Microdroplets, aerosol flows. This isn't a matter of stopping large droplets, where masks are more effective provided they don't nebulize. See also https://lin

Microdroplets, aerosol flows. This isn't a matter of stopping large droplets, where masks are more effective provided they don't nebulize. See also https://link.springer.com/article/10.1007/BF01658736 https://link.springer.com/article/10.1007/BF01658736 https://ncbi.nlm.nih.gov/pmc/articles/PMC2493952/pdf/annrcse01509-0009.pdf https://ncbi.nlm.nih.gov/pmc/articles/PMC2493952/pdf/annrcse... for surprising null or even slightly negative result of surgeons masking vs. surgery patient wound infections. Assuming linear scaling or other naive-model benefits from masks runs into large-scale gen-pop experience, which has now been studied by enough RCTs to cast doubt on the assumption. Long-term WHO and CDC policy guidance was based on empirical science showing non-pharmaceutical interventions are not very effective against respiratory viruses.