It is a contamination of the atmosphere by gaseous, liquid, or solid wastes or byproducts that can endanger human health and the health and welfare of plants and animals, or can attack materials, reduce visibility, or produce undesirable odors. While some air pollution is caused by natural sources, such as radon (q.v.) gas emitted from the earth, this article is concerned with air pollution caused by human activities. See also Environment . Each year industrially developed countries generate billions of tons of pollutants. The most prevalent and widely dispersed air pollutants are described in the accompanying table. The level is usually given in terms of atmospheric concentrations (micrograms of pollutants per cubic meter of air) or, for gases, in terms of parts per million, that is, number of pollutant molecules per million air molecules. Many come from directly identifiable sources; sulfur dioxide, for example, comes from electric power plants burning coal or oil. Others are formed through the action of sunlight on previously emitted reactive materials (called precursors). For example, ozone, a dangerous pollutant in smog (q.v.), is produced by the interaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Currently more than 100 million Americans live in cities that do not meet public health standards for ozone. On the other hand, ozone in the upper atmosphere is a protection from the sun's ultraviolet rays. The discovery in the 1980s that air pollutants such as fluorocarbons were destroying the ozone layer (q.v.) has caused the phasing out of these materials.
Meteorology and Health Effects:
Pollutant concentrations are reduced by atmospheric mixing, which depends on such weather conditions as temperature, wind speed, amount of sunlight, and the movement of high and low pressure systems and their interaction with the local topography, for example, mountains and valleys. Normally, temperature decreases with altitude. But when a colder layer of air settles under a warm layer, producing a temperature or thermal inversion, atmospheric mixing is retarded and pollutants may accumulate near the ground. Inversions can become sustained under a stationary weather system coupled with low wind speeds. Periods of poor atmospheric mixing of only a few days, and sometimes only a few hours, can lead to high concentrations of hazardous materials in high-pollution areas and, under severe conditions, can result in illness and even death. An inversion in Donora, Pa., in 1948 caused respiratory illness in over 6000 persons and led to the death of 20. Severe pollution in London took 3500 to 4000 lives in 1952 and another 700 in 1962. Release of methyl isocyanate into the air during a temperature inversion caused the disaster at Bhopal, India, in December 1984, with at least 3300 deaths and more than 20,000 illnesses. The effects of long-term exposure to low concentrations are not well defined; however, those most at risk are the very young, the elderly, smokers, workers whose jobs expose them to toxic materials, and persons with heart or lung disease. Other adverse effects of air pollution are potential injury to livestock and crops. Often, the first noticeable effects of pollution are aesthetic and may not necessarily be dangerous. These include visibility reduction due to tiny particles suspended in air, or bad odors, such as the rotten egg smell produced by hydrogen sulfide emitted from pulp and paper mills.

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