Environmental toxicology is a relatively recent field that examines the occurrence of, exposure to, and form of toxicants in the environment, and the comparative effects of these toxicants on different organisms. DDT, for example, is a pesticide that has been used to control mosquitoes responsible for spreading malaria. Although this pesticide is effective in combating the spread of malaria, DDT and its chemical products have also been found to affect reproduction in birds by causing egg shell thinning, and in other organisms (e.g., alligators) by altering their estrogen balance. Consequently, studies of toxicology now extend well beyond dose-response assays of toxicants on specific target organisms to analyses of their impact on entire ecosystems.
In addition to anthropogenic toxicants like pesticides, environmental tox-icologists also study naturally occurring toxicants, such as metals and metalloids. Selenium, for example, is a naturally occurring element that is essential at low concentrations in the diet of many animals. Excessive intake of selenium, however, can be toxic to organisms. In the 1980s scientists working at Kester-son Slough in the San Joaquin Valley, California, observed a large number of deformed and dying waterfowl. The slough was part of a water project designed to receive and evaporate excess irrigation water and remove pesticides from the highly productive agriculture regions in the San Joaquin Valley. The observed effects on the waterfowl were eventually linked to an excess of selenium in the water. The selenium accumulated in the slough because the soils and runoff from the valley were naturally rich in selenium, and because evaporation in the slough further increased its concentration in the water. In this example, it was discovered that a rare, but naturally occurring and essential element was unwittingly concentrated to toxic levels in the environment by human activity. see also Cancer; DDT (Dichlorodiphenyl trichloro-ethane); Hazardous Waste; Health, Human; Lead; Risk.
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Society of Toxicology. Available from http://www.toxicology.org.
A. Russell Flegal and Christopher H. Conaway
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