According to WHO ( World Health Organization), air pollution may be defined as “ the presence of materials in the air in such concentration which is harmful to man and his environment”
Major sources of atmospheric pollution are
Combustion of fissile fuels in homes, factories, thermal plants, automobiles, railways, etc.
Mining and processing.
Welding, stone crushing.
Construction and demolition.
52% of air pollution is caused by CO, 18% by SO2 12% by hydrocarbon, 10% by particulates, 6% by nitrogen oxides, 2% by remaining.
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Air pollutants and their effects
Carbon dioxide (CO2) – A major pollutant in the atmosphere comes from burning of fossil fuels.
Carbon monoxide (CO) – It is produced due to incomplete combustion, metallurgical operations and naturally by plants and animal. Normally CO has a brief residence in the atmosphere and gets oxidized to CO2. CO combines with haemoglobin, produces carboxyhaemoglobin which impairs oxygen transport resulting in headaches, decreased vision, cardiovascular disease, asphyxia.
Particulate matter (PM) – It consists of soot, flyash, the dust of various types, fur, hair, spores, pollen grain etc. Particulate matter is differentiated into settleable ( larger than 10 μm,) remaining in the air for less than one day, and suspended ( lesser than 10 μm) remains in the air for more than one day to several weeks. SPM ( suspended particulate matter) is maximum in Kolkata. SPM is differentiated in aerosol ( less than1 μm), dust ( more than 1 μm) and mist ( liquid, more than 1 μm) According to Central Pollution Control Board, particles most harmful to human health are of 2.5 μm diameter. Dust and smoke produce smog. Smoke is similar to dust but consists of a visible suspension of carbon and another particle suspension of carbon and other particles given off by burning or smouldering organic matter. PM causes respiratory disease, tuberculosis, byssinois ( due to cotton dust), allergy.
Nitrogen oxide (NOX) – They are produced naturally through biological and non-biological activities from nitrates, nitrites etc. They cause fading and deterioration of textiles, produce lesions, necrosis, defoliation. They also cause eye irritation, respiratory troubles, lung edema, blood congestion, dilation of arteries and cancer.
Sulfur dioxide ( SO2) – It is produced during combustion of fossil fuels, refining of petroleum and melting of sulfur-containing ores. Sulfur dioxide produces smog. Maximum SO2 pollution is formed in Kolkata. It causes membrane damage, destruction of chlorophylls. Lichens are most sensitive to SO2 pollution. SO2 corrodes metal, impairs sensitive equipment, damages building, marble, paper,and textiles. It produces eye irritation, damages respiratory tract, produces asthma, bronchitis. SO2 produces acid rain which destroys vegetation and degrades articles.
Fluorides – They are produced during the refining of minerals. Fluorides cause fluorosis. It causes chlorosis, necrosis of leaf tips and margins. It causes abnormal calcification of bones and teeth, frequent diarrhea, neuromuscular disorders.
Hydrocarbons – These are produced naturally as well as due to incomplete combustion. Hydrocarbons are carcinogenic, cause irritation of eyes and mucous membrane. There is increased mucus secretion and tearing of alveoli. Methane is naturally occurring hydrocarbon produced due to decomposition of organic matter, paddy fields and incomplete combustion in automobiles, industries.
Chlorofluorocarbon / Freons ( CFC) – This chemicalare used in the refrigerator, propellants CFC reacts with ozone and depleted it.
Other atmospheric pollutants – Mercury ( burning of coal, smelting), methyl isocyanate ( pesticide manufacturer), phosgene ( pesticide manufacture, dye industries) and lead ( automobile exhausts ) are pollutants added to the atmosphere. Bhopal gas tragedy was due to the releaseof phosgene and methyl isocyanate.
Smog ( Des Voeux, 1905) – Smog is opaque dark fog having condensed water vapors, dust, smoke and gases.
Smog is of two types
contains sulfur gases, smoke and dust particles. Classical smog has the reducing environment. It is dark brown and opaque. The smog is formed by condensation of water vapour with H2S and SO2 over dust and smoke particles. Classical smog occurred in London during December 1952 when it affected 50% population and killed over 4000 persons.
Photochemical ( Los Angeles ) smog – It was first reported over Los Angeles in 1940’s. Photochemical smog is produced at high temperature over cities and town due to still air, emission of nitrogen oxides. Nitrogen dioxide splits into nitric oxide and nascent oxygen. Nascent oxygen combines with oxygen to form ozone. Ozone reacts with a hydrocarbon to form aldehydes and ketone. Nitrogen oxides, oxygen, ketones combine to form peroxyacyl nitrates (PAN)
Peroxyacyl nitrate(PAN) – It is produced due to the reaction between NOX and hydrocarbons under the effect of UV – radiation of sunlight. It causes eye irritation, respiratory tract disease in human. Several enzymes are deactivated by PAN.