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Air pollution occurs when wastes dirty the air. Artificially created wastes are the main sources of air pollution. They can be in the form of gases or particulates. Such wastes result chiefly from the burning of fuel to power motor vehicles and to heat buildings. They also come from the industrial processes and the burning of solid wastes. Natural pollutants include dust, pollen, and soil particles.

The rapid growth of population and industry, and the increase in the number of automobiles and airplanes, has made air pollution an increasingly serious problem in many big cities since the 1950’s. The air over these cities often become so filled with pollutants that it harms the health of people, and also harms plants, animals, fabrics, building materials, and the economy.

The damage caused by air pollution costs the people of the United States alone about $16 billion a year – or $75 per person. This money is spent for increased maintenance of building, cleaning and replacement of clothing, and health care.

The major sources of air pollution vary from city to city. Automobiles produce nearly all the pollution in Los Angeles. Furnaces in apartment and office buildings cause most of the contamination in New York City. Chicago’s air pollution comes equally from industry, motor vehicles, and heating plants.

Weather conditions called thermal inversions may enable pollutants to build up over a certain area. A thermal inversion occurs when a layer of warm air settles over a layer of cool air that lies near the ground. This prevents pollutants from rising and scattering, and causes them to collect near the ground. The thermal inversion continues until rain or wind breaks up the layer of stationary warm air, allowing the impurities to rise.