You’ve heard that it’s important to #WearAMask. That’s true – masks save lives. But, you might not have heard that some masks are better than others. And, new research shows that some masks might even be counterproductive. Wondering how masks work and why this matters? A thread.
So, how do masks offer protection? You can think of a mask as a filter, and the smaller the holes in the filter, the fewer things can pass through. But, the coronavirus is tiny – it has a diameter of 125 nanometers, or nm for short. Here’s a picture of it under a microscope. 
How small is that? Look at piece of your hair – it has a width of about 50-100 microns. 1 micron is equal to 1000 nanometers. That means that you would have to line up 400-800 virus particles to span the miniscule width of one of your hairs. That’s small.
If the virus is that tiny, how does a mask offer protection? That has to do with how the virus spreads. It spreads from the water droplets that you breathe, sneeze, or cough. These droplets vary in size but are much bigger – some are even big enough to see.
This highlights an important point that’s often missed. A mask gives you some protection, but much more to the people around you. It catches the water droplets that you breathe, cough, or sneeze so that they can’t make someone else sick. Wearing a mask is about caring for others.
And, masks vary in the protection they offer. As scientists try to understand the relative efficacy of different types of masks, one challenge has been that masks are often tested using solid latex particles, which can behave differently than the water droplets we breathe out.
Scientists @DukeU tackled this challenge, and in a recent report described an inexpensive setup using a laser and a cell phone camera to measure the number of water droplets that can pass through a mask as someone is talking. advances.sciencemag.org/content/early/…
Their study was mostly about the measuring device and they encourage others to then do more detailed studies to understand how effective different masks are under different conditions. But, they do make some interesting observations. Here’s a picture of the masks they tested.
And here is a graph that shows the relative ability of the different masks to stop water droplets as someone is talking. On the vertical axis, 1.0 represents the number of droplets for someone not wearing a mask, so the lower the number for a mask, the better.
While N95 masks are the most effective in this study, it’s critical to say – please save these masks for healthcare workers! They are highly effective, but only when fitted and worn correctly. And, the people saving lives by working with Covid patients most need that protection.
And, the study shows that many cloth masks are almost as effective at stopping water droplets as N95 masks. What is surprising is the result for the fleece neck gaiter. The researchers found that wearing this actually produced *more* water droplets. The opposite of what we want!
They didn’t investigate this in depth, but it’s possible to make a few hypotheses. They propose that the fleece breaks down bigger droplets into smaller ones as they pass through the material, which results in overall more droplets going into the air.
This makes sense if you think about the material. If you’re an outdoorsy type of person, you might have heard the phrase “cotton kills.” That’s because cotton absorbs water, which is bad if you’re hiking and can get hypothermia from wearing wet clothes.
This is why fleece is so popular for outdoor sports. It’s essentially made from plastic, which repels water. But, what’s good for hikers isn’t necessarily good for face masks. The ability of fleece to repel water means it isn’t the best for catching water droplets you breathe.
But, the ability of cotton to absorb water means it could be quite effective in catching the water droplets you breathe out, which would explain why the cotton masks are among the best performing in this study.
There is still much to be learned here. One of the limitations of the study that the researchers describe is that the cell phone camera isn’t able to capture the smallest water droplets, or aerosols. Also, they only looked at someone talking, not coughing or sneezing.
However, these preliminary data suggest that perhaps it’s best to save the fleece for the hiking trail and choose a tightly woven cotton face covering instead. Above all, these data show that those homemade cloth masks likely make a real difference to protect others. #WearAMask
