Note: EPA no longer updates this information, but it may be useful as a reference or resource.
Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.
Nature and Sources of the Pollutant: Carbon monoxide is a colorless, odorless, and at higher levels, a poisonous gas formed when carbon in fuel is not burned completely. It is a product of motor vehicle exhaust, which contributes about 60 percent of all CO emissions nationwide. High concentrations of CO generally occur in areas with heavy traffic congestion. In cities, as much as 95 percent of all CO emissions may emanate from automobile exhaust. Other sources of CO emissions include industrial processes, non-transportation fuel combustion, and natural sources such as wildfires. Peak CO concentrations typically occur during the colder months of the year when CO automotive emissions are greater and nighttime inversion conditions (where air pollutants are trapped near the ground beneath a layer of warm air) are more frequent.
Health and Environmental Effects: Carbon monoxide enters the bloodstream through the lungs and reduces oxygen delivery to the body's organs and tissues. The health threat from CO is most serious for those who suffer from cardiovascular disease. At higher levels of exposure, healthy individuals are also affected. Visual impairment, reduced work capacity, reduced manual dexterity, poor learning ability, and difficulty in performing complex tasks are all associated with exposure to elevated CO levels.
Trends in CO Levels: Long-term improvements continued between 1987 and 1996. Ambient CO concentrations decreased 37 percent, and the estimated number of accedences of the 8-hour standard decreased 92 percent. In addition, CO emissions decreased 18 percent, and CO emissions from highway vehicles decreased 26 percent. Long-term air quality improvement in CO occurred despite a 28 percent increase in vehicle miles traveled in the U.S. during this 10-year period. Between 1996 and 1996, ambient CO concentrations decreased 7 percent, while CO emissions decreased 1 percent. Transportation sources (includes highway and off-highway vehicles) now account for 79 percent of national total CO emissions.