The US National Ambient Air Quality Standards (NAAQS) are the
centerpiece of the US Clean Air Act (CAA) and establish allowable
concentration levels for six "criteria air pollutants":
ozone, particulate matter, lead, carbon monoxide, nitrogen dioxide,
and sulfur dioxide. The CAA requires the US Environmental
Protection Agency (EPA) to review and, as appropriate, revise the
NAAQS at least every five years, and EPA has, since 1970, regularly
adopted increasingly stringent standards. Whether those revisions
have gone far enough or too far has become a predictably
contentious issue, with each review involving debates over science,
the role of EPA's Clean Air Science Advisory Committee (CASAC),
the discretion of the EPA Administrator, and the format of the
review process itself, among many other issues.
