Air pollution is one of the major environmental problems throughout the world.
The Environment (Protection) Act, 1986 of India has provisions to regulate environmental pollution {e.g., the standards of quality of air, water and soil for various areas and purposes) and the maximum allowable limits of concentrations of various environmental pollutants for different areas are also laid down.
These National Ambient Air Quality Standards (Tables 6.1 & 6.2) are prescribed for three distinct areas, viz:
(a) Industrial,
(b) Residential, and rural and other areas, and
(c) Sensitive areas.
The open air around us is called ambient air. Ambient air pollution is also referred to as tropospheric air pollution. Since the troposphere begins at the earth’s surface, tropospheric air pollution is also called ground level air pollution or lower atmospheric pollution. Since humans are most easily affected by the quality of air in the lowest region of die earth’s atmosphere, tropospheric air pollution has drawn attention of scientists, administrators and layman.
Ambient air pollution is caused by the following three classes of pollutants:
(a) Criteria air pollutants
(b) Hazardous air pollutants, and
(c) Volatile organic chemicals
Criteria Air Pollutants:
There are six pollutants, which are considered criteria air pollutants (CAPs) or most serious air pollutants that account for the large majority of air pollution in the lower atmosphere. These are: carbon monoxide (CO), sulphur” dioxide (SO2), Nitrogen oxides (NOx), Particulate Matter (PM10) Ozone (O3) and lead (Pb).
Carbon Monoxide (CO):
Carbon monoxide is an incomplete combustion product of fossil fuel and accounts about 50% of air pollution worldwide. In developed countries, 70 to 75% of CO emissions are from motor vehicles. Other sources of CO include coal, natural gas and biomass burning. Atmospheric oxidation of methane gas and other hydrocarbons also produce CO. It has 200 times greater affinity for the iron in hemoglobin than does oxygen.
As a result, it reduces the normal oxygen carrying capacity of the blood. A 30 ppm concentration of CO for 4 hours is known to convert 5% of the blood’s hemoglobin into carboxyhaemoglobin, thus starving the body of oxygen and causing CO poisoning. Other co-related illnesses are headaches, dizziness and drowsiness. In elderly people, CO problem may cause congestive heart failure.
Sulphur dioxide (SO2):
Sulphur dioxide accounts for about 15 to 18% ambient air pollution. Sulphur dioxide is produced by burning of coal and from metal smelters. Other smaller sources are pulp and paper mills and oil refineries. Petroleum also contains sulphur, but it can be more easily removed from petroleum than from coal, so motor vehicles account for only a small percentage of sulfur dioxide emissions.
SO2 causes direct effects on humans and vegetation. It readily reacts with moisture in the lungs, other mucous membranes and eyes to form an irritating acid. Since it is so reactive, major part of SO2 inhaled by humans is removed in the upper respiratory tract. As a result, exposure to SO2 can aggravate already existing respiratory or heart problems. Exposure to low concentrations of SO2 can also damage vegetation.
Nitrogen Oxides (NOx)
Nitrogen oxides are formed when fuel is burnt at very high temperatures, such as in industrial plants and transportation vehicles. During combustion in the engine cylinder, a reaction occurs between atmospheric oxygen and nitrogen to produce nitrogen monoxide (nitric oxide, NO). This gas passes out of the engine in the exhaust phase, cools down, and then combines with more oxygen to form nitrogen dioxide (NO2) and nitrogen tetroxide (N2O4).
The mixture of all these gases is generally described as nitrogen oxides (NO2) and they account for about 6 to 10% of air pollution. Like SO2, nitrogen oxides contribute to a number of problems. Direct exposure to nitrogen oxides irritates the eyes and lungs and aggravates asthma. Nitrogen dioxide is poisonous to plant life. Like sulphur dioxide, nitrogen oxides are precursors of acid rain. When moisture is present, chemical reactions in the atmosphere convert nitrogen oxides to nitric acid.
Particulate Matter:
Particulates are emitted into the atmosphere during combustion of fossil fuel. Air borne particulates are also generated from burning biomass and from windblown dirt, fertilizer, dried manure, or dried crop residues. Construction sites release large amounts of dust. Particulates in coastal areas contain high levels of chloride, which can corrode buildings and other items.
Particulates account for about 10 to 13% of air pollution. The size of particulates varies greatly (0.01 to 500 μm) and they can be either dry or liquid. Very fine particles of either solid or liquid that are suspended in the air are referred to as aerosols (diameter less than 1.0 μm).
Sulphate particles and some soot and dust particles can be as small as 0.01 nm. Small particles, especially those in the size range of 0.01 to 50 μm diameter are of most significance in air pollution, as they are not obviously visible. They can remain in the atmosphere for varying lengths of time and undergo chemical reactions to produce secondary air pollutants like smog (smoke + fog). The famous 1905 smog of London was the result of sulphur dioxide, soot and tarry materials released by the uncontrolled burning of sulphur containing coal.
The photochemical smog, results from the reaction of volatile organic chemicals (VOCs) and nitrogen oxides in the presence of sunlight to form ozone and other photochemical oxidants, including NO2 and peroxyacy1 nitrates (PANs). Acute effects of photochemical smog are eye irritation and headaches. It adversely effects breathing and can also damage lungs and other tissues.
Thus, particulate air pollution causes respiratory problems worldwide. People suffering from asthma, bronchitis and emphysema are worst affected by particulate air pollution. A recent study indicates that an increase in air particulates aggravates heart problems like ischemia and arrhythmias.
