Does Polluted Air Increase COVID-19 Severity? A Critical Review of the Evidence and Proposals to Clarify a Potentially Dramatic Interaction

Several broad ecological analyses have been conducted, mostly in urban settings in Europe and North America, suggesting that air pollution may be associated with greater severity of SARS-CoV-2 infection. Following the identification of possible measurement and confounding biases, we review published studies using alternative study designs, whose main finding was a crude association between COVID-19 severity and PM 2.5 long-term exposure. These preliminary studies are lacking adequate control for confounders and data for other major pollutants. Their results are inconsistent regarding short-term exposures, and virtually all were from high-income countries, limiting their generalizability. We consider the role of alternative study designs in elucidating further such a potential association, by using individual baseline and health outcome data and epidemiological methods to control for potential confounders. To further investigate the role of air pollution in COVID-19 severity between early 2020 and late 2021, we propose to design retrospectively case–control and case-crossover studies using data from public health and air pollution registries, as these may represent the best compromise between validity, reproducibility, and cost. Public health and air pollution registries may provide adequate data sources in industrialized countries and some middle-income countries, facilitating the study of air pollution and COVID-19.