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

Daily deaths are associated with air pollution. This association might be c on*hhy;founded by uncontrolled risk factors. In order to estimate the poten tial 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: Chi cago, IL; Detroit, MI; Minneapolis, MN; Pittsburgh, PA; and Seattle, WA. Da ily levels of particles with a 50% cut-off aerodynamic diameter of 10 mum ( PM10) and weather measurements were obtained. City-specific analysis was ca rried out using Poisson regression, adjusting for time trend, ambient tempe rature, dew point, barometric pressure and day of the week. A cubic polynom ial was used for each epidemic period (greater than or equal to 10 days of excessive pneumonia hospital admissions), and a dummy variable was used to control for isolated epidemic days, A 10-mug.m(-3) increase in PM10 concentration (lag 0-1) was associated with increased daily deaths in Chicago (0.81%, 95% confidence internal (CI) 0.5 4-1.09); 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 PM10 effect were observed (Chicago 9%, Detroit 11%, Minneapolis 3%, Pittsbu rgh 5%, and Seattle 15%). The overall effect of PM10 concentration was 0.85 % (95% CI 0.60-1.10) per 10 mug.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 deat hs is not due to failure to control for influenza or pneumonia epidemics.