Have you been wearing a face mask when going out in public recently? There’s been a lot of debate recently about whether they are effective in keeping people safe from COVID-19 infection, and it’s been really challenging to find good information. After reading several things over the past few days, I have concluded that wearing a mask in public is a helpful step I can take to help keep myself and others safe, with the important caveat that healthcare workers need access to masks before the rest of us (see below). In particular, I found this extensive review of the medical and scientific literature on mask & respirator use helpful, including why research on mask efficacy is so hard to do and speculation on why the CDC and WHO generally don’t recommend wearing them.

I was able to find one study like this outside of the health care setting. Some people with swine flu travelled on a plane from New York to China, and many fellow passengers got infected. Some researchers looked at whether passengers who wore masks throughout the flight stayed healthier. The answer was very much yes. They were able to track down 9 people who got sick on the flight and 32 who didn’t. 0% of the sick passengers wore masks, compared to 47% of the healthy passengers. Another way to look at that is that 0% of mask-wearers got sick, but 35% of non-wearers did. This was a significant difference, and of obvious applicability to the current question.

See also this review of relevant scientific literature, this NY Times piece, this Washington Post opinion piece by Jeremy Howard (who is on a Twitter mission to get everyone to wear masks):

When historians tally up the many missteps policymakers have made in response to the coronavirus pandemic, the senseless and unscientific push for the general public to avoid wearing masks should be near the top. The evidence not only fails to support the push, it also contradicts it. It can take a while for official recommendations to catch up with scientific thinking. In this case, such delays might be deadly and economically disastrous. It’s time to make masks a key part of our fight to contain, then defeat, this pandemic. Masks effective at “flattening the curve” can be made at home with nothing more than a T-shirt and a pair of scissors. We should all wear masks — store-bought or homemade — whenever we’re out in public. At the height of the HIV crisis, authorities did not tell people to put away condoms. As fatalities from car crashes mounted, no one recommended avoiding seat belts. Yet in a global respiratory pandemic, people who should know better are discouraging Americans from using respiratory protection.

I have to admit that I have not been wearing a mask out in public — I’ve been to the grocery store only three times in the past two weeks, I go at off-hours, and it’s rural Vermont, so there’s not actually that many people about (e.g. compared to Manhattan). But I’m going to start wearing one in crowded places (like the grocery store) because doing so could a) safeguard others against my possible infection (because asymptomatic people can still be contagious), b) make it less likely for me to get infected, and c) provide a visible signal to others in my community to normalize mask wearing. As we’ve seen in epidemic simulations, relatively small measures can have outsize effects in limiting later infections & deaths, and face masks, even if a tiny bit effective, can have a real impact.

Crucially, the available research and mask advocates stress the importance of wearing masks properly and responsibly. Here are some guidelines I compiled about responsible mask usage:

Don’t buy masks (or use new masks you might have at home) while there is a shortage for healthcare workers, especially not N95 respirators (which are difficult to use properly anyway). Make a mask at home. Skiers & snowboarders, wear your buffs or ski masks. Donate any unused masks or respirators you may have to healthcare workers.

Make sure your mask fits properly — limit any gaps between the mask and your face as much as you can. (Facial hair can limit mask effectiveness.)

While wearing your mask in public, don’t fuss with it — touching your face is bad, remember? Wear it at home for a few hours to get used to the sensation. Then when you’re ready to go out, put it on properly and don’t touch it again until you’re back home (or in the car or whatever). Part of the point of the mask is for you to touch your face less.

Limit reuse of potentially contaminated masks. Discard or, if possible, wash or disinfect masks after public usage or at the end of the day.

Wearing a mask doesn’t mean you can safely go do a bunch of things without fear of getting infected. The idea here is to protect yourself while engaging in necessary activities in public. Wearing a mask doesn’t mean you can visit grandma safely or discard the six-feet-away rule.

Don’t do anything stupid like spraying your mask with a household cleaner that contains bleach and put it on. Come on.

So that’s what I’ve personally concluded from all my reading. I hope wearing masks can help keep us a little safer during all of this.

Update: From Ferris Jabr at Wired, It’s Time to Face Facts, America: Masks Work.

It is unequivocally true that masks must be prioritized for health care workers in any country suffering from a shortage of personal protective equipment. But the conflicting claims and guidelines regarding their use raise three questions of the utmost urgency: Do masks work? Should everyone wear them? And if there aren’t enough medical-grade masks for the general public, is it possible to make a viable substitute at home? Decades of scientific research, lessons from past pandemics, and common sense suggest the answer to all of these questions is yes.

Update: The Atlantic’s Ed Yong weighs in on masks:

In Asia, masks aren’t just shields. They’re also symbols. They’re an affirmation of civic-mindedness and conscientiousness, and such symbols might be important in other parts of the world too. If widely used, masks could signal that society is taking the pandemic threat seriously. They might reduce the stigma foisted on sick people, who would no longer feel ashamed or singled out for wearing one. They could offer reassurance to people who don’t have the privilege of isolating themselves at home, and must continue to work in public spaces. “My staff have also mentioned that having a mask reminds them not to touch their face or put a pen in their mouth,” Bourouiba noted.

He also writes about something I’ve been wondering about: is the virus airborne, what does that even mean, when will we know for sure, and how should that affect our behavior in the meantime?

These particles might not even have been infectious. “I think we’ll find that like many other viruses, [SARS-CoV-2] isn’t especially stable under outdoor conditions like sunlight or warm temperatures,” Santarpia said. “Don’t congregate in groups outside, but going for a walk, or sitting on your porch on a sunny day, are still great ideas.” You could tie yourself in knots gaming out the various scenarios that might pose a risk outdoors, but Marr recommends a simple technique. “When I go out now, I imagine that everyone is smoking, and I pick my path to get the least exposure to that smoke,” she told me. If that’s the case, I asked her, is it irrational to hold your breath when another person walks past you and you don’t have enough space to move away? “It’s not irrational; I do that myself,” she said. “I don’t know if it makes a difference, but in theory it could. It’s like when you walk through a cigarette plume.”

And from the WHO, here’s a video on how to wear a mask properly.

Update: One of the reasons I started to wear a mask when I go out in public was to “provide a visible signal to others in my community to normalize mask wearing”. Maciej Cegłowski’s post touches on this and other reasons to wear a mask that don’t directly have to do with avoiding infection.