Impact of Oxidant Gases on the Relationship between Outdoor Fine Particulate Air Pollution and Nonaccidental, Cardiovascular, and Respiratory Mortality

Outdoor fine particulate air pollution (PM(2.5)) is known to increase mortality risk and is recognized as an important contributor to global disease burden. However, less is known about how oxidant gases may modify the chronic health effects of PM(2.5). In this study, we examined how the oxidant capacity of O(3) and NO(2) (using a redox-weighted average, O(x)) may modify the relationship between PM(2.5) and mortality in the 2001 Canadian Census Health and Environment Cohort. In total, 2,448,500 people were followed over a 10.6-year period. Each 3.86 µg/m(3) increase in PM(2.5) was associated with nonaccidental (Hazard Ratio (HR) = 1.095, 95% CI: 1.077, 1.112), cardiovascular (HR = 1.088, 95% CI: 1.059, 1.118), and respiratory mortality (HR = 1.110, 95% CI: 1.051, 1.171) in the highest tertile of O(x) whereas weaker/null associations were observed in the middle and lower tertiles. Analysis of joint non-linear concentration-response relationships for PM(2.5) and O(x) suggested threshold concentrations between approximately 23 and 25 ppb with O(x) concentrations above these values strengthening PM(2.5)-mortality associations. Overall, our findings suggest that oxidant gases enhance the chronic health risks of PM(2.5). In some areas, reductions in O(x) concentrations may have the added benefit of reducing the public health impacts of PM(2.5) even if mass concentrations remain unchanged.