Is there an association between the level of ambient air pollution and COVID-19?

Epidemiological studies suggest that environmental factors (e.g., air pollution) can influence the spread and infectivity of coronavirus disease 2019 (COVID-19); however, very few papers have investigated or discussed the mechanism behind the phenomenon. Given the fact that pollution will increase as social distancing rules are relaxed, we summarized the current understanding of how air pollution may affect COVID-19 transmission and discussed several possible mechanisms. Air pollution exposure can dysregulate the human immune response and make people more susceptible to infections, and affect infectivity. For example, in response to exposure to air pollution, angiotensin-converting enzyme 2 will increase, which is the receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This may increase the efficiency of viral infection. It is also possible that air pollution can facilitate SARS-CoV-2 spread by increasing the transmission, and potentially, SARS-CoV-2 can also survive longer when attached to a pollutant.