Ambient Air Quality Standards

In accordance with the CAA, as amended, the EPA has established the NAAQS for criteria pollutants. The NAAQS is based on background studies, including information on health effects, control technology, costs, energy requirements, emission benefits, and environmental impacts.

The pollutants selected as criteria pollutants are sulfur dioxide, particulate matter (now PM10 and previously TSP or total suspended particulates), nitrogen oxides, carbon monoxide, photochemical oxidants (ozone), volatile organic compounds, and lead. The NAAQS represents the maximum allowable concentration of pollutants allowed in the ambient air at reference conditions of 25°C and 760 mm Hg. Table 4.1.1 shows the pollutant levels of the national primary and secondary ambient air quality standards.

States are responsible for ensuring that the NAAQS is met. They can establish statewide or regional ambient air quality standards that are more stringent than the national standards. To achieve and maintain the NAAQS, states develop state implementation plans (SIPs) containing emission standards for specific sources. When an area fails to meet an NAAQS, it is considered a nonattainment area. More stringent control requirements, designed to achieve attainment, must be applied to nonattainment areas.

The 1990 amendments to the CAA (1) require states to submit revised SIPs for nonattainment areas, (2) accelerate attainment timetables, and (3) require federally imposed controls if state nonattainment plans fail to achieve attainment. In addition, the amendments expand the number and types of facilities that are regulated under SIPs.

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