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

States and tribes submit recommendations to the EPA as to whether or not an area is attaining the national ambient air quality standards for a criteria pollutant. The states and tribes base these recommendations on 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 as attainment or nonattainment for 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 “unclassifiable/attainment”); 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 information.  Those areas are designated "unclassifiable."

Once designations take effect, state and local governments 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.