The Clean Air Act requires EPA to set national ambient air quality standards for “criteria pollutants.” Currently, carbon monoxide and five other major pollutants are criteria pollutants. The others are ozone, lead, sulfur oxides, nitrogen oxides, and particulate matter. The law also requires EPA to review the standards periodically and revise them if appropriate to ensure that they provide the requisite amount of health and environmental protection and to update those standards as necessary.
Everywhere in the country has air quality that meets the current CO standards. Most sites have measured concentrations below the national standards since the early 1990s, since which time, improvements in motor vehicle emissions controls have contributed to significant reductions in ambient concentrations.