechanisms.

Other important research topics to be pur­sued include nanoparticle ageing, surface modifications, and change in aggregation state after interaction with bystander substances in the environment and with biomolecules and other chemicals within the organisms.

How do these interactions modify the toxicity of nano­particles?

Do they render toxic nanoparticles less toxic?

Or can they render benign nano­particles more toxic?

What about the benefi­cial properties of some nanoparticles?

Do they change in the short and long term after un­dergoing chemical interactions?

Research should also be directed toward finding ways to reduce nanoparticle toxicity (such as anti­oxidants provided from dietary sources and supplements, metals chelators, anti-inflamma­tory agents). Understanding and rationally dealing with the potentially toxic effects of nanoparticles requires a multidisciplinary ap­proach, necessitating a dialogue between those involved in the disparate aspects of nanopar­ticle fabrication and their effects, including but not limited to nanomaterial fabrication scien­tists, chemists, toxicologists, epidemiologists, environmental scientists, industry, and policy makers.

In order to achieve an interdiscipli­nary dialogue, systematic summaries should be prepared, discussing current knowledge in the various Nano-fields, and using a common vocabulary. This will help bring together sci­entists in different fields, as well as policy mak­ers and society at large.

These summaries should include periodic written reviews, con­ferences, and accessible databases that contain the collected knowledge of nanoparticle syn­thesis, characterization, properties, and toxic­ity, in a format easily comprehensible to a wide audience of scientists. A database initiative has already begun, led by the National Institute for Occupational Safety and Health, as the ‘Nano­particles Information Library’.

We also suggest several directions for mini­mizing human exposure to nanoparticles, and thereby reducing associated adverse health ef­fects. National governments and international organizations should enact stringent air quality policies, with standardized testing methods and low exposure limits. With such compelling existing evidence of the correlation between particle pollution levels, mortality, and a wide range of diseases (comprising car­diovascular, respiratory diseases, and malig­nant tumors), the primary source of atmo­spheric nanoparticles in urban areas—com­bustion based vehicles should be mandated to have lower nanoparticle emission levels.

In the light of their potential toxicity, the commer­cialization of dietary and cosmetic nanoparti­cles, as well as other consumer products in­corporating nanoparticles, must be strictly reg­ulated. In particular, they must be regulated in distinct materials from their bulk constitu­ents. Before using these nanoparticles several questions should be answered: Are they bio­compatible? Do they translocate and accumu­late in the body (including skin)? What are the long-term effects of uptake and accumulation? In general, consumer products containing Nano-materials should be recycled.

A model ini­tiative began in 2001 in Japan for electrical appliances, where the retailers, manufacturers, and importers are now responsible for recy­cling the goods they produce or sell. There is limited existing research regarding ecological and environmental implications of natural and anthropogenic nanoparticles pol­lution, though the role of nanoparticles in some form of environmental degradation is well-known, e.g. atmospheric nanoparticles play a central role in ozone depletion.

Nano-particulate pollution is likely to play an important role in global climate balance, despite the fact that current anthropogenic climate changes are attributed solely to greenhouse gases. This is dangerous as it encourages the misconception that wood burning does not contribute to pollution and/or climate change.

In a simple calculation of carbon liberation and fixation, it appears that wood burning, as a so- called ‘renewable’ source of energy, is benign to the environment. A proper accounting of nanoparticle pollution in addition to CO₂ re­veals the naivety of this analysis. Advances in nanotechnology are driven by rapid commercialization of products contain­ing nanostructures and nanoparticles with re­markable properties. This is reflected in the enormous number of publications on nano­technology.

In comparison, the number of publications on nanoparticle toxicity is much smaller, as the funding available for toxicity studies are mostly government related. One way of increasing funding for Nano toxicity re­search might be via international regulations requiring that a fraction of the revenues of each company involved in their production and commercialization to be dedicated to this field of research.

Without this level of com­mitment it is likely that a current or future industrial nanoparticle product, with non-obvious or delayed toxicity, will cause signif­icant human suffering and/or environmen­tal damage. The field of nanotechnology has yet to have a significant public health haz­ard, but it is a real possibility that can and should be prevented.

We conclude that the development of nan­otechnology and the study of Nano toxicology have increased our awareness of environ­mental particulate pollution generated from natural and anthropogenic sources, and hope that this new awareness will lead to significant reductions in human exposure to these potentially toxic materials.

With increased knowledge, and on-going study, we are more likely to find cures for diseases associated with nanoparticle exposure, as we will un­derstand their causes and mechanisms. We foresee a future with better-informed, and Exercise hopefully more cautious manipulation of engineered nanomaterial’s, as well as the de­velopment of laws and policies for safely managing all aspects of nanomaterial manu­facturing, industrial and commercial use, and recycling.