Chart 3 - Ambient Carbon Monoxide

When fuel and other substances containing carbon burn without sufficient oxygen, they produce carbon monoxide (CO), a colorless, odorless, and, at high levels, poisonous gas. Although trace amounts of CO occur naturally in the atmosphere, transportation sources account for 79 percent of the nation's total emissions. In cities, automobile exhaust may be responsible for as much as 95 percent of all CO emissions. Industrial processes, non-transportation fuel combustion, and natural sources such as wildfires are other sources of CO emissions.

Average ambient CO concentrations fell seven percent in 1997, and have declined 66 percent since 1976. Ambient CO levels have met the EPA's target "good" range since 1993. It is noteworthy that these reductions occurred despite a 100-percent increase in vehicle miles traveled (VMT) and at sites across all monitoring environments—urban, suburban, and rural.