PM2.5 and NO2 levels tend, in particular, to be more closely related to COVID-19 than PM10,, and their association with COVID-19 mortality and incidence may be attributed to the impossibility of contacting alveolar type II cells with a PM greater than 5 μm, where the cell input receiver for SARS-CoV-2, angiotensin-converting enzyme 2 (ACE2), is located. This evidence concerns the gene ACE2 and COVID-19.