above these cutoff values were derived from 1991 census figures for each county (U.S. Bureau of the Census, unpublished data, 1992).

For this report, a population at risk was defined as persons who have a “significantly higher probability of developing a condition, illness, or other abnormal status,” as described by EPA (4). Five at-risk populations were included: preadolescent children (aged ≤13 years), the elderly (aged ≥65 years), persons aged <18 years with asthma, adults (aged ≥18 years) with asthma, and persons with chronic obstructive pulmonary disease (COPD) (e.g., chronic bronchitis and emphysema). Age-specific county populations for 1991 were estimated by applying the population age distribution of each state (U.S. Bureau of the Census, unpublished data, 1992) to the counties within that state. The number of persons with asthma or COPD in each county was estimated by applying age-specific prevalences from CDC’s National Health Interview Survey (5) to age-specific population estimates for each county. Although PM₁₀ levels are presented on a county basis, they do not indicate that all areas of the county were subject to that level or that all persons in the county were exposed to the recorded concentration.

During 1992, PM₁₀ levels were ≥155 µg/m³ in 16 counties; an estimated 23 million persons (9.1% of the total U.S. population) resided in these counties (Table 1). Approximately 92 million additional persons (36% of the U.S. population) resided in counties in which PM₁₀ levels were 55 µg/m³−154 µg/m³. Overall, an estimated 115 million persons (45% of the U.S. population) resided in counties with PM₁₀ levels ≥55 µg/m³ (Table 1). In the United States during 1992, 46% of persons with asthma lived in communities with levels of particulate air pollution higher than the California standard.