"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. (The SARS-CoV-2 virus that causes COVID-19 is about 0.1 micrometers. — ed.)"

https://scitechdaily.com/some-masks-can-be-worse-than-not-wearing-one-at-all-physics-of-how-masks-affect-airflow-and-covid-19-protection/