OBJECTIVE: Early ecological studies have suggested links between air pollution and risk of COVID-19, but evidence from individual-level cohort studies is still sparse. We examined whether long-term exposure to air pollution is associated with risk of COVID-19 and who is most susceptible.

METHODS: We followed 3 721 810 Danish residents≥30 years on March 1st 2020 in the National COVID-19 Surveillance System until the date of 1st positive test (incidence), COVID-19 hospitalization or death until April 26th 2021. We estimated residential annual mean particulate matter<2·5 µm in diameter (PM 2·5), nitrogen dioxide (NO 2), black carbon (BC), and ozone (O 3) in 2019 by the Danish DEHM/UBM model, and used Cox proportional hazards regression models to estimate the associations of air pollutants with COVID-19 outcomes, adjusting for age, sex, individual- and area-level socioeconomic status, and population density.

RESULTS: 138 742 were infected, 11 270 hospitalized and 2557 died from COVID-19 during 14 months. We detected associations of PM 2.5 (per 0·5 µg·m -3) and NO 2 (per 3·6 µg·m -3) with COVID-19 incidence (hazard ratio; 95% confidence interval: 1·10; 1·05-1·14 and 1·18; 1·14-1·23), hospitalizations (1·09; 1·01-1·17 and 1·19; 1·12-1·27), and death (1·23; 1·04-1·44 and 1·18; 1·03-1·34), which were strongest in the lowest socioeconomic groups and among patients with chronic respiratory, cardiometabolic, and neurodegenerative disease. We found positive associations with BC and negative with O 3.

CONCLUSION: Long-term exposure to air pollution may contribute to increased risk of contracting SARS-CoV-2 infection as well as developing severe COVID-19 disease demanding hospitalization or resulting in death. Keyword: air pollution, COVID-19, SARS-CoV-2, hospitalization, incidence, mortality.