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Air Pollution contamination of the atmosphere by gaseous liquid or solid wastes or by-products that can endanger human health and the health and welfare of plants and animals or can attack materials reduce visibility or produce undesirable odors Among air pollutants emitted by natural sources only the radioactive gas radon is recognized as a major health threat A byproduct of the radioactive decay of uranium minerals in certain kinds of rock radon seeps into the basements of homes built on these rocks According to recent estimates by the US government 20 percent of the homes in the US harbor radon concentrations that are high enough to pose a risk of lung cancer 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 is produced by the interaction of hydrocarbons and nitrogen oxides under the influence of sunlight Ozone has also caused serious crop damage On the other hand the discovery in the 1980s that air pollutants such as fluorocarbons are causing a loss of ozone from the earths protective ozone layer has caused the phasing out of these materials Pollutant concentrates are reduced by atmospheric mixing which depends on such weather conditions as temperature wind speed 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
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