An anti-masker informed the cloth masks don't work, because the virus is so small it passes right. The same person claimed that cloth masks block oxygen and hold in CO2. I suggested looking up the sizes of a SARS-CoV-2, Oxygen molecule, and carbon dioxide molecule.
I tried to explain that masks reduce the risk of spreading covid-19 by serving as barrier for virus laden respiratory droplets. Also, how aerosolization is suspected to occur -- crowded indoor settings, poor ventilation ...
The reaction is usually to cite the allegedly low death rate (as if people either live or die, no other outcomes), bring up comorbidities and direct causes of death, compare gross numbers to cancer or flu, ...