Ozone (O3):
People are not easily convinced about the role of tropospheric ozone as a pollutant. Ozone is a more reactive chemical than molecular oxygen and often adversely affects both plants and animals. Ozone is not typically emitted as ozone, but it is formed from the ozone precursors, VOCs, and nitrogen oxides. Ozone is a summer pollutant as it is formed during summer when VOCs (especially formaldehyde and others) and NO2 react with atmospheric oxygen to form ozone in the summer heat and the sun’s strong UV rays. Motor vehicles are a strong source of ozone precursors.
Ozone levels up to 0.06 ppm in ambient air are not considered harmful, but ozone levels from 0.12 to 0.20 ppm or more are considered unhealthful. Most ground level ozone is anthropogenic. But in the troposphere excess ozone is the problem, whereas in the stratosphere, ozone depletion is the problem.
Its acute effects cause eye irritation and adversely affect lungs and respiratory membranes of nose and throat. Ozone may also increase susceptibility to infection. Ozone also adversely affects tree growth and damages forests. Sensitive crops are damaged at ozone level 0.05 ppm and estimated crop losses from ozone pollution may be 5 to 10%.
Lead(Pb):
Lead is an airborne particulate. The heavy metals lead, cadmium and mercury are quite common pollutants found at hazardous waste sites. Lead emissions are from metal mining and smelting, coal burning, electric power plants and municipal solid waste combustors. Lead in water pipes also remains in place for many years. In motor vehicles, lead-acid battery is used worldwide. The household dust may also contain lead when leaded paint flakes and crumbles into small airborne particles.
Until 1970s motor vehicle exhaust was the major source of lead to the environment in many countries of the world since lead was added to gasoline as antiknock agent. As the use of lead in gasoline has been banned and many countries are using unleaded petrol, lead emissions to the air have fallen more than 90%. Blood levels of lead in 1 to 5 year old children should not be more than 4 to 5 μg per 100 ml and if the level is slightly above 10 μg per 100 ml, necessary steps to reduce lead are immediately required.
Lead poisoning of children has been described as a serious problem. Lead adversely affects the nervous system of developing foetus and small child. Boys whose bones contained high levels of lead were more likely to be juvenile delinquents. However, adults are not as sensitive to lead exposure as are children but do suffer adverse effects, including damage to the nervous system and kidney failure and chances of high blood pressure. Other potential adverse effects may be aneamia and infertility.
Hazardous Air Pollutants:
A large number of pollutants are grouped under the category of hazardous air pollutants (HAPs) or toxic air pollutants. In US, there are 189 federally regulated HAPs (Hill, 1997). Important Examples of organic chemical HAPs are benzene, chloroform, and formaldelyde. Other examples of organic chemical HAPs are toluene, ethylene glycol, methanol, methyl bromide, parathion, styrene and vinyl chloride. Examples of inorganic chemical HAPs are asbestos and arsenic and the metals (cadmium, chromium, mercury, nickel, and beryllium).
The HAP benzene is produced in large volumes and is also found in gasoline. It can irritate the eyes and skin, cause dizziness and headache and has also been associated with aplastic anaemia and leukemia. Formaldehyde (HCHO) is another important HAP, which is released from factories manufacturing furniture. It can irritate the lungs and eyes and is a carcinogen at high concentrations. The HAP chloroform is formed during water chlorination and other chlorine uses. It was used as an anesthetic for many years until its ability to damage the liver was discovered. Exposure to chloroform can also damage the kidneys.
The HAP asbestos, an inorganic chemical, is a natural fibrous mineral material widely used to insulate furnaces. Exposure to asbestos is associated with asbestosis, lung cancer and mesothelioma. When airborne fibres of asbestos are inhaled, they are permanently trapped into the lungs. The HAP Arsenic is a nonmetallic element used in metal alloys and glass making. Chronic arsenic ingestion is associated with skin cancer and chronic inhalation with lung cancer. The HAP Cadmium is released into the air by combustion of coal and mining and smelting operations.
However, plants take up cadmium more readily than other metals. Because tobacoo plants concentrate cadmuim, an adult absorbs more than 90% of the cadmium inhaled through smoking. Cadmium can accumulate in kidney and liver. Because cadmium affects calcium metabolism, bone degeneration has also been observed. In laboratory studies with rodents, cadmium, depending on concentration, shows numerous toxic effects; including birth defects, and is a carcinogen.
The HAP Mercury is a volatile liquid metal, sometimes called quicksilver. Mercury emitted into the atmosphere is washed out into water bodies where bacteria transform it to methylmercury. Methylmercury is more toxic than elemental mercury as most of it is absorbed in human gastrointestinal tract.
The mercury level in fish may range from 0.01 to 0.5 ppm but in large ocean fish may accumulate up to 3 ppm or more. Thus, about 95% of human exposure results from eating mercury contaminated fish. Mercury, like lead, is toxic to the nervous system and foetus. High exposure to mercury in adults results in a number of toxic effects, including kidney damage, deafness and blindness and other problems of nervous system.
Volatile Organic Chemicals (VOCs):
These are a large number of organic chemicals that evaporate easily and some of them significantly contribute to smog formation. About 70% of the HAPs are among the VOCs and there are hundreds of other chemicals in this category. Motor vehicle exhausts are a major source of VOCs, emitting about 50% of the total VOCs. Other small sources of VOCs are auto maintenance shops, wood-drying and wood-painting operations and sewage treatment plants.
In homes and other places, people are exposed to VOCs from solvents, paints, charcoal broiler starters, aerosol sprays and deodorants and cosmetics. Some VOCs contribute to the formation of ground level photochemical ozone. VOCs also contribute too many other problems. Some time people may develop asthma or other respiratory problems due to VOCs or ozone-containing smog.