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…