The key to national air pollution control policy since the passage of the Clean Air Act amendments of 1970 are the National Ambient Air Quality Standards (NAAQSs). Under this law, the Environmental Protection Agency (EPA) had to establish NAAQSs for six of the most common air pollutants, sometimes referred to as criteria pollutants, by April 1971. Included are carbon monoxide, lead (added in 1977), nitrogen oxides, ozone, particulate matter, and sulfur dioxide. Hydrocarbons originally appeared on the list of pollutants, but were removed in 1978 since they were adequately regulated through the ozone standard. The provisions of the law allow the EPA to identify additional substances as pollutants and add them to the list.

For each of these pollutants, primary and secondary standards are set. The primary standards are designed to protect human health. Secondary standards are to protect crops, forests, and buildings if...