The Clean Air Act is the main force behind the control of air pollution in the United States. The Act was originally passed in 1963, but large and important amendments were added in both 1970 and 1990.  The Act enforces a comprehensive program for reducing air pollution.

As passed by the U.S. Congress in 1963, the Clean Air Act was a moderate bill that offered federal research aid, urged the development of state control agencies, and involved the federal government in inter-state pollution issues. In 1965, an amendment was added to the bill requiring the U.S. Department of Health, Education and Services to create and enforce auto emission standards.  This marked the federal government's first active role in clean air policy, though it was the Clean Air Act of 1970 that, for the first time, put real power in the hands of the federal government instead of the states. 

This 1970 law remains the basis for air pollution control policy.  It has four major components.  First, it put into place National Ambient Air Quality Standards. Targeted at major polluting chemicals, such standards were intended to protect human health as well as the environment. 

These standards were to be developed by the Environmental Protection Agency. Second, the EPA was to establish New Source Performance Standards to determine how much pollution should be allowed by different industries in different regions.  Third, the Act specified standards for controlling auto emissions with the aim of reducing various gases by almost 90 percent.  Finally, the law encouraged states to develop plans to achieve such standards and then required that state plans be approved by the EPA.  If a state chose not to form such a plan or did not complete it by a specified date, the EPA would take over the administration of the law for that state.  The states were also required to enforce the Clean Air Act.

In 1977, more amendments were added to the Act; these dealt with states that were not achieving national objectives, with auto emissions, and with measures to prevent air quality deterioration in areas where the air had previously been clean. The Clean Air Act was last amended in 1990.  This time the additions addressed acid rain, toxic pollutants, areas still not at regulation standards, and ozone layer depletion.  Under the Act, massive decreases in certain gas emissions were mandated in order to control acid rain; toxic pollutants were to be regulated even more; deadlines were set for the noncompliant areas; and three major chemical contributors to ozone layer depletion were phased out.