Association between Short-Term Exposure to Air Pollution and COVID-19 Mortality: A Population-Based Case-Crossover Study Using Individual-Level Mortality Registry Confirmed by Medical Examiners

BACKGROUND: Studies have suggested links between ambient air pollution and coronavirus 2019 (COVID-19) mortality, yet confirmation by well-designed epidemiological studies with individual data is needed. OBJECTIVES: We aimed to examine whether short-term exposure to air pollution is associated with risk of mortality from COVID-19 for those infected with COVID-19. METHODS: The Cook County Medical Examiner’s Office reports individual-level data for deaths from COVID-19 that occur in its jurisdiction, which includes all confirmed COVID-19 deaths in Cook County, Illinois. Case-crossover analysis was conducted to estimate the associations of estimated short-term exposures to particulate matter (PM) with aerodynamic diameter [Formula: see text] ([Formula: see text]) and ozone ([Formula: see text]) on the day of death and up to 21 d before death at location of death with COVID-19. A total of 7,462 deaths from COVID-19 that occurred up to 28 February 2021 were included in the final analysis. We adjusted for potential confounders by time-stratified case-crossover design and by covariate adjustments (i.e., time-invariant factors, meteorological factors, viral transmission, seasonality, and time trend). RESULTS: Of the 7,462 case and 25,457 self-control days, almost all were days with exposure levels below the [Formula: see text] 24-h National Ambient Air Quality Standard (NAAQS) ([Formula: see text]); 98.9% had [Formula: see text] levels below the maximum 8-h NAAQS ([Formula: see text] or 70 parts per billion). An interquartile range (IQR) increase ([Formula: see text]) in cumulative 3-wk [Formula: see text] exposure was associated with a 69.6% [95% confidence interval (CI): 34.6, 113.8] increase in risk of COVID-19 mortality. An IQR increase ([Formula: see text]) in 3-d [Formula: see text] exposure was associated with a 29.0% (95% CI: 9.9, 51.5) increase in risk of COVID-19 mortality. The associations differed by demographics or race/ethnicity. There was indication of modification of the associations by some comorbid conditions. DISCUSSION: Short-term exposure to air pollution below the NAAQS may increase the mortality burden from COVID-19. https://doi.org/10.1289/EHP10836