Daily Summer Temperatures and Hospitalization for Acute Cardiovascular Events: Impact of Outdoor PM2.5 Oxidative Potential on Observed Associations Across Canada

Abstract

Background: Oxidative stress plays an important role in the health impacts of both outdoor fine particulate air pollution (PM2.5) and thermal stress. However, it is not clear how the oxidative potential of PM2.5 may influence the acute cardiovascular effects of temperature. Methods: We conducted a case–crossover study of hospitalization for cardiovascular events in 35 cities across Canada during the summer months (July–September) between 2016 and 2018. We collected three different metrics of PM2.5 oxidative potential each month in each location. We estimated associations between lag-0 daily temperature (per 5ºC) and hospitalization for all cardiovascular (n = 44,876) and ischemic heart disease (n = 14,034) events across strata of monthly PM2.5 oxidative potential using conditional logistical models adjusting for potential time-varying confounders. Results: Overall, associations between lag-0 temperature and acute cardiovascular events tended to be stronger when outdoor PM2.5 oxidative potential was higher. For example, when glutathione-related oxidative potential (OPGSH) was in the highest tertile, the odds ratio (OR) for all cardiovascular events was 1.040 (95% confidence intervals [CI] = 1.004, 1.074) compared with 0.980 (95% CI = 0.943, 1.018) when OPGSH was in the lowest tertile. We observed a greater difference for ischemic heart disease events, particularly for older subjects (age >70 years). Conclusions: The acute cardiovascular health impacts of summer temperature variations may be greater when outdoor PM2.5 oxidative potential is elevated. This may be particularly important for ischemic heart disease events.