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Carbon Monoxide (CO)

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, poisonous gas formed when carbon in fuels is not burned completely. It is a byproduct of highway vehicle exhaust, which contributes about 60 percent of all CO emissions nationwide. In cities, automobile exhaust can cause as much as 95 percent of all CO emissions. These emissions can result in high concentrations of CO, particularly in local areas with heavy traffic congestion. Other sources of CO emissions include industrial processes and fuel combustion in sources such as boilers and incinerators. Despite an overall downward trend in concentrations and emissions of CO, some metropolitan areas still experience high levels of CO.

Health and Environmental Effects:
Carbon monoxide enters the bloodstream and reduces oxygen delivery to the body's organs and tissues. The health threat from exposure to CO is most serious for those who suffer from cardiovascular disease. Healthy individuals are also affected, but only at higher levels of exposure. Exposure to elevated CO levels is associated with visual impairment, reduced work capacity, reduced manual dexterity, poor learning ability, and difficulty in performing complex tasks. EPA's health-based national air quality standard for CO is 9 parts per million (ppm) measured as an annual second-maximum 8-hour average concentration.

Trends in Carbon Monoxide Levels:
Long-term improvements continued between 1986 and 1995. National average CO concentrations decreased 37 percent while CO emissions decreased 16 percent. Long-term air quality improvement in CO occurred despite a 31 percent increase in vehicle miles traveled in the U.S. during the past 10 years. Between 1994 and 1995, national average CO concentrations decreased 10 percent, while total CO emissions decreased 7 percent. Transportation sources (includes highway and off-highway vehicles) now account for 81 percent of national total CO emissions.

CO Concentrations
CO Emissions

This document is provided for historical purposes only. The most recent version can be found at AIRTrends

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