Masks for Dummies
They set up a bunch mannequins in an actual classroom (as pictured above). They configured some of the mannequins' pie-holes to exhale aerosols and others to inhale the surrounding air. They actually tried several different scenarios for placing the exhalers and inhalers, which you can read about in the paper if you're interested.
The aerosols they used were salt particles engineered to be about the same size as aerosolized SARS-CoV-2 particles. They note that the concentration of particles was higher than someone with COVID-19 would actually emit, but they had to do this to ensure they could measure the test aerosols (signal) above the aerosols naturally present in the room (noise).
To test transmission, they put different masks on the dummies: 4-layer commercial cotton masks, cloth masks designed by and produced for UW, 3-layer "procedure masks" made of melt-blown polypropylene (like you yourself might have worn recently), and a heavy-duty surgical mask.
They also supplemented these masks with a couple of gadgets called "fitters." One is Badger Seal, apparently designed by UW, and the other is a Fix The Mask brace. I have not taken any money to promote these products. In fact, I just learned about them in this article, and plan to test them (look for a future post on that). As you'll see if you follow the links, they both hold the masks down around your nose and sides of your face to prevent leaks.
Data presented in the paper suggest that all the masks except the UW one have 50-80% leakage without the fitters. That's a lot. Here's a picture included in the paper illustrating the difference the fitters make:
Now, here is a chart showing their findings:
Those are some pretty stark differences. My practical conclusions from this are as follows. First, the commercial multi-layer cloth masks aren't very effective. They are better then nothing, but only about 20% better than nothing. You could buy the fancy UW cloth masks, which are about three times better. But why would you do that when you can pair a plain old, cheap procedure mask (about 16 cents each in quantity on Amazon), add a fitter, and get better than 90% efficiency?
This is an important study because for a long time, experts have told us masks protect others but they don't protect you. These results seem to show that they do protect you from inhaling the virus (as well as others from exhaling them), very effectively if you have a good fitter.
This is yet another piece of evidence that masks work and people should wear them to protect themselves and others.
