ACE2 and COVID-19: At a nanoscale, the viral load and virulence through the SARS-CoV-2 spike proteins and ACE2 human cells receptors fusion mechanisms, correlate directly with infectivity, disease phenotype, morbidity, and mortality, which should be accounted for correlating conventional aerosol metrics, such as PM2.5 or PM10 concentrations to COVID-19 cases, or deaths, and provide a direct explanation of the variability of the COVID-19 severity through the Case Fatality Ratio [18].