Essay on Air Pollution: Sources, Types and Effects!
The earth’s atmosphere contains various gases, water vapor and suspended particles. The dry air of the atmosphere is composed of four major gases nitrogen, oxygen, argon and carbon dioxide that account for more than 99.5% (Table 55.1).
The other gases found in traces in the air include helium, methane, krypton, hydrogen, carbon monoxide, nitrous oxide (N2O), nitrogen dioxide (NO2), ammonia, ozone, sulfur dioxide and hydrogen sulfide.
The lower part of the atmosphere (up to about 12 km) also contains water vapor at a concentration, ranging from 0.01 to 5.0%. This water is mostly contributed by evaporation from the hydrosphere.
The air is getting contaminated by pollution due the natural and unnatural activities of man. Air pollution is basically the presence of foreign substances in the air at a concentration that will adversely affect the health and property of the individual.
As per the World Health Organization (WHO) criteria, air pollution refers to ‘the substances put into the air by the activity of mankind into concentration sufficient to cause harmful effects to his health, vegetables, property or to interfere with the enjoyment of his property.’
According to Indian Standards Institution ‘air pollution is the presence in ambient atmosphere of substances generally resulting from the activity of man, in sufficient concentration, present for sufficient time and under circumstances which interfere significantly with the comfort, health or welfare of a person or with the full use or enjoyment of his property.’
The term pollutant refers to a substance which increases in quantity in the air and adversely affects the environment e.g. carbon monoxide, sulfur dioxide, lead. On the other hand, a contaminant is a substance which is not present in nature, but released due to human activity e.g. methyl isocyanate. DDT, malathion. However, this distinction is not very rigid, and most authors use the term pollutant to represent both (pollutant as well as contaminant).
Sources of Air Pollutants:
The sources that contribute to air pollution may be broadly categorized into two types:
1. Natural sources:
These include volcanic eruptions, sand storms, and decomposition of organic matter, forest fire, pollen grains and cormic dust. The problem of pollution due to natural sources in general, is considered to be minimal.
2. Anthropogenic (man-made) sources:
Air pollution due to human-induced activities is very high. The sources of pollution include industries, burning of fossil fuels, emissions from vehicles, agricultural activities and warfare’s. The sources of air pollution may also considered as stationary (industries, open combustion) or mobile (motor vehicles, trains, aircraft) in nature.
Industrial pollutants:
The major problem of air pollution is due to industrial activities. Among the several industries, nine types of industries are considered to be the major air pollutant generating industries (Table 55.2). Among these, thermal power plants, steel industries, petroleum refineries and metal smelters are the most dangerous polluting industries.
Classification of Air Pollutants:
Air pollutants are classified based on their origin, chemical composition and the state of the matter.
Classification based on origin:
Air pollutants are divided into two categories, based on their origin-primary and secondary.
Primary air pollutants:
These pollutants are directly emitted into the atmosphere and present there as such (i.e. in the form they are originally emitted). Primary air pollutants contribute to as much as 90% global air pollution. Particulates, carbon monoxide, (CO), oxides of sulfur (SOx), oxides of nitrogen (NOx), hydrocarbons (HCs), radioactive compounds, pollen and bacteria are the major primary air pollutants.
Secondary air pollutants:
These are produced in the air as a result of interaction among the primary pollutants, or by a reaction that occurs between a primary pollutant and a normal constituent of the atmosphere. Good examples of secondary air pollutants are ozone (O3), peroxyacetyl nitrate (PAN), formaldehyde and smog.
Classification based on chemical composition:
According to chemical composition, air pollutants are categorized as organic and inorganic.
Organic air pollutants:
These pollutants are mainly composed of carbon and hydrogen. In addition, oxygen, nitrogen, sulfur and phosphorus may also be present e.g. hydrocarbons, organic sulfur compounds, aldehydes, ketones, carboxylic acids.
Inorganic air pollutants:
These are purely inorganic in nature e.g. carbon dioxide, carbon monoxide, oxides of sulfur, oxides of nitrogen, ozone.
Classification based on the state of matter:
Air pollutants may be divided into two types, based on the state in which they exist-particulate and gaseous.
Particulate air pollutants:
The solids and liquids dispersed in the atmosphere constitute the particulate air pollutants. Solid particulates e.g. smoke, dust, fly ash. Liquid particulates e.g. mist, fog, spray.
Gaseous air pollutants:
These are the organic and inorganic gases that are present in the air as pollutants. Organic gases e.g. methane, butane, aldehydes. Inorganic gases e.g. CO2, SO2, NO2, NH3, and H2S.
