National Ambient Air Quality Standards (NAAQS) (US)
Under the authority of the Clean Air Act and NAAQS, the EPA created national air quality standards for what are known as the seven “criteria” air pollutants. These pollutants include: (1) sulfur dioxide (SO2; which contributes to acid rain); (2) lead (Pb; lead exposure is associated with numerous adverse health consequences including brain dysfunctions and other central nervous system and behavioral disruptions); (3) carbon monoxide (CO; which contributes to the formation of smog), (4) ozone (O-3; which contributed to the formations of smog); (5) nitrogen oxides (NO(x), which are associated with smog and global warming); (6) particle matter -10 (or PM-10, or particles less than 10 microns in size, an inhalation and lung irritant); and (7) particle matter -2.5 (PM-2.5, particle matter less than 2.5 microns; which is related to respiratory irritation and illness).
Under NAAQS, the EPA sets general air pollution standards. However, air quality across regions of the US varies, and in some locations requires stricter regulations in order to protect public health and safety. To reflect these different needs, NAAQS standards may vary by air quality control regions (AQCRs; see, AQCRs in the law). There are currently 250 different AQCRs in the United States (US AQCRs).
NAAQS regulates existing sources of air pollution using “reasonably available control technology” (RACT; Clean Air Act § 172), which is the lowest level of pollution emission attainable through the application of reasonable and cost effective technology.
An area that meets the requires of the NAAQS is called an attainment area since it has satisfied national air quality standards. An area that fails to meet to those standards is called a nonattainment area. The US EPA may require nonattainment areas to submit plans for bringing any portion of the nonattainment area into attainment. A list of nonattainment areas in the US can be found here.