An actual study on masks this week.
https://nypost.com/2020/12/16/wearing-a-used-mask-could-worse-than-no-mask-amid-covid-19-study/Quote:
Researchers from the University of Massachusetts Lowell and California Baptist University say that masks slow down airflow, making people more susceptible to breathing in particles and a dirty face mask can't effectively filter out the tiniest of droplets.
"It is natural to think that wearing a mask, no matter new or old, should always be better than nothing," said author Jinxiang Xi.
"Our results show that this belief is only true for particles larger than 5 micrometers, but not for fine particles smaller than 2.5 micrometers."