An attainment area is a politically or geographically defined region that is in compliance with the National Ambient Air Quality Standards, as set by the Environmental Protection Agency (EPA). A region is considered to be an attainment area if it does not exceed the specified thresholds for air pollutants that may be a threat to public health. The standards are set for a variety of ambient air pollutants, including particulates, ozone, carbon monoxide, sulfur oxides, and nitrogen oxides. A region can be an attainment area for one pollutant and a nonattainment area for another.