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 O₃ and NO₂ (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/m³ 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.