Effects of Air Pollution:
Majority of the air pollutants are present normally in the atmosphere, although at low concentrations. Such pollutants are unlikely to cause any significant harmful effects. When the concentrations of the air pollutants go beyond the acceptable limits (variable for each pollutant) they are dangerous and cause several harmful effects. The WHO has set guidelines and fixed threshold limit values (TLV) for each air pollutant. The general effects of air pollutants on humans, plants, animals, materials and global climate are briefly described.
Effects of air pollutants on humans:
On an average, man breathes around 22,000 times and inhales about 16 kg of air each day. Primarily, the air we breathe is life sustaining. However, pollutants present in the air often cause harmful effects. The nature of the pollutant, its concentration and duration of exposure are among the factors that affect the health of humans.
In general, infants, elderly people and those with respiratory diseases are more susceptible to air pollution. Further, adverse health effects of air pollutants are maximum during winter compared to other seasons. Some of the important air pollutants and their effects on human health are given in Table 55.3. Sulfur dioxide is the most dangerous pollutant gas to man. It is produced in many industries and causes respiratory disorders.
Oxides of nitrogen and carbon monoxide reduce O2 supply to the tissues. Lead pollution is associated with tissue damage and abnormal behavioural pattern. Suspended particulate matter (SPM) mainly causes chronic pulmonary diseases. Prolonged exposure to radioactive isotopes results in anemia, besides a heavy risk of cancer and genetic defects.
Effects of air pollutants on plants:
At low concentration, the air pollutants may not cause any visible damage to the plants. However, even at this level, the pollutants may be stored and introduced into the food chain. This in turn, may affect animals as well as humans.
The entry of the gas air pollutants (SO2, NO2) predominantly occurs through the stomata, openings on the leaves. Stomata are located at the bottom of the leaves through which CO2 enters for photosynthesis. In the same way as CO2 enters the leaves, the gaseous pollutants also enter and cause various effects.
The solid particles are adsorbed on the surfaces of leaves. This may result in clogging of the stomata and a reduced intake of CO2. Further, the suspended particles deposited on the leaves, may adversely affect the leaf functions (reduced exposure to sun light, decrease in chlorophyll content, interruption of gaseous exchange).
The effects of common air pollutants on leaves are given in Table 55.4. The sensitivity of plants to pollutants depends on many factors such as climatic conditions (light, temperature, and humidity), soil and water, and the individual plant’s response to a particular pollutant. The major air pollutants affecting plants are SO2, NO2, O3, fluorides, ammonia and ethylene, besides the particulates. They may damage the plants to varying degrees as outlined in Table 55.4.
The sensitivity of certain selected plants to air pollutants can be used for bio-monitoring of air pollution.
Effects of air pollutants on animals:
The effect of air pollution on animals is mostly indirect, as it occurs after they eat polluted plants or foliage. Fluorine, lead and arsenic are three main air pollutants that cause harmful effects to livestock.
Fluorine:
Fluorine causes loss of weight, muscular weakness, diarrhea, wearing of teeth, and even death. Fluorosis mainly affects the ruminants, particularly dairy cows.
Lead:
Lead poisoning is commonly observed in animals grazing near lead mines. It is associated with loss of appetite, difficulty in breathing, diarrhea and paralysis.
Arsenic:
Arsenic toxicity in animals is associated with increased salivation, thirst, irregular pulses, abnormal body temperature and paralysis. Chronic poisoning may result in anemia, diarrhea, paralysis and even death.
Effects of air pollutants on materials:
Air pollutants cause immense damage to various materials — stone, metal, paint work, fibre materials, glass, ceramic, textiles, rubber, architecture etc. The adverse effects of air pollution on materials and properties are associated with severe economic losses throughout the world.
Effects of air pollutants on global climatic changes:
Air pollution is associated with significant changes in global climate and the related processes e.g., ozone depletion, green house effects, acid rain.
Bio Monitoring of Air Pollution:
Plants are used to monitor (bio monitoring) air pollution and such plants are referred to as indicator plants. This is based on the principle of sensitivity and response of the plants to air pollutants. A list of the plants used for bio monitoring and the corresponding air pollutants is given in Table 55.5. Among these, lichens and mosses are most commonly used to check the quality of air. The pattern of occurrence of patches on lichens serves as an index for bio monitoring of air pollution.
Bioluminescence in air pollution monitoring:
Bioluminescence is the phenomenon of emission of visible light by an organism. It occurs as an enzymatically catalysed light emitting reaction in living cells. Bioluminescence is an indicator for the analysis of atmospheric gases such as SO2, formaldehyde and ethyl acetate. The microorganism, Photo bacterium phosphoreum can be used as a special photo detector. The changes in the emission of light due to a pollutant can be detected by a sensor, amplified and recorded.