Independent associations of short- and long-term air pollution exposure with COVID-19 mortality among Californians

The growing literature demonstrating air pollution associations on COVID-19 mortality contains studies predominantly examining long-term exposure, with few on short-term exposure, and rarely both together to estimate independent associations. Because mechanisms by which air pollution may impact COVID-19 mortality risk function over timescales ranging from years to days, and given correlation among exposure time windows, consideration of both short- and long-term exposure is of importance. We assessed the independent associations between COVID-19 mortality rates with short- and long-term air pollution exposure by modeling both concurrently. Using California death certificate data COVID-19-related deaths were identified, and decedent residential information used to assess short- (4-week mean) and long-term (6-year mean) exposure to particulate matter <2.5µm (PM(2.5)), nitrogen dioxide (NO(2)), and ozone (O(3)). Negative binomial mixed models were fitted on weekly census tract COVID-19 mortality adjusting for potential confounders with random effects for county and census tract and an offset for population. Data were evaluated separately for two time periods March 16, 2020–October 18, 2020 and October 19, 2020–April 25, 2021, representing the Spring/Summer surges and Winter surge. Independent positive associations with COVID-19 mortality were observed for short- and long-term PM(2.5) in both study periods, with strongest associations observed in the first study period: COVID-19 mortality rate ratio for a 2-μg/m(3) increase in long-term PM(2.5) was 1.13 (95%CI:1.09,1.17) and for a 4.7-μg/m(3) increase in short-term PM(2.5) was 1.05 (95%CI:1.02,1.08). Statistically significant positive associations were seen for both short- and long-term NO(2) in study period 1, but short-term NO(2) was not statistically significant in study period 2. Results for long-term O(3) indicate positive associations, however, only marginal significance is achieved in study period 1. These findings support an adverse effect of long-term PM(2.5) and NO(2) exposure on COVID-19 mortality risk, independent of short-term exposure, and a possible independent effect of short-term PM(2.5).