As some communities begin to lift #COVID19 stay-at-home orders, let’s talk about #masks. There is extensive evidence that masks protect you, and thus protect those around you. For example, this meta-analysis of 51 studies and 49 publications: bmj.com/content/bmj/33… (1/9) 
But what type of mask should you wear? Any type of mask that serves as a barrier between your nose/mouth and the environment will likely help and is better than no mask, but data has been published on cloth masks vs surgical (medical) masks vs N95 masks (respirators). (2/9)
How does protection from wearing cloth masks—recommended by @CDCgov—compare to protection from surgical masks? This study of 1,607 healthcare workers from 14 hospitals compared cloth masks and surgical masks for preventing respiratory infection: bmjopen.bmj.com/content/bmjope… (4/9)
Cloth masks in this study were only 63% as effective as surgical masks for preventing any respiratory illness, and only 8% as effective as surgical masks for preventing influenza-like illness (note that the control group here was a mix of cloth and surgical masks). (5/9)
N95 masks on average reduced the odds of being infected by 91%, but with high variability, likely because tightly fitting an N95 mask to the face is a strong determinant of its effectiveness, and because N95 masks can be uncomfortable to wear continuously for an entire day. (6/9)
In this study of 1,669 hospital healthcare workers, N95 masks offered better protection than surgical masks only if the N95 mask was worn continuously—likely impractical for most people. Surgical masks offer a good compromise of practical protection. atsjournals.org/doi/pdf/10.116… (7/9)
Why are cloth masks less effective than surgical masks? “Laboratory tests showed the penetration of particles through the cloth masks to be very high (97%) compared with [surgical] masks (44%) and N95 (<0.1%)” #SARSCoV2 is only ~0.1 µm, and cloth masks have much larger holes. 8/9
Take-home: providing masks for the ill and those who treat them must be prioritized. But as surgical masks are increasingly available, especially from overseas, their use should more effectively reducing risk of #COVID19 transmission than cloth masks.(9/9)
