Acid rain occurs when compounds such as sulfur dioxide and nitrogen oxides are released into the air, causing a chemical reaction. These compounds mix and react with water, oxygen and other chemicals to form acidic pollutants. The compounds mix easily with water, and the wind can carry them great distances.
Traveling sulfur dioxide and nitrogen oxides become part of rain, snow, sleet or fog. Although rain is always slightly acidic because it mixes with naturally-occurring oxides in the air, acid rain can have a pH value of four, and some acid rain has been recorded with a pH of two. Acid rain contributes to the damage of high-elevation trees, such as red spruce, and it causes acidification of lakes and streams. Acid rain contributes to degradation of buildings, and it poses harm to public health when the sulfur dioxide, nitrogen oxides and particulate matter fall to the earth.
Acid rain is a result of air pollution, such as the smoke produced by fires or the fumes that emanate from car exhaust. Power stations and factories that burn fuel produce polluting gases that contribute to the formation of acid rain. Approximately two-thirds of sulfur dioxide and one-quarter of nitrogen oxides in the United States come from electric power generation where fossil fuels, such as coal, are burned.