Tldr: Even a homemade mask is better than nothing.
The idea is that the main mode of spread of this virus is through tiny droplets of moisture breathed out of the lungs of an infected person. If you breathe on a mirror or outside on a freezing day, you can see the moisture, otherwise the fine mist is invisible. If you inhale enough virus particles, they can establish an infection.
If you've ever worn an N95 mask, it's pretty obvious that they at least partially block movement of that moisture, since it gets pretty humid in the mask from your own breathing. But there have been plenty of studies showing that as long as you're using them correctly, they block the relevant sized particles, and also reduce risk of respiratory infections.
The idea is that the main mode of spread of this virus is through tiny droplets of moisture breathed out of the lungs of an infected person. If you breathe on a mirror or outside on a freezing day, you can see the moisture, otherwise the fine mist is invisible. If you inhale enough virus particles, they can establish an infection.
If you've ever worn an N95 mask, it's pretty obvious that they at least partially block movement of that moisture, since it gets pretty humid in the mask from your own breathing. But there have been plenty of studies showing that as long as you're using them correctly, they block the relevant sized particles, and also reduce risk of respiratory infections.
In general, the N95 are better than the surgical-type masks, but the surgical masks are better than nothing. Wearing a mask does not bring your risk of infection to zero, but it reduces it by 50-75%, depending on the study. Given that the new virus has an R0 around 2-3 (i.e. each person infects 2 or 3 others), this level of reduction of transmission could bring that rate down to below 1, which would mean the virus would die out, even without lockdowns. That’s even more true if both the transmitter and potential transmittee are both wearing masks, because if you are infectious, the mask traps the virus particles that you’re breathing out.
I suspect that part of the reason the virus hasn’t taken hold in Japan is that it’s very common for people to wear surgical masks in public. Japan has had only 33 covid-19 deaths out of a population of 126 million people, despite having a high population density, proximity to China, and the second-oldest population in the world. I also suspect that the Japanese wash their hands a lot more than we do in the US, so I’m not willing to say it’s only the masks. But given the data, it’s hard to see how it wouldn’t make a significant difference.
So that brings us to the question of the shortage of masks. Medical professionals are running out, and obviously they should get first dibs on newly produced masks. But there has actually been a study done on the effectiveness of homemade masks, and there are lots of webpages out there with instructions to make them. There are even groups collecting handmade ones to be used by medical professionals. The epidemiologists have been predicting “the big one” for years, although most of them thought it would be flu. So that raises the question of whether or not a mask made out of a tea towel would offer any protection. The study measured particle transmission across the masks, not infection. The study was done in the Netherlands, and compared the European equivalent of an N95 mask, a surgical mask, and the handmade mask.
The result was that the tea towel mask pretty consistently filtered out half to 2/3 of particles for everyone, including children. The surgical masks filtered between half and 90%, depending on the person, and the N95 masks filtered at least the 95% that they’re rated for, and quite a bit more for some people. The commercial masks worked much better for adults than children, which makes sense given the mask design.
In the studies of transmission of actual infections, rather than particles in a lab, the difference between N95 and surgical masks is less pronounced. Unfortunately, no one has tested the tea towel mask against actual infection. But even in the worst case here, filtering out half of the viruses should make a real difference for infectivity.
Tea towels are made out of woven material, with little holes in it, whereas N95 masks are made out of a felted fabric. If you look closely at a high-grade furnace filter, rated for virus filtration, it is made of a similar material, and I suspect if you disassembled one, you could repurpose the material as face masks that would be better than a tea towel. But as you might suspect there is no literature on this, and I haven’t tried it myself. I think if I were going to try, I would layer the furnace filter material with something sturdier, as it is pretty thin and might tear easily.
The fit of the mask is as important as the material itself, as contaminated air can go around a loose-fitting mask. The idea is that when you breathe, the fabric should pull in and out as the air passes through it. I own one mask that is made out of molded rubbery material, with replaceable filter pads. It’s clearly much better than even a well-fitted N95 mask, because it creates a truly air-tight seal around the face. So if you happen to have one of those lying around, go for it.
I wore an N95 mask for my one excursion to the store post-social-distancing, even though there are no known cases in my town. Part of my strategy is to try to normalize it. I'm not someone who cares what people think of me, but it still feels a little weird. Hopefully those who want to wear a mask but feel social pressure against it will be emboldened if they are not the first.
Finally, if you have or make a mask, don't forget to disinfect it. The virus particles stick to the material. 10% bleach, 30+% alcohol, or lysol are all fine.
