Long-term exposure to fine particle air pollution associated with emergency COVID-19 hospitalizations: A nationwide retrospective Korean cohort study

Objectives Emerging evidence suggests an association between ambient air pollution and severe coronavirus disease 2019 (COVID-19) outcomes. We aimed to assess whether long-term exposure to fine particulate matter (PM 2.5 ) is associated with acute hospitalization after COVID-19 infection and whether this association is modified by COVID-19 vaccination. Study design Retrospective nationwide cohort study of confirmed COVID-19 cases in the Republic of Korea, with municipality-level PM 2.5 exposure assigned and time to emergency admission modeled using a Cox's proportional hazards model. Methods Using national health insurance data in the Republic of Korea, we enrolled 556,767 individuals confirmed with COVID-19 infection from October 8, 2020, to December 31, 2021. Municipality-level annual average PM 2.5 estimates for 2019 were linked to each COVID-19 case based on their residence. The outcome was defined as hospital admission via an emergency department visit. We employed the Cox's proportional hazards model to assess the association between PM 2.5 exposure and the risk of COVID-19 hospitalization after adjusting for potential individual- and municipality-level confounders. Results Higher exposure to PM 2.5 was associated with an increased risk of COVID-19 hospitalization. In the overall population, an increase in PM 2.5 of 5 μg/m 3 was associated with a 10% (95% confidence interval [CI], 3–17%) increase in the risk of hospitalization. In the unvaccinated group, a 5 μg/m 3 increase was associated with a 21% increase in hospitalizations (95% CI, 13–30%). However, we found no association among vaccinated individuals (one vaccine dose or more). Conclusions We found evidence that long-term exposure to fine particulates was associated with the increased risk of acute hospitalization from COVID-19 among unvaccinated individuals.