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This revised criteria document consolidates and updates the current scientific basis for another reevaluation of the National Ambient Air Quality Standards for carbon monoxide (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 when the last scientific review was completed. 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 uncontrolled indoor and 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. The greater concern has focused on subpopulations who are particularly susceptible to CO, especially the subpopulation with coronary artery disease.
U.S. EPA. Air Quality Criteria for Carbon Monoxide (1999) (Second External Review Draft). U.S. Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, Washington Office, Washington, DC, EPA/600/P-99/001B, 1999.