Acid rain, is a form. of air pollution, currently a subject of great controversy because of widespread environmental damage for which it has been blamed. It forms when oxides of sulfur and nitrogen combine with atmospheric moisture to yield sulfuric and nitric acids, which may then be carded long distances from their source before they are deposited by rain. The pollution many 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 century — it is derived from atmospheric studies that were made in the region of Manchester, England — the more accurate scientific term would be "acid deposition." The dry form. of such precipitation is just as damaging as the liquid form, especially to trees and structures. Furthermore, some of the pollutants also associated with acid rain are not themselves acidic.

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 recognized in local settings, as exemplified by the spells of acid smog in heavily industrialized areas. The wide destructiveness of acid rain, however, has come to be realized 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, and other areas of the two countries are also showing increasing signs of damages, as are 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 tended to challenge such assessments and to stress the need for further studies; and because of the cost of pollution reduction, governments have tended to support this attitude. Studies released by the U.S. government in the early 1980s, however, strongly implicated industries as the main source of acid rain.