The association between short and long-term exposure to PM_2.5 and temperature and hospital admissions in New England and the synergistic effect of the short-term exposures

Background: Particulate matter < 2.5 μm in diameter (PM_2.5) and heat are strong predictors of morbidity, yet few studies have examined the effects of long-term exposures on non-fatal events, or assessed the short and long-term effect on health simultaneously. Objective: We jointly investigated the association of short and long-term exposures to PM_2.5 and temperature with hospital admissions, and explored the modification of the associations with the short-term exposures by one another and by temperature variability. Methods: Daily ZIP code counts of respiratory, cardiac and stroke admissions of adults ≥65 (N = 2,015,660) were constructed across New-England (2001−2011). Daily PM_2.5 and temperature exposure estimates were obtained from satellite-based spatio-temporally resolved models. For each admission cause, a Poisson regression was fit on short and long-term exposures, with a random intercept for ZIP code. Modifications of the short-term effects were tested by adding interaction terms with temperature, PM_2.5 and temperature variability. Results: Associations between short and long-term exposures were observed for all of the outcomes, with stronger effects of long-term exposures to PM_2.5. For respiratory admissions, the short-term PM_2.5 effect (percent increase per IQR) was larger on warmer days (1.12% versus −0.53%) and in months of higher temperature variability (1.63% versus −0.45%). The short-term temperature effect was higher in months of higher temperature variability as well. For cardiac admissions, the PM_2.5 effect was larger on colder days (0.56% versus −0.30%) and in months of higher temperature variability (0.99% versus −0.56%). Conclusions: We observed synergistic effects of short-term exposures to PM_2.5, temperature and temperature variability. Long-term exposures to PM_2.5 were associated with larger effects compared to short-term exposures.