The Physics of Masks and Moral Suasion

There is a war of ideas out there which often seems to boil down to "wear a mask" v. "masks are worthless" For the record, I think both of these positions are utterly stupid.

On the "wear a mask" side: Should I wear one at home, when I'm sleeping, out jogging on otherwise deserted roads, while driving? On the "worthless" side: It is intuitively obvious that as long as there is some retention, there is some protection. Two additional points here: 1. What is being filtered is not just a virion by itself but tiny droplets of saliva or mucous containing viruses. 2. The size of the infectious load (infectious particles) is a large determinant of whether you will get an infection.

What about the idea that if you don't care about yourself, you need to protect those around you: This is really stupid too: Masks mostly protect the person wearing it*. Regardless of how retentive the filter, when you inhale you force the mask to tighten around your face, when you exhale, you push it away from your face. There is bound to be more bypass around the filter material when you exhale than when you inhale. The second reason is distance: You are exposed to all the air you inhale, but the air you exhale does not go directly into the lungs of anyone else (unless you are giving mouth-to-mouth). The air you exhale becomes rapidly mixed with other air and this dilutes any infectious particles. Finally, the chances that you are infectious is very low, which means that the chances that your mask is doing anything useful is also, very low.

*Here is where it gets tricky: If you get infected, you increase the chances of infecting people you live with and some of them may be old or otherwise vulnerable. So it makes sense to protect yourself to protect them. No. Not all the time. If you are in crowded places where you can't socially distance it makes sense to wear a mask.

Common sense, not glib slogans.