The CAA requires that any area that does not meet the air quality standards for each pollutant be designated nonattainment for that pollutant. Some nonattainment areas, such as many with CO problems, may be localized and have relatively small boundaries. Other nonattainment areas, such as those with ozone problems, may be more regional in nature and have larger boundaries.
Transportation agencies have a vital role in the establishment of nonattainment area boundaries. Commuting and travel patterns can be an important element in setting the boundaries, and transportation agencies are the best source for this information. Currently, EPA is considering an implementation strategy for the revised ozone and particulate matter air quality standards, and will be considering boundaries for both the revised standards in the future.
In the case of ozone, EPA believes it is best to consider a larger area due to the pervasive nature of ground level ozone and the regional transport of ozone. EPA believes this approach will best ensure public health protection from the adverse effects of ozone pollution caused by population density, traffic and commuting patterns, and urban development. The 1990 Amendments to the CAA established the Metropolitan Statistical Area or the Consolidated Metropolitan Statistical Area (C/MSA) as the presumptive boundary for ozone nonattainment areas classified as serious, severe and extreme. Also, EPA has recommended that the C/MSA serve as the presumptive boundary for any new nonattainment areas under the eight-hour ozone and PM2.5 standards.