Air Pollution and Mortality

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Abstract
Because of the possibility of residual confounding, relative risks of the order of 1.3 or less, as reported by Dockery et al. in their study of air pollution and mortality (Dec. 9 issue),1 are difficult to interpret in epidemiologic investigations. As is common in epidemiologic studies, Dockery et al. adjusted for age by stratifying subjects into five-year age groups. Because of the strong influence of age on lung cancer and cardiovascular mortality, this stratification system may not be fine enough to detect small relative risks. For example, the mortality rate from lung cancer among smokers increases roughly in association with the seventh power of age2. The average age of the study subjects from Steubenville, Ohio, was 51.6 years, whereas it was 48.4 in the subjects from Portage, Wisconsin. Among smokers, the risk of lung cancer at the age of 51.6 relative to that at the age of 48.4 is roughly (51.6/48.4)7, or 1.57. A similar computation for nonsmokers (the mortality rate from lung cancer increases roughly in association with the fourth power of age) yields a relative risk of 1.29. Thus, if the average difference in age reported in Table 1 of the article by Dockery et al. results from the Steubenville subjects' being somewhat older than the other subjects in many of the age strata, then the modest “Steubenville effect” reported in the paper may be fully explained by this difference in age alone.

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