"Under provisions of the Clean Air Act, which is intended to improve the quality of the air we breathe, EPA is required to set National Ambient Air Quality Standards for six common air pollutants. These commonly found air pollutants (also known as "criteria pollutants") are found all over the United States. They are particle pollution (often referred to as particulate matter), ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. For each criteria pollutant, there are specific procedures used for measuring ambient concentrations and for calculating long-term (quarterly or annual) and/or short-term (24-hour) exposure levels. The methods and allowable concentrations vary from one pollutant to another, and within NAAQS revisions for each pollutant. These pollutants can harm your health and the environment, and cause property damage. Of the six pollutants, particle pollution and ground-level ozone are the most widespread health threats. EPA calls these pollutants "criteria" air pollutants because it regulates them by developing human health-based and/or environmentally-based criteria (science-based guidelines) for setting permissible levels. The set of limits based on human health is called primary standards. Another set of limits intended to prevent environmental and property damage is called secondary standards. A geographic area that meets or does better than the primary standard is called an attainment area; areas that don't meet the primary standard are called nonattainment areas. "
"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."
References and Further Reading
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