Acid rain consists of any form of precipitation (rain, snow, fog) which has an unnaturally low (acidic) pH level. The acid consists primarily of sulfuric and nitric acids which are produced from the reaction of water molecules in the air with SO2 (sulfur dioxide) and NOx (nitrogen oxides). Acid deposition refers to the actual acids which are dropped (deposited) on surfaces by precipitation.

Coal-burning energy plants and internal combustion engines are widely considered to be the primary causes of acid rain. Harmful effects of acid rain include respiratory disorders in humans and other animals, damage to vegetation, acidification of ground water resulting in changes in biological balance, corrosion of buildings and monuments, and in some cases accelerated wear to vehicle surfaces and exposed parts.

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