Do respiratory epidemics confound the association between air pollution and daily deaths?

Daily deaths are associated with air pollution. This association might be con*hhy;founded by uncontrolled risk factors. In order to estimate the potential confounding caused by respiratory epidemics of the association between air pollution and health effects, a time series study of air pollution and daily deaths was carried out. Daily records of deaths for all ages were obtained from five US cities: Chicago, IL; Detroit, MI; Minneapolis, MN; Pittsburgh, PA; and Seattle, WA. Daily levels of particles with a 50% cut-off aerodynamic diameter of 10 microm (PM10) and weather measurements were obtained. City-specific analysis was carried out using Poisson regression, adjusting for time trend, ambient temperature, dew point, barometric pressure and day of the week. A cubic polynomial was used for each epidemic period (> or =10 days of excessive pneumonia hospital admissions), and a dummy variable was used to control for isolated epidemic days. A 10-microg x m(-3) increase in PM10 concentration (lag 0-1) was associated with increased daily deaths in Chicago (0.81%, 95% confidence internal (CI) 0.54-1.09); Detroit (0.87%, 95% CI 0.60-1.15), Minneapolis (1.34%, 95% CI 0.78-1.90), Pittsburgh (0.84%, 95% CI 0.51-1.18) and Seattle (0.52%, 95% CI 0.11-0.94). When controlling for respiratory epidemics, small decreases in the PMlo effect were observed (Chicago 9%, Detroit 11%, Minneapolis 3%, Pittsburgh 5%, and Seattle 15%). The overall effect of PM10 concentration was 0.85% (95% CI 0.60-1.10) per 10 microg x m(-3) before controlling for epidemics and 0.78% (95% CI 0.51-1.05) after. This study showed that the association between air pollution and daily deaths is not due to failure to control for influenza or pneumonia epidemics.