https://slatestarcodex.com/2020/03/23/face-masks-much-more-than-you-wanted-to-know/

There's been recent controversy about the use of face masks for protection against coronavirus. Mainstream sources, including the CDC and most of the media say masks are likely useless and not recommended. They've recently been challenged, for example by Professor Zeynep Tufekci in the New York Times and by Jim and Elizabeth on Less Wrong. There was also some debate in the comment section here last week, so I promised I'd look into it in more depth.

As far as I can tell, both sides agree on some points.

They agree that N95 respirators, when properly used by trained professionals, help prevent the wearer from getting infected.

They agree that surgical masks help prevent sick people from infecting others. Since many sick people don't know they are sick, in an ideal world with unlimited mask supplies everyone would wear surgical masks just to prevent themselves from spreading disease.

They also agree that there's currently a shortage of both surgical masks and respirators, so for altruistic reasons people should avoid hoarding them and give healthcare workers first dibs.