Since exposure to air pollution was shown to result in ACE2 overexpression in the lung [224,225,226,227] and to negatively affect the protective mechanisms of the respiratory system [513], these mechanisms together gave rise to the “double-hit hypothesis” where their combined effects increase susceptibility to SARS-CoV-2 infection, as well as COVID-19 severity [514]. Here, ACE2 is linked to COVID-19.