After EPA sets a new National Ambient Air Quality Standard or revises an existing standard for a criteria air pollutant, the Clean Air Act requires EPA to determine if areas of the country meet the new standards. 

Within one year of setting a new or revised national ambient air quality standard for a criteria pollutant, States and tribes submit recommendations to the EPA as to whether or not an area is attaining the standard. The states and tribes base these recommendations on available air quality data collected from monitors at locations in urban and rural settings as well as other information characterizing air quality such as modeling. After working with the states and tribes and considering the information from air quality monitors, and/or models, EPA will "designate" an area based on whether or not it is meeting the standard. 

If the air quality in a geographic area meets or is cleaner than the national standard, it is called an attainment area (designated “attainment/unclassifiable”); areas that don't meet the national standard are called nonattainment areas.  In some cases, EPA is not able to determine an area's status after evaluating the available informationand those areas are designated "unclassifiable."

Once designations take effect, state and local governments with nonattainment areas must develop implementation plans outlining how areas will attain and maintain the standards by reducing air pollutant emissions. Tribes may elect to develop tribal implementation plans but are not required to do so.