Volatile organic carbons (VOC), sulfuric dioxide (SO2), and nitrogen oxides (NOx) are produced both naturally and anthropogenically (by people). Natural sources, including volcanoes and the decomposition of plants, contribute to the natural acidity of rain water (pH 5.6 -- a pH of 7 is neutral.). Anthropogenic sources include automobile exhaust and electric power plants, especially those that burn coal. These man-made sources result in precipitation that in 2001 had an average pH of 4.5 in the Adirondacks. Because the pH scale is logarithmic, a drop of one on the pH scale equals a ten-fold change in acidity. Therefore, rain and other precipitation in the Adirondack region in 2001 was over ten times more acidic than natural precipitation.

Pollutants that cause acid deposition enter the atmosphere both as gas and as tiny particles. These gases and particles can fall from the air as "dry deposition," or combine with water in the air to form acid rain, fog, or snow (wet deposition).

Both wet and dry deposition can do harm where they fall, both to living things, such as fish and forests, and to objects such as gravestones. Acid deposition also has indirect effects. When a lake or stream becomes too acidic for fish, the number of mosquitoes and black flies, which are not as sensitive to acidity, increases. When metal structures corrode, such as equipment in water treatment plants, human communities face additional economic burden.