Excess of COVID-19 cases and deaths due to fine particulate matter exposure during the 2020 wildfires in the United States

Zhou, Xiaodan; Josey, Kevin; Kamareddine, Leila; Caine, Miah C.; Liu, Tianjia; Mickley, Loretta J.; Cooper, Matthew; Dominici, Francesca

The year 2020 brought unimaginable challenges in public health, with the confluence of the COVID-19 pandemic and wildfires across the western United States. Wildfires produce high levels of fine particulate matter (PM(2.5)). Recent studies reported that short-term exposure to PM(2.5) is associated with increased risk of COVID-19 cases and deaths. We acquired and linked publicly available daily data on PM(2.5), the number of COVID-19 cases and deaths, and other confounders for 92 western U.S. counties that were affected by the 2020 wildfires. We estimated the association between short-term exposure to PM(2.5) during the wildfires and the epidemiological dynamics of COVID-19 cases and deaths. We adjusted for several time-varying confounding factors (e.g., weather, seasonality, long-term trends, mobility, and population size). We found strong evidence that wildfires amplified the effect of short-term exposure to PM(2.5) on COVID-19 cases and deaths, although with substantial heterogeneity across counties.