Face masks only work in the one case of where one person, who HAS THE VIRUS is talking, coughing or sneezing in close proximity to another person.
If people do not have the virus wear a face covering, they are touching there face WAY MORE than they would if they were not wearing a mask.
For somebody who does not have the virus, a face shield or even safety glasses would protect them more than a mouth/nose covering.
Masks only work when everyone wears them, that’s the point.
35% of people infected with SARS-CoV-2 will show no symptoms. The only way they’d know they were infected is if they happened to get tested during the window when it would show up on a test. Many others will show symptoms so mild they could easily be mistaken for allergies. So with that in mind, how exactly do you know if you’re infected (and need to wear a mask to protect others when you go out) or if you don’t have it?
That’s the point of the guidance: that nobody really knows for certain that they aren’t currently infected with this.