From early on in the pandemic, we heard that N95 masks—the sturdier kind that medical professionals often wear—were the gold standard for protecting a person from the novel coronavirus that causes COVID-19.
A little later on, it became clear that a huge reason to wear a mask isn’t just to protect yourself from incoming germs, it’s to prevent your germs from being spread to others. Experts also started recommending against wearing versions of N95 masks that have exhalation valves, suggesting that they don't slow the spread of the disease.
And now, two amazing new videos, created by Matthew Staymates, a fluid dynamicist at the National Institute of Standards and Technology show why.
First, he looked at how much exhaled air the valves allow to escape. On the left, he’s wearing an N95 with a valve; on the right, an N95 without a valve:
Staymates had suspected that the valves let out quite a bit of what the wearer is exhaling when he went to get takeout and saw someone wearing an N95 with a valve. “I’m a fluid dynamicist, and the curse of that is that I’m constantly thinking about airflow. And when I saw this person, I thought, ‘you know, if that valve is doing what I think it’s doing, that’s not a very good mask to be wearing right now.’”
And since his lab moved home with him at the outset of the pandemic, he decided to go downstairs and test it.
Since the way the first video is done is designed to only show airflow from the valve, he did another test to show the movement of exhaled droplets (not just air). And it’s clear that the N95 let plenty of droplets through:
“The valve is doing exactly what it’s intended to do,” says Staymates. For people working in construction, for instance, the mask can keep out dust while the valve makes breathing feel easier. “But during the pandemic, we’re actually kind of switching what we want the masks to do,” he explains. Instead of just keeping particles from flowing into the wearer, “we want them to do what we call source control, which is block droplets from getting from me onto you,” he says. And his experiments confirm that the best way to do that is a mask that doesn’t contain a valve. Check out other mask options here.
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