Designation of an area as an attainment, nonattainment, or unclassifiable area is made by the EPA administrator in consultation with the relevant state governors and is based on data gathered from the NAMS/SLAMS air quality monitoring networks (which are discussed in Chapter 6). The designation process engenders two fundamental assumptions: (1) that attainment and maintenance of a NAAQS can be established using data from a limited number of surface air quality monitoring sites, and (2) that geographical areas can be effectively regulated with a single, uniform designation and a common plan for achieving and/or maintaining an acceptable level of air quality.

An area’s attainment or nonattainment status with regard to a criteria pollutant is determined by comparing the NAAQS with the area’s “design value”^a for the pollutant. The design value is derived from air quality monitoring data gathered by local or state authorities following guidelines specified by EPA. If the design value exceeds the NAAQS, the area is designated as being in nonattainment. For CO and O₃, an additional, more detailed classification scheme is used to indicate the severity of the nonattainment (see Table 3-1). Because the statistical form of the standard varies from one pollutant to another, the method used to determine the design value also varies by pollutant. For example, in the case of the old 1-hr O₃ standard, the design value is calculated as the fourth highest 1-hr averaged concentration observed at any monitoring site in the area over three consecutive years. The new 8-hr O₃ design value, on the other hand, is defined as the 3-year average of the annual fourth highest 8-hr averaged concentration observed each year. The new 24-hr averaged PM_2.5 design value is derived from the 3-year average of the 98th percentile measured concentrations in an area. The annual PM_2.5 design value is obtained from an average of all measurements made in the area. These various protocols involving multiyear and spatial averages and selecting concentrations below the absolute maximum observed have been chosen to limit the impact of statistical outliers and extreme and anomalous meteorological events. Nevertheless, as discussed elsewhere, the use of design values in the attainment demonstration of the SIP presents important challenges.

In addition to providing a mechanism for identifying and designating nonattainment areas, the CAA (in Section 175(a)) also provides a process by which a nonattainment area can be redesignated an attainment area. To be redesignated, an area must file a “maintenance SIP.” In addition to showing that the area is in compliance with the NAAQS on the basis of relevant air quality monitoring data, the maintenance SIP must provide a plan for ensuring that the standard will not be violated in the future. Once such a plan is approved by EPA, the area is generally referred to as a maintenance area. EPA currently lists 62, 59, 30, and 16 maintenance areas for O₃, CO, SO₂, and PM₁₀ respectively. There are a number of other nonattainment areas that have demonstrated achievement of the standards, but either because they have not completed the process of filing a maintenance SIP or because EPA has not yet accepted it, they have not been redesignated.