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Populations at Risk from Air Pollution -- United States, 1991

The Clean Air Act of 1970 required the U.S. Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQSs) for six of the most widespread air pollutants in the outdoor environment: particulate matter with a diameter less than or equal to 10 um, sulfur dioxide, nitrogen dioxide, carbon monoxide, ozone, and lead. Congress directed EPA to establish these national air quality standards with an adequate margin of safety to account for limitations in scientific information on the health effects of these pollutants, with particular concern for those populations at potentially higher risk for adverse outcomes from exposure to these pollutants. This report provides estimates of the number of persons who may have been at risk for exposure to unhealthy levels of these pollutants during 1991.

For this report, a population at risk was defined as having a "significantly higher probability of developing a condition, illness, or other abnormal status," as described by EPA (1). For example, elderly persons may not be particularly sensitive to the effects of sulfur dioxide pollution but are considered to be at risk because lowered respiratory function may reduce their ability to withstand the additional reduction in respiratory function caused by exposure to sulfur dioxide. When developing the NAAQSs, EPA identified specific at-risk population subgroups for each pollutant (Table 1). Communities that fail to meet the NAAQS for one or more of these six pollutants are considered nonattainment areas.

A projection of the prevalence of populations at risk was derived by the synthetic estimation technique developed by the Bureau of the Census (2). Age-specific national prevalence rates for the medical conditions under study were obtained from CDC's National Health Interview Survey (NHIS) (3). These prevalence figures were multiplied by the 1990 U.S. census population figures, enumerated for each nonattainment community on an age-specific basis (4). Expected estimates of the local prevalence of chronic diseases reported in the NHIS were scaled in direct proportion to the base population of the area and its age distribution and then used to compute the national figures. No adjustments were made for other factors that might affect local prevalence because such data are rarely available on a county level. Estimates of populations at risk should be quoted individually and should not be added together because the populations at risk are not mutually exclusive (e.g., persons may be aged greater than or equal to 65 years and have chronic obstructive pulmonary disease).

During 1991, a total of 514 counties and 20 cities were designated as nonattainment areas (5; EPA, unpublished data, 1991), representing an estimated 164 million persons (66% of the resident U.S. population) (6). Persons residing in nonattainment counties included 63% (approximately 31 million) of preadolescent (aged less than or equal to 13 years) children, 60% (approximately 19 million) of persons aged greater than or equal to 65 years, and 64% (approximately 9 million) of persons with chronic obstructive pulmonary disease (Table 1).

Reported by: P Paris, S Rappaport, MPH, K Lieber, MPH, Epidemiology and Statistics Unit, R White, MST, American Lung Association, New York. Air Pollution and Respiratory Health Br, Div of Environmental Hazards and Health Effects, National Center for Environmental Health, CDC.

Editorial Note

Editorial Note: The findings in this report assist in quantifying the potential public health impact of air pollution, particularly among persons at increased risk. This risk is greatest for persons residing in nonattainment areas, including more than 60% of U.S. children and adults with asthma and persons with chronic bronchitis and emphysema. In addition, this risk is higher for more than one third of pregnant women and persons with coronary heart disease.

This report does not include estimates for healthy persons who exercise, an important group at risk for the health effects of ozone. Although numerous clinical studies indicate that persons who exercise vigorously are at increased risk for acute effects from exposure to ozone at levels above the national ambient standard (7), the population in this report determined to be at risk from ozone was limited to groups with inherent physiologic risk factors. Thus, the estimates of at-risk populations in ozone nonattainment areas may substantially underestimate the number of persons potentially exposed to unhealthy ozone levels.

A national health objective for the year 2000 is to increase the proportion of persons who live in counties that have not exceeded any air quality standard during the previous 12 months from 49.7% in 1988 to 85.0% (objective 11.5) (8). The description and estimates of high-risk populations presented in this report underscore the importance of improved air quality for disease prevention and health promotion. In addition, because an estimated 61% of children with asthma and 65% of adults with asthma reside in communities with potentially unhealthy air quality, efforts to improve air quality should assist in achieving the national health objective for reducing morbidity associated with asthma.

Estimates of populations at risk presented in this report were derived by applying national prevalence estimates to county-specific population figures. These estimates could be refined through the collection and analysis of data for at-risk populations by state and local health agencies. In addition, a recent survey of air pollution information activities at state and local air pollution control agencies (9) indicated the need for additional education methods for air pollution control and improved coordination between health and environmental agencies.

On May 4, the ALA will release a new report, Breath in Danger II (6), that estimates the populations at risk from air pollution exposure in the United States, as well as the results of a survey of U.S. attitudes on air pollution. These reports are available from the American Lung Association, 1740 Broadway, New York, NY 10019-4374; telephone (212) 315-8700.


  1. US Environmental Protection Agency. Air quality criteria document for lead. Washington, DC: US Environmental Protection Agency, 1977.

  2. Irwin R. Guide for local area population projections. Washington, DC: US Department of Commerce, Bureau of the Census, 1972; technical paper no. 39.

  3. NCHS. Current estimates from the National Health Interview Survey, 1990. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, CDC, 1991; DHHS publication no. (PHS)92-1509. (Vital and health statistics; series 10, no. 181).

  4. Bureau of the Census. Age-specific population of U.S. counties. Washington, DC: US Department of Commerce, Bureau of the Census, 1990. (Current population reports).

  5. US Environmental Protection Agency. Ozone and carbon monoxide areas designated nonattainment. Research Triangle Park, North Carolina: US Environmental Protection Agency, Office of Air Quality Planning and Standards, 1991.

  6. American Lung Association. Breath in danger II: estimation of populations-at-risk of adverse health consequences in areas not in attainment with National Ambient Air Quality Standards of the Clean Air Act. New York: American Lung Association, 1993.

  7. Lippmann M. Health effects of ozone: a critical review. Journal of the Air Pollution Control Association 1989;39:672-94.

  8. Public Health Service. Healthy people 2000: national health promotion and disease prevention objectives. Washington, DC: US Department of Health and Human Services, Public Health Service, 1991; DHHS publication no. (PHS)91-50213.

  9. CDC. Air pollution information activities at state and local agencies -- United States, 1992. MMWR 1993;41:967-9.

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