Criteria Air Pollutants (generally and in US)
The US Environmental Protection Agency (EPA) sets criterion air pollutant measures for the US under the Clean Air Act (see in this dictionary, Clean Air Act) and the National Ambient Air Quality Standards provisions in those regulations (see, NAAQS).
Under the NAAQS, the following six pollutants are identified as criterion air pollutants. The list below shows 7 pollutants because we have separated the PM2.5 and PM10 measures:
(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 (O3; which contributed to the formations of smog);
(5) nitrogen oxides (NO(x), which are associated with smog and global warming);
(6) particle matter -10 (PM10, or particles less than 10 microns in size, an inhalation and lung irritant); and
(7) particle matter -2.5 (PM2.5, particle matter less than 2.5 microns; which is related to respiratory irritation and illness).
For those interested, the legal requirements or emission limits for these pollutants may be found here: Legal Limits.
In the US, controversy surrounding the designation of criteria air pollutants emerged when the US EPA Administrator sought to modify the list of criteria air pollutants to include pollutants that affect climate change. The EPA specifically referred to section 202(a) of the Clean Air Act (see, CAA) as a rationale for including several pollutants that contribute to climate change as criteria air pollutant. Significant controversy ensured in response to this plan, and the EPA adapted its strategy and instead sough to regulate these greenhouse gas pollutants under Clean Air Act provisions that impact vehicle fuel economy (see, Corporate Average Fuel Economy entry in this dictionary). During the process of attempting to regulate greenhouse gas missions in the US, the EPA granted California a waiver from the Clean Air Act to pursue stricter greenhouse gas regulations (see, California Waiver).
Efforts to add carbon dioxide as a criteria air pollutant are ongoing.