ACID RAIN, form of air pollution (q.v.) , currently a subject of great controversy because of widespread environmental damage for which it has been blamed. It forms when oxides of sulphur and nitrogen combine with atmospheric moisture to yield sulphuric and nitric acids, which may then be carried long distances from their source before they are deposited by rain. The pollution may also take the form of snow or fog or be precipitated in dry forms. In fact, although the term "acid rain" has been in use for more than a centuryæit is derived from atmospheric studies that were made in the region of Manchester, Englandæthe more accurate scientific term would be "acid deposition." The dry form of such precipitation is just as damaging to the environment as the liquid form. The problem of acid rain may be said to have originated with the Industrial Revolution, and it has been growing ever since. The severity of its effects has long been recognised in local settings, as exemplified by the spells of acid smog in heavily industrialised areas. The wide destructiveness of acid rain, however, has come to be realised only in recent decades. One large area that has been studied extensively is northern Europe, where acid rain has eroded structures, injured crops and forests, and threatened or depleted life in freshwater lakes. In 1983, for example, published reports indicated that 34 percent of the forested areas of West Germany had been damaged by acid rain. The northeastern U.S. and eastern Canada have also been affected by this form of pollution, as well as other areas of these countries and other regions of the world.

Industrial emissions have been blamed as the major cause of acid rain. Because the chemical reactions involved in the production of acid rain in the atmosphere are complex and as yet little understood, industries have challenged such assessments and stressed the need for further studies; and because of the cost of pollution reduction, governments have tended to support this attitude. U.S. government studies released in the early 1980s, however, strongly implicated industries as the main source of acid rain in the eastern U.S. and Canada. In 1988, as part of the UN-sponsored Long-Range Transboundary Air Pollution Agreement, the U.S., along with 24 other nations, ratified a protocol freezing the rate of nitrogen oxides emissions at 1987 levels. The 1990 amendments to the Clean Air Act of 1967 put in place regulations to reduce the release of sulphur dioxide from power plants to 10 million tons per year by Jan. 1, 2000. This amount is about one-half the emissions of 1990.