ACE2 and COVID-19: In conclusion, ACE2 is very significant because (a) the Nrf2 (anti-inflammatory) pathway is activated and the NFKB (inflammatory) pathway is switched off to prevent inflammatory response disorders; (b) COVID-19 modifies this mechanism by combining it with ACE2; (c) ACE2 is overexpressed after exposure to PM2.5, thereby increasing the risk of COVID-19 infection, where ACE2 is the key to the virus's entry (Figure 4).