https://www.sciencedaily.com/releases/2020/12/201218084151.htm

In Physics of Fluids, by AIP Publishing, researchers from the University of Massachusetts Lowell and California Baptist University examined the effect of wearing a three-layer surgical mask on inspiratory airflows and the mask's effects on the inhalation and deposition of ambient particles in the upper respiratory airways.

"It is natural to think that wearing a mask, no matter new or old, should always be better than nothing. 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," said author Jinxiang Xi.