It didn't take long after the pandemic started for facemasks to evolve into a fashion accessory, with retailers offering a variety of fashion-forward, utilitarian, or even custom-silkscreened facemasks. But with many masks being produced by textile companies rather than highly-regulated medical industry manufacturers, how do we know for sure how well that mask is working? For instance, do filters really help? Should masks be cotton, or silk, or nylon, or something else? Ear loops or behind-the-head elastic or ties? Bandanas or gaiters or surgical masks or KN95s or DIYs or Amazon bestsellers? Of these endless options, which masks will keep us, and the people around us, safest?
Researchers have pivoted from their usual projects to look into these questions. The answers they've found are both simpler and more complicated than we might expect.
First, to know what makes a mask work, it's vital to understand at what it's physically supposed to be doing. We all know the basics of that answer: masks block virus particles, both from entering the mouth and nose, and from scattering into the air. But it turns out that up close, this gets much more complicated.
Northeastern researchers Loretta Fernandez and Amy Mueller studied the efficacy of various masks, specifically for the wearer—meaning, how many virus particles can get into a particular mask. "Masks don't really act as a sieve," Fernandez explains. "We're not asking them to be like a strainer." Instead, masks need to slow down airflow. "If the air paths [into the mask] can be made to have twists and turns, that increases the chance of particles coming into contact with the fibers in the mask, [where they'll] stick and get trapped."
That's why more layers of fabric tend to make masks more effective. Additional layers mean that air has to travel through more convoluted pathways, and virus particles have additional chances to catch on the mask's fibers, instead of reaching the wearer's mouth and nose. The same for exhalation, and protecting others: more layers "certainly help the filtering efficiency" in terms of particles escaping into the air, says Martin Fischer. Fischer researched a low-cost mechanism to measure face mask efficacy for expelling virus particles.
But that doesn't mean you should go buy or make a mask with as many layers as possible. Fischer, Fernandez, and Mueller all reference a "tradeoff": the more layers a mask has, the more particles it will catch, but the less breathable it will become. Past a certain point, you get diminishing returns. "You can have a 12 layer mask that removes 99.9% of particles [for the wearer], but it's almost entirely impossible to breathe through, and nobody is going to adopt that habit," Fernandez says. Fischer agrees: "a mask that is superthick—not transmissive at all to air—you probably wouldn't even wear, because it would feel so uncomfortable."
And the fabric of your mask isn't the only variable affecting its efficacy—fit is just as important, if not more so. Fernandez and Mueller found a wide variation in efficacy of masks they tested, and, Mueller explains, "we found that the variation was often more related to the seal of the mask, rather than the material itself. A very large fraction of the base efficacy of any mask comes from having a good seal around it. Within the range above that efficacy, different materials have different properties, but we found that a very wide range of materials could achieve a decent level of filtration."
By "fit," Fernandez and Mueller don't mean that an effective mask ought to press against the mouth and nose. They mean that the edges of the mask should form an effective seal with the face, so that the breath has to go through the mask, rather than past it. Fernandez says, "If you want to improve the fit, you have to get rid of all those routes that air could take to bypass the mask—around the nose, or around the cheeks."
What about filters? "You can probably intuit what's going to happen when you stick a filter inside of a mask," says Fernandez. "If it doesn't cover the whole area, then the air you breathe in is going to find the easiest path across, which is not through the filter. It's around the filter."
Some of the masks Mueller and Fernandez tested performed better than expected; others didn't do so well. They tested a DIY mask made with microfiber shop towels, a paperclip, hairbands, and staples, which they found blocked 86% of particles. On the other hand, they note, "we have tested out some masks that we did buy from clothing manufacturers, like Onzie and Reformation, and those didn't perform particularly well—[they removed] 50% or so of particles [for the mask wearer], which is standard."
How is the average person supposed to figure out which mask to get? "It's so hard," Mueller says, "and most people can't go buy 25 different things and see what they think." But, she says, it's also simple: most fabric masks work well enough for most people. "We're not comparing apples to apples" if we talk about fabric masks as equivalent to N95s, Mueller says. "They're not. But we never claimed that they were. For what they're being made to do, lots of them do a really good job."
So can regular people make sure we're at least getting into the range of 60% of all particles blocked — and ideally, higher? Researchers have shared some principles to keep in mind, in addition to fabric and fit.
Many people—and clothing companies—reference the "flashlight test," where you shine a light on your mask, and the amount of light that you can see through it tells you how effectively it filters the air. Fischer cautions against relying on that test alone, as it involves too many tricky variables. "It gives you an idea, but if one person has a strong flashlight and another person has a weak one, or if the mask is black versus white, I'm sure that makes a difference" with how much light shows through. He prefers "the famous candle test, where you try to blow out a candle. If you can easily blow through the mask, that means that air easily goes through, and of course, with air come particles."
Mueller points out, too, that as the pandemic wears on, our masks may wear out.
"I think everybody has an experience of watching fabric age. We all have that favorite tee shirt that we've washed 75 times, and it starts to get thinner and thinner. Nobody has really comprehensively measured how well [mask] materials will continue to [perform] over time. If you start with something that's a fluffy cotton, that's more like flannel, and over time you watch it get threadbare, that process is probably telling you that you're losing efficacy. Know what your starting point is, and then see how [your mask] changes." Indeed, vintage tees are comfier, and well-loved masks probably will be, too, but that doesn't mean we should wear them.
Mueller also cautions people to avoid valve masks. "Those masks are protecting you, because when you breathe in, the air is still going through the mask, but when you breathe out, 100% of the air is going through that valve, so [valve masks] aren't contributing anything to the public health effort."
Researchers also suggest wearing different masks for different settings.
"If I'm walking around the city, and pretty far away from people, or if I'm standing in the classroom, and my students are 12 feet away from me, I'll wear my regular cloth mask," Fernandez says, "because it's easier for them to understand me, and it's easy for me to breathe in it." On the train, where it's impossible to socially distance, Fernandez wears a (non-N95) respirator with two filters.
"I'm not convinced that I'm doing myself any favors—it is a little hard to breathe through," Fernandez says. "As soon as I get off the train, I take it off, and I put my regular cloth mask back on." And, when she's running, "I've got a bandana around my face."
It's frustrating not to have a clearer answer ("these masks from this company are best," or "always use a mask with a filter," or "three layers of cotton is most protective"). The answers we do have are less tidy. Mueller thinks that's in part because masks serve a dual function. We wear life jackets and seatbelts, for example, only to protect ourselves, but they are also a social good – in that they're protecting others, too.
"We're looking at this one item to serve two purposes. In discussing how effective it is at protecting you, and how effective it is at protecting other people, the discussion becomes complex quickly, and it can be confusing to people," Mueller told Salon.
Fernandez says that she isn't as worried about specific mask efficacy, as long as people are actually wearing them.
"My dad's in his 80s," she says, and "he's wearing a standard medical mask. I'm just glad he's participating in putting something on his face. I'm satisfied with that, and as long as he's socially distancing and washing his hands, I'm not too worried."