Title I of the Clean Air Act requires that after we set a new National Ambient Air Quality Standard (NAAQSNAAQSStandards established by EPA for maximum allowable concentrations of six "criteria" pollutants in outdoor air. The six pollutants are carbon monoxide, lead, ground-level ozone, nitrogen dioxide, particulate matter, and sulfur dioxide. The standards are set at a level that protects public health with an adequate margin of safety.) or we revise an existing standard, we must then designate areas in the United States as being in “attainment” or “nonattainment” with the standard. 

This website provides information on the process EPA follows to designate areas as being in attainment or nonattainment with the standards for particle pollutionparticle pollutionFine liquid or solids such as dust, smoke, fumes, or smog found in air or emissions. leadA heavy metal that is hazardous to health if breathed or swallowed. Its use in gasoline, paints, and plumbing compounds has been sharply restricted or eliminated by federal laws and regulations.established in 2012.