Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes. Nationally—and particularly in urban areas—the majority of CO emissions to ambient air come from mobile sources. CO can cause harmful health effects by reducing oxygen delivery to the body's organs (like the heart and brain) and tissues. At extremely high levels, CO can cause death.
EPA first set air quality standards for CO in 1971. For the protection of both public health and welfare, EPA set an eight-hour primary standard at 9 parts per million (ppm) and a one-hour primary standard at 35 ppm. In a review of the standards completed in 1985, EPA revoked the secondary standards (for public welfare) due to a lack of evidence of adverse effects on public welfare at or near ambient concentrations. The last review of the CO NAAQS was completed in 1994 and the Agency chose not to revise the standards at that time.
The air quality in the United States meets the current CO standards. Most sites have measured concentrations below the national standards since the early 1990s, and since then, improvements in motor vehicle emissions controls have contributed to significant reductions in ambient concentrations.