Abstract
BACKGROUND: Climate change is anticipated to have profound effects on human health, including in infectious diseases. Wildfires have been increasing in frequency and intensity due to climate change and have been linked to worsening respiratory disease outcomes. We aimed to demonstrate whether there was an association between wildfire smoke and coronavirus disease 2019 (COVID-19) in California during 2020. METHODS: We used an ecologic cohort study with a spatial autoregressive model to test for associations between wildfire smoke, measured as particulate matter <2.5 µg/m(3) and COVID-19 cases and deaths at the county level in California in 2020. All data was downloaded from open sources that were freely available to the public. In our spatial autoregressive model, we adjusted for demographic, environmental factors and spatial autocorrelation that could be associated with the exposure and outcome. RESULTS: In an adjusted analysis, we found a 1-month lag increase of 203 COVID-19 cases per 10 000 persons per 10 µg/m(3) of smoke exposure (P < .001) at the county level. There was a 1-month lag increase of 2.75 COVID-19 deaths per 10 000 persons per 10 µg/m(3) of smoke exposure (P < .001) at the county level. These findings were attenuated in the second month after smoke exposure, with a 2-month lag increase of 80.6 COVID-19 cases per 10 000 persons per 10 µg/m(3) of smoke exposure (P = .002) and no 2-month lag association with COVID-19 deaths. CONCLUSIONS: The year 2020 was particularly strong for wildfires in California and a unique year for infectious diseases with the COVID-19 pandemic. Our findings demonstrate that wildfire smoke exposure likely increased the spread of COVID-19 and worsened the mortality rate.