the cases of major stationary sources and area sources by assessment of residual risk. The responsibility for setting the standards for both types of pollutants is assigned to the EPA administrator.

In addition to the programs to control criteria pollutants and HAPs, the CAA includes provisions to control emissions from mobile sources, protect areas with good air quality, reduce the effects of acid deposition (or acid rain), safeguard stratospheric ozone (O3), and reduce visibility impairment resulting from regional haze. However, the tenor of these provisions is substantially different from those relating to the programs for criteria pollutants and HAPs. Although the CAA directs the EPA administrator to set air quality and emission standards for criteria pollutants and HAPs, it is more explicit about setting standards for the other provisions, with Congress itself often setting the standards. For example, the CAA now includes specific standards for evaporative and exhaust emissions from light-duty and heavy-duty on-road vehicles and engines. Controls are also required for a wide range of nonroad engines (such as lawnmowers, construction equipment, and locomotives), and programs are mandated for clean fuel and inspection and maintenance of light-duty vehicles. Similarly, in the case of acid rain mitigation, the CAA Amendments of 1990 contain language that establishes a specific nationwide cap for SO2 emissions and standards for nitrogen oxides (NOx) emissions from electric utilities.


Criteria Pollutants

Criteria pollutants were first defined in the 1970 Amendments to the CAA, which directed the administrator of EPA to identify those widespread ambient air pollutants that are reasonably expected to present a danger to public health or welfare.2 On the basis of air quality criteria3—that is, the current state of scientific knowledge on the effects of these pollutants on health and welfare—the administrator is directed to develop and promulgate primary and secondary NAAQS for each criteria air pollutant. In addition to specifying a maximum ambient concentration for each pollutant, promulgation of a standard must also include descriptions of the moni-


Within the framework of the CAA, “welfare” refers to the viability of agriculture and ecosystems (such as forests and wildlands), the protection of materials (such as monuments and buildings), and the maintenance of visibility (EPA 2002d).


Air quality criteria are defined in Section 108 of the CAA as a summary of the “latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of such pollutant in the ambient air.”

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