Out of this comes a variety of emissions:
(1) Fine particles (less than 100 in diameter), which include carbon particles, metallic dusts, tars, resins, aerosols, solid oxides, nitrates, and sulphates;
(2) Coarser particles (over 200/m), largely carbon particles and heavy dust that is quickly removed by gravity from the air;
(3) Sulphur compounds;
(4) Nitrogen compounds;
(5) Oxygen compounds;
(6) Halogens; and
(7) Radioactive substances.
These pollutants are artificial pollutants and they are poured in air mainly by at least five major fuel-burning sources. Automobiles (cars, scooters, motorcycles) are the greatest sources of air pollution. They produce nearly two-thirds of the carbon monoxide and one-half of the hydrocarbons and nitrous oxides.
The automobile exhaust also contains leaded gas and particulate lead. Electrical power plants burning fossil fuels, particularly coal and sometimes petrol or diesel, produce two-thirds of the sulphur dioxides.
Industrial processors such as metallurgical plants and smelters, chemical plants, petroleum refineries pulp and paper mills, sugar mills, cotton mills, and synthetic rubber manufacturing plants are responsible for about one-fifth of the air pollution.
Heating plants for homes, apartments, schools, and industrial buildings-are the fourth largest source of air pollution. The transportation industry, exclusive of automobiles and including railroads, ships, aircrafts, trucks, buses, tractors, etc., contribute the same type of pollutants as cars.
Other sources of air pollution are minor in quantities but bear significance due to the harmful substances they release, are agriculture, which is responsible for pesticides, dust from agriculture practices and field burning, and the construction industry. Nature too adds few natural pollutants such as pollen, hydrocarbons released by vegetation, dusts from deserts, storms, and volcanic activity.
Some of the most common air pollutions, their sources and their effects on human health have been tabulated in Table 25.1.
To measure and control the magnitude of air pollution in various industrial centres of India, National Environmental Engineering Research Institute (NEERI) has set air monitoring stations in Bombay, Calcutta, Delhi, Madras, Hyderabad, Kanpur, Jaipur, Ahmedabad and Nagpur.
In one of the Survey conducted by NEERI in 1970 to measure the air pollution by sulphur dioxide (SO2) and suspended particles in some major cities of India (Table 25.2), it is found that Chembur-Trombay area of Bombay has highest SO2 pollution, while, New Delhi has highest air pollution of suspended particulate matter. In another survey, Calcutta is reported to have highest carbon monoxide pollution during peak traffic hours.