Long-term Exposure to Fine Particulate Matter Air Pollution and Mortality among Canadian Women

Background: Long-term exposure to fine particulate matter (PM_2.5) has been associated with increased mortality, especially from cardiovascular disease. There are, however, uncertainties about the nature of the exposure-response relation at lower concentrations. In Canada, where ambient air pollution levels are substantially lower than in most other countries, there have been few attempts to study associations between long-term exposure to PM_2.5 and mortality. Methods: We present a prospective cohort analysis of 89,248 women who enrolled in the Canadian National Breast Screening Study between 1980 and 1985, and for whom residential measures of PM_2.5 could be assigned. We derived individual-level estimates of long-term exposure to PM_2.5 from satellite observations. We linked cohort records to national mortality data to ascertain mortality between 1980 and 2005. We used Cox proportional hazards models to characterize associations between PM_2.5 and several causes of death. The hazard ratios (HRs) and 95% confidence intervals (CIs) computed from these models were adjusted for several individual and neighborhood-level characteristics. Results: The cohort was composed predominantly of Canadian-born (82%) and married (80%) women. The median residential concentration of PM_2.5 was 9.1 μg/m³ (standard deviation = 3.4). In fully adjusted models, a 10 μg/m³ increase in PM_2.5 exposure was associated with elevated risks of nonaccidental (HR: 1.12; 95% CI = 1.04, 1.19), and ischemic heart disease mortality (HR: 1.34; 95% CI = 1.09, 1.66). Conclusions: The findings from this study provide additional support for the hypothesis that exposure to very low levels of ambient PM_2.5 increases the risk of cardiovascular mortality.