This tweet raised the question of whether masks really are more effective if placed on sick people (blocking outgoing droplets) or if placed on healthy people (blocking incoming droplets). Everyone in public or in a risky setting should have a mask, of course, but we still need to allocate the higher-quality vs lower-quality masks somehow. When sick people are few and are obvious, and masks are scarce, masks should obviously go on the sick people. However, COVID-19 transmission is often presymptomatic, and masks (especially lower-quality improvised masks) are not becoming less scarce over time.
If you have two people in a room and one mask, one infected and one healthy, which person should wear the mask? Thinking about the physics of liquid droplets, I think the answer is that the infected person should wear it.
Overall, it seems like for a given contact-pair, a mask does more good if it's on the sick person. However, mask quality also matters in proportion to the number of healthy-sick contacts it affects; so, upgrading the masks of all of the patients in a hospital would help more than upgrading the masks of all the workers in that hospital, but since the patients outnumber the workers, upgrading the workers' masks probably helps more per-mask.
Wearing a surgical mask, I get the sense it tends to form more of a seal when inhaling, less when exhaling. (like a valve). If this is common, it would be a point in favour of having the healthy person wear them.
This post is a container for my short-form writing. See this post for meta-level discussion about shortform as an upcoming site feature.