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The Clean Air Act (CAA) of 1970, and its subsequent amendments, is a complex, comprehensive law that recognizes the existence of serious air pollution, establishes standards to protect public health and the environment, and authorizes the U.S. Environmental Protection Agency (EPA) to set binding national standards for common and widespread outdoor air pollutants (also called criteria pollutants).

The goal of the Act was to set and achieve National Ambient Air Quality Standards (NAAQS) in every state by 1975. The setting of maximum pollutant standards was coupled with directing the states to develop state implementation plans (SIP's) applicable to appropriate industrial sources in the state.

The Act was amended in 1977 primarily to set new goals (dates) for achieving attainment of NAAQS since many areas of the country had failed to meet the deadlines. The 1990 amendments to the Clean Air Act in large part were intended to meet unaddressed or insufficiently addressed problems, such as air toxics, acid rain, ground-level ozone, and stratospheric ozone depletion.

The 1990 amendments tightened pollution control requirements in cities that had not reached attainment, mandated a schedule for control technology for all major toxic air emitters, and listed 189 substances (hazardous air pollutants [HAP's] -- also called air toxics) subject to special controls. The EPA can add other pollutants that may present a threat of adverse health effects or environmental effects, but they cannot be listed as hazardous unless they meet certain conditions.

The 1990 amendments also required reductions in acid rain emissions, tighter auto emissions standards, mandated cleaner gasoline and clean-fueled vehicles in the nation's most polluted cities, and phased out production of chemicals that contribute to depletion of the stratospheric ozone layer.