When fossil fuels-oil, coal, and gas-are burned in industrial plants or automobile engines, large amounts of sulfur oxides and nitrogen oxides are released into the air. One industrial smokestack may produce as much as 500 metric tons of sulfur oxides each day. Most of it is sulfur dioxide, which becomes sulfuric acid in moist air.

Nitrogen oxides and carbon particles produced by automobiles and other motor vehicles are so plentiful in some large cities that the air is colored by them. Nitrogen oxides also enter the air from the breakdown of agricultural fertilizers. One of the nitrogen oxides becomes nitric acid corrodes many metals, and strong concentrations of nitric acid can cause burns on skin.

Millions of metric tons of sulfuric acid and nitric acid fall in rain on the United States and Canada each year. The concentration of these acids are too weak to cause burns, but they do produce other severe effects. Other acids also occur but in smaller amounts. Hydrochloric acid often comes directly from smokestacks. Carbon monoxide and carbon dioxide are produced by automobiles. These become carbonic acid.

All the acids in the air have changed the normal pH of rain from 5.6 to an average of 4.5 over the entire eastern Unites States and Canada. A pH of 4.5 is more than ten times as acidic as a pH of 5.6. Some areas have received acid rain with a pH of 3.0, which is as acidic as vinegar.

Certain heavy metals-cadmium, lead, and mercury-are also present in acid rain. These poisonous metals, together with compounds called hydrocarbons, are produced when fossil fuels are burned. They are found in air samples taken everywhere in North America and Europe.

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