Previous studies have suggested a relationship between outdoor air pollution and the risk of coronavirus disease 2019 (COVID-19). However, there is a lack of data related to the severity of disease, especially in China. This study aimed to explore the association between short-term exposure to outdoor particulate matter (PM) and the risk of severe COVID-19.

We recruited patients diagnosed with COVID-19 during a recent large-scale outbreak in eastern China caused by the Delta variant. We collected data on meteorological factors and ambient air pollution during the same time period and in the same region where the cases occurred and applied a generalized additive model (GAM) to analyze the effects of short-term ambient PM exposure on the risk of severe COVID-19.

A total of 476 adult patients with confirmed COVID-19 were recruited, of which 42 (8.82%) had severe disease. With a unit increase in PM₁₀, the risk of severe COVID-19 increased by 81.70% (95% confidence interval [CI]: 35.45, 143.76) at a lag of 0–7 days, 86.04% (95% CI: 38.71, 149.53) at a lag of 0–14 days, 76.26% (95% CI: 33.68, 132.42) at a lag of 0–21 days, and 72.15% (95% CI: 21.02, 144.88) at a lag of 0–28 days. The associations remained significant at lags of 0–7 days, 0–14 days, and 0–28 days in the multipollutant models. With a unit increase in PM_2.5, the risk of severe COVID-19 increased by 299.08% (95% CI: 92.94, 725.46) at a lag of 0–7 days, 289.23% (95% CI: 85.62, 716.20) at a lag of 0–14 days, 234.34% (95% CI: 63.81, 582.40) at a lag of 0–21 days, and 204.04% (95% CI: 39.28, 563.71) at a lag of 0–28 days. The associations were still significant at lags of 0–7 days, 0–14 days, and 0–28 days in the multipollutant models.

Our results indicated that short-term exposure to outdoor PM was positively related to the risk of severe COVID-19, and that reducing air pollution may contribute to the control of COVID-19.