Acid rain is a term for rain, snow, sleet, or other wet precipitation that is polluted by such acids as sulfuric acid and nitric acid. Acid rain harms thousands of lakes, rivers, and streams worldwide, killing fish and other wildlife. It also damages buildings, bridges, and statues. High concentrations of acid rain can harm forests and soil.

Acid rain forms when water vapor in the air reacts with certain chemical compounds. These compounds, including sulfur dioxide and nitrogen oxides, come largely from the burning of coal, gasoline, and oil. Most automobiles, factories, and power plants burn such fuels for energy. Regions affected by acid rain include large parts of eastern North America, Scandinavia and central Europe, and parts of Asia. Since about the 1950s, the problem has increased in rural areas. This has occurred because the use of taller smokestacks in urban areas has enabled the winds to transport pollutants farther from their sources.

Scientists and engineers have developed ways to reduce the acidity of rain. For example, several kinds of devices remove sulfur and nitrogen compounds from fuels or industrial emissions before they reach the atmosphere. Adding lime to lakes and rivers and their drainage areas temporarily neutralizes their acidity. But the neutralization may have harmful side effects.

In 1990, the United States Congress amended the Clean Air Act of 1970 to reduce acid rain in the United States and Canada. The amendments tightened standards for emissions, required fuels that burn more cleanly, and called for power plants to cut their sulfur dioxide emissions.