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Acid rain is a common term for pollution caused when sulfur and nitrogen dioxides combine with atmospheric moisture to produce a rain, snow, or hail of sulfuric and nitric acids. Such pollution may also be suspended in a fog, or the pollutants may be deposited in dry form. Environmental damage from acid rain has been reported in northern Europe and North America. High levels of acid rain have also been detected in other areas of the world, such as above the tropical rain forest of Africa. Acid rain has destroyed plant and animal life in lakes, damaged forests and crops, endangered marine life in coastal waters, eroded structures, and contaminated drinking water.
Research has shown that although some of the damage attributed to acid rain is a result of natural causes, sulfur dioxide from oil and coal combustion and nitrogen oxides produced from automobile engines have greatly intensified the acid rain problem. Winds can carry the pollutants thousands of kilometers away from their source. The British government has recognized that sulfur emissions from power plants in the United Kingdom are contributing to acid deposition in Scandinavia. Canadian emissions contribute substantially to acid rain in the northeastern United States, for example, and much of the sulfur falling in eastern Canada is believed to originate in the United States. In 1986 the U.S. National Academy of Sciences acknowledged that acid rain from U.S. sources had become a serious problem in the eastern United States and Canada. Although the Canadian government has agreed to reduce sulfur dioxide emissions, the United States has not placed limitations on its sulfur emissions that may drift into Canada.
Scientists agree that acid rain is harmful, but reports concerning its severity conflict. A U.S. government report issued in September 1987 minimized the environmental damage caused by acid rain and concluded that the acid-rain problem is not increasing. A 1988 survey conducted by the Environmental Protection Agency, however, indicated that streams in the eastern United States were more acidic than was previously believed. In 1990 the National Acid Precipitation Assessment Program (NAPAP), created by Congress in 1980, issued a report on the results of its study. The report indicated that acidic waters also occur in the southern and midwestern United States but downplayed acid-rain damage to forests.
Many scientists urge that measures to control acid rain begin immediately. The most direct action would be to cut off pollution at the source. Regulations require that new coal-burning plants must install expensive scrubbers in their smokestacks to remove most of the dioxides. Other possible measures include burning only low-sulfur oil or coal, or removing the sulfur from coal with high sulfur content. The 1990 revision of the Clean Air Act focuses on reducing urban smog, acid rain, and emissions. The costs of such measures are considerable, however, and who should pay these costs continues to arouse controversy.