Within-city spatial variations in long-term average outdoor oxidant gas concentrations and cardiovascular mortality: Effect modification by oxidative potential in the Canadian Census Health and Environment Cohort.

Health effects of oxidant gases may be enhanced by components of particulate air pollution that contribute to oxidative stress. Our aim was to examine if within-city spatial variations in the oxidative potential of outdoor fine particulate air pollution (PM_2.5) modify relationships between oxidant gases and cardiovascular mortality.

Methods: We conducted a retrospective cohort study of participants in the Canadian Census Health and Environment Cohort who lived in Toronto or Montreal, Canada, from 2002 to 2015. Cox proportional hazards models were used to estimate associations between outdoor concentrations of oxidant gases (O_x, a redox-weighted average of nitrogen dioxide and ozone) and cardiovascular deaths. Analyses were performed across strata of two measures of PM_2.5 oxidative potential and reactive oxygen species concentrations (ROS) adjusting for relevant confounding factors.

Results: PM_2.5 mass concentration showed little within-city variability, but PM_2.5 oxidative potential and ROS were more variable. Spatial variations in outdoor O_x were associated with an increased risk of cardiovascular mortality [HR per 5 ppb = 1.028, 95% confidence interval (CI): 1.001, 1.055]. The effect of O_x on cardiovascular mortality was stronger above the median of each measure of PM_2.5 oxidative potential and ROS (e.g., above the median of glutathione-based oxidative potential: HR = 1.045, 95% CI: 1.009, 1.081; below median: HR = 1.000, 95% CI: 0.960, 1.043).

Conclusion: Within-city spatial variations in PM_2.5 oxidative potential may modify long-term cardiovascular health impacts of O_x. Regions with elevated O_x and PM_2.5 oxidative potential may be priority areas for interventions to decrease the population health impacts of outdoor air pollution.