What information does EPA use to calculate nonattainment?
The Clean Air Act defines a nonattainment area as an area that is violating the national ambient air quality standard or contributing to a violation of the standard in a nearby area. The 8-hour ozone standard is based on averaging air quality measurements over 8-hour blocks of time. EPA uses the average of the annual 4th highest 8-hour daily maximum concentrations from each of the last three years of air quality monitoring data to determine a violation of the ozone standard. In addition to air quality data, EPA also considers other key factors, including emissions, traffic and commuting patterns, population density, and expected growth as to whether or not additional counties should be included in the consolidated/metropolitan statistical area (C/MSA). The C/MSA is the presumptive nonattainment default boundary for areas classified serious and above as defined in the Clean Air Act.
