A geographic area in which levels of a criteria air pollutant meet the health-based primary standard (national ambient air quality standard, or NAAQS) for the pollutant.. It is important to note that, because there are several pollutants, an area can be in attainment for one pollutant standard while still being in non-attainment for others.
Source: U.S. Environmental Protection Agency. 2013. Terminology Service Terms of Environment. Accessed on 9 September 2013. Available at http://iaspub.epa.gov