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The U.S. Environmental Protection Agency (EPA) promulgates the National Ambient Air Quality Standards (NAAQS) on the basis of scientific information contained in criteria documents. The last air quality criteria document for carbon monoxide (CO) was completed by EPA in 1991. This revised criteria document consolidates and updates the current scientific basis for another reevaluation of the NAAQS for CO, currently set at 9 ppm (10 mg/m3) for 8 h and 35 ppm (40 mg/m3) for 1 h. Although emphasis is placed on the presentation of health effects data, other scientific data are presented and evaluated to provide a better understanding of the nature, sources, distribution, measurement, and concentration of CO in the environment, as well as the measurement of population exposure to CO. Recent air quality analyses show that ambient CO concentrations measured at central, fixed-site monitors in metropolitan areas of the United States have decreased significantly since the late 1980s. The decline in ambient CO follows approximately the decline in motor vehicle emissions. A significantly larger portion of an individual's total CO exposure may come from tobacco smoke, from unvented or partially vented combustion appliances indoors, and from uncontrolled outdoor sources. Health assessment provided in this document supports and substantiates the conclusions drawn in the previous document. Young, healthy individuals are not at immediate risk from ambient CO exposure because only limitations at maximal exercise performance have been demonstrated and not at submaximal exercise levels that are more germane to typical human activity. The greater concern of low-level ambient exposure to CO has, therefore, focused on subpopulations who are particularly susceptible to CO, especially the subpopulation with coronary artery disease.
U.S. EPA. AIR QUALITY CRITERIA CARBON MONOXIDE, EXTERNAL REVIEW DRAFT. U.S. Environmental Protection Agency, Washington, DC, EPA 600/P-99/001A, 1999.