Formed when sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions from the burning of fossil fuels (primarily from coal-burning utility plants, automobiles and trucks) mix with water vapor in the atmosphere. The acidified water then falls to the Earth as rain or snow, where it can damage trees at high altitudes, leach nutrients from soils, damage buildings and public sculptures and, most notably, acidify lakes, where, in sufficient concentrations, it can kill off all aquatic life. The Northeastern United States, particularly the Adirondack mountains in northern New York State, have suffered the greatest harm from acid rain.

Acid rain first became the focus of scientific and political attention in the 1980s, and the Clean Air Act Amendments of 1990 contained the first legislative attempt to deal with the problem. The law set a cap on sulfur dioxide emissions from power plants and reduced the amount of nitrogen oxides that could be emitted per unit of energy.