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.

  1. A mask on a sick person prevents the creation of fomites; masks on healthy people don't.
  2. Outgoing particles have a larger size and shrink due to evaporation, so they'll penetrate a mask less, given equal kinetic energy. (However, kinetic energies are not equal; they start out fast and slow down, which would favor putting the mask on the healthy person. I'm not sure how much this matters.)
  3. Particles that stick to a mask but then un-stick lose their kinetic energy in the process, which helps if the mask is on the sick person, but doesn't help if the mask is on the healthy person.

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.

Jimrandomh's Shortform

by jimrandomh 1 min read4th Jul 201964 comments

This post is a container for my short-form writing. See this post for meta-level discussion about shortform as an upcoming site feature.