Atmospheric pollutants are substances that accumulate in the air to a degree that is harmful to living organisms or to materials exposed to the air. Common air pollutants include smoke, smog, and gases such as carbon monoxide, nitrogen and sulfur oxides, and hydrocarbon fumes. While gaseous pollutants are generally invisible, solid or liquid pollutants in smoke and smog are easily seen. One particularly noxious form of air pollution occurs when oxides of sulfur and nitrogen combine with atmospheric moisture to produce sulfuric and nitric acid. When the acids are brought to Earth in the form of acid rain, damage is inflicted on lakes, rivers, vegetation, buildings, and other objects. Because sulfur and nitrogen oxides can be carried for long distances in the atmosphere before they are removed in precipitation, damage may occur far from pollution sources.

Smoke is an ancient environmental pollutant, but increased...