Does air pollution cause respiratory illness? A new look at Canadian cities

It is routinely asserted that urban air pollution is a major cause of acute respiratory conditions, leading to thousands of hospitalizations each year. The claim is based on inferences from partial correlations between ambient air pollution levels and hospitalization rates. Yet questions persist about the statistical robustness of the epidemiological findings, and controlled experiments have not confirmed the statistical findings. In this paper we present and analyze a new monthly data base showing concentrations of five major air contaminants in 11 large Canadian cities from 1974 to 1994, matched with monthly hospital admission rates by age group for all lung diagnostic categories; as well as a comprehensive set of socioeconomic and meteorological covariates. We compare two estimation approaches: model selection and Bayesian model averaging. Almost all of our estimates of the health effects of air pollution are insignificant. Two pollutant types have significantly negative coefficients, indicating, if interpreted in the standard way, that these pollutants are actually beneficial for health. We do not claim this, but we conclude that the perceived statistical relationship between air pollution and health is not robust.