Do socioeconomic characteristics modify the short term association between air pollution and mortality? Evidence from a zonal time series in Hamilton, Canada

Study objective: To assess the short term association between air pollution and mortality in different zones of an industrial city. An intra- urban study design is used to test the hypothesis that socioeconomic characteristics modify the acute health effects of ambient air pollution exposure. Design: The City of Hamilton, Canada, was divided into five zones based on proximity to fixed site air pollution monitors. Within each zone, daily counts of non- trauma mortality and air pollution estimates were combined. Generalised linear models ( GLMs) were used to test mortality associations with sulphur dioxide ( SO2) and with particulate air pollution measured by the coefficient of haze ( CoH). Main results: Increased mortality was associated with air pollution exposure in a citywide model and in intra- urban zones with lower socioeconomic characteristics. Low educational attainment and high manufacturing employment in the zones significantly and positively modified the acute mortality effects of air pollution exposure. Discussion: Three possible explanations are proposed for the observed effect modification by education and manufacturing: ( 1) those in manufacturing receive higher workplace exposures that combine with ambient exposures to produce larger health effects; ( 2) persons with lower education are less mobile and experience less exposure measurement error, which reduces bias toward the null; or ( 3) manufacturing and education proxy for many social variables representing material deprivation, and poor material conditions increase susceptibility to health risks from air pollution.