Within-city spatiotemporal variations in outdoor ultrafine particles and emergency room visits for cardiovascular outcomes

Few epidemiological studies have examined the acute cardiovascular health impacts of within-city spatiotemporal variations in outdoor ultrafine particles (UFPs). We conducted a time-stratified case-crossover study of within-city spatiotemporal variations in outdoor UFPs and emergency room visits for cardiovascular outcomes in Toronto, Canada, between 2019 and 2020. Outdoor UFP data (lag-0, 3-day mean, and 7-day mean) were assigned to residential locations using exposure models trained using data collected over a 1 year period (2020-2021). Conditional logistic regression models were used to estimate odds ratios (ORs) (95% confidence intervals [CIs]) for outdoor UFP number concentrations (per 10,000/cm3) and mean UFP size (per 5 nm), adjusting for potential time-varying confounders. Effect modification by temperature was also examined. In total, our analysis included 89,694 cases of acute cardiovascular events. Seven-day mean outdoor UFP number concentrations and mean UFP size were most strongly associated with cardiovascular outcomes. Specifically, 7-day mean outdoor UFP number concentrations were associated with increased risks of all cardiovascular events (OR = 1.058, 95% CI = 1.007, 1.111) and ischemic heart disease events (OR = 1.140, 95% CI = 1.020, 1.274). Likewise, 7-day mean outdoor UFP size was associated with increased risks of all cardiovascular events (OR = 1.026, 95% CI = 1.013, 1.039) and ischemic heart disease events (OR = 1.059, 95% CI = 1.029, 1.090). Submultiplicative and subadditive interactions were observed between temperature and both outdoor UFP number concentrations and mean UFP size. Within-city spatiotemporal variations in outdoor UFPs are associated with an increased risk of cardiovascular events independent of other common outdoor air pollutants.