By air we refer to a mixture of gases that envelops the surface of our planet. Remarkably the composition of this mixture is nearly constant from the ground level to a height of 80 km. The two major constituents of air are nitrogen (78 per cent by volume) and oxygen (approximately 21 per cent). Argon and carbon dioxide are also present but together they constitute only about 1 per cent. The remaining major constituent, water vapour, fluctuates between 0.01 and 5 per cent. A large number of gases, the minor constituents of air, make up about 0.01 per cent by volume. When some of these are present in sufficient quantity to affect the physical well-being of humans, animals, vegetation and materials, they are considered pollutants of air. Such pollutants may exist as solid particles, as liquid droplets, in a gaseous state or as a mixture. Usually if air is polluted, it is simultaneously by more than one kind of pollutant. Local concentrations of such pollutants, as are often found in industrial and urban areas, are accentuated by meteorological and topographic conditions (Figure 5.1) which prevent the mixing of air and the associated dilution effects. This occurs, for example, over Mexico City and Kuala Lumpur.
The spectre of air pollution hangs over industrial and urban centres. The air pollution over London in the 1950s and several disastrous instances of pollution over New York in the 1950s and 1960s are textbook examples. Both London and New York are now much cleaner, as far as their air is concerned, due to concerted efforts put in after such disastrous episodes. Unfortunately, as the developing countries go through industrial and urban development, such attempts to keep the air clean have not yet started to happen.