This turns out to be an unexamined error made back in the day when it did not matter and the topic was TB for which the 5 micron level clearly applies. Not so for anything else but this was not important then.
The 60-Year-Old Scientific Screwup That Helped Covid Kill
All pandemic long, scientists brawled over how the virus spreads. Droplets! No, aerosols! At the heart of the fight was a teensy error with huge consequences.
PHOTOGRAPH: NAILA RUECHEL