Examination of effect modification of the relationship between PM2.5 and cardiovascular mortality by particle oxidative potential within two Canadian cities

Abstract

BACKGROUND AND AIM There is evidence that particle oxidative potential modifies effects of fine particulate matter air pollution (PM2.5) on health outcomes in between-cities studies in Canada. Additionally, previous work shows that oxidative potential of PM2.5 varies considerably at the within-city spatial scale. The aim of the present study was to determine if differences in the oxidative potential of PM2.5 at the within-city spatial scale modify the relationship between PM2.5 and cardiovascular mortality. METHODS We conducted a cohort study in the Canadian Census Health and Environment Cohort, a population-based cohort based on the Canadian longform Census. We followed participants who lived in the cities of Toronto or Montreal, Canada, at least 2 years in the period 2001-2016 (approximately 12 271 100 person-years in 1 101 000 individuals, rounded for confidentiality). Annual estimates of PM2.5 exposures were assigned from satellite data calibrated with ground-based observations, and oxidative potential was measured on samples of PM2.5 taken at 234 sites across the study area. We used time-varying Cox proportional hazards models to estimate hazard ratios for the effect of PM2.5 on cardiovascular mortality within strata of PM2.5 oxidative potential. Additionally, we estimated hazard ratios for oxidative burden (the product of oxidative potential and PM2.5 exposures). RESULTS We did not find evidence that oxidative potential modifies the effect of PM2.5 on mortality from cardiovascular causes (N = approximately 6100 events). The overall relationship between PM2.5 exposures and cardiovascular mortality was not significantly different from the null, and the same was true for oxidative burden measures. CONCLUSIONS Spatial variability in PM2.5 and the oxidative potential of PM2.5 may be insufficient for OP to act as a modifier of the effect of PM2.5 on cardiovascular mortality within the cities of Toronto and Montreal. KEYWORDS : fine particulate matter, oxidative potential, outdoor air pollution, cardiovascular disease