Today's air quality standards have emerged from sections 109 and 112 of the 1970 Clean Air Act (CAA) Amendments and Title III of the 1990 CAA Amendments.


The 1970 CAA Amendments define two primary types of air pollutants for regulation: criteria air pollutants and hazardous air pollutants. Under section 108, criteria pollutants are defined as those that "cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare . . . the presence of which in the ambient air results from numerous or diverse mobile or stationary sources." Under section 109, the EPA identifies pollutants that meet this definition and prescribes national primary air quality standards, "the attainment and maintenance of which . . . allowing an adequate margin of safety, are requisite to protect the public health."

National secondary air quality standards are also prescribed, "the attainment and maintenance of which ... is requisite to protect the public welfare from any known or anticipated effects associated with the presence of the air pollutant." Welfare effects include injury to agricultural crops and livestock, damage to and the deterioration of property, and hazards to air and ground transportation. The National Ambient Air Quality Standards (NAAQS) are to be attained and maintained by regulating stationary and mobile sources of the pollutants or their precursors.


Under section 112, the 1970 amendments also require regulation of hazardous air pollutants. A hazardous air pollutant is defined as one "to which no ambient air standard is applicable and that . . . causes, or contributes to, air pollution which may reasonably be anticipated to result in an increase in serious irreversible, or incapacitating reversible, illness." The EPA must list substances that meet the definition of hazardous air pollutants and publish national emission standards for these pollutants providing "an ample margin of safety to protect the public health from such hazardous air pollutant[s]." Congress has provided little additional guidance, but identified mercury, beryllium, and asbestos as pollutants of concern.


Although the control of criteria air pollutants is generally considered a success, the program for hazardous air pollutants was not. By 1990, the EPA regulated only seven of the hundreds of compounds believed to meet the definition of hazardous air pollutants.

Title III of the 1990 CAA Amendments completely restructured section 112 to establish an aggressive new program to regulate hazardous air pollution. Specific programs have been established to control major-source and area-source emissions. Title III establishes a statutory list of 189 substances that are designated as hazardous air pollutants. The EPA must list all categories of major sources and area sources for each listed pollutant, promulgate standards requiring installation of the maximum achievable control technology (MACT) at all new and existing major sources in accordance with a statutory schedule, and establish standards to protect the public health with an ample margin of safety from any residual risks remaining after MACT technology is applied.

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