During World War II, nylon stockings disappeared from store shelves as the valuable synthetic material was diverted to make critical wartime supplies such as parachutes, flak jackets and aircraft fuel tanks. Now, new research suggests that nylon stockings could once again play a critical role in a national battle — this time by making homemade cloth masks significantly more protective.

Researchers at Northeastern University have found that adding an outer layer made from nylon stockings to a homemade face covering can boost its ability to filter out small particles in the air by creating a tighter seal between the mask and the wearer's face. In some cases, that extra nylon layer helped homemade cloth masks match or exceed the filtering capability of medical-grade surgical masks.

"It really improved the performance of all of the masks, and it brought several of them up and over the baseline mask we were using, which was a 3M surgical-type mask," says Loretta Fernandez, an assistant professor of civil and environmental engineering at Northeastern University and one of the scientists who conducted the research.

Even the 3M surgical mask performed better with stockings in their study: Testing showed that it went from blocking out 75% of small particles to 90% with the addition of a pantyhose overlayer. By comparison, an N95 respirator, which is designed to create a tight seal around the face, blocks out at least 95% of small particles when worn properly.

"Adding a layer that keeps the mask tight to the face is going to improve the function of any of these masks," Fernandez explains, "because how well they protect us is not only a matter of what material we're using to do the filtering but also how well [the mask] seals to the face, so that we're trying to avoid air making it around the mask into our breathing zone." The pantyhose layer, she says, helps creates a tighter seal around the face to reduce how much air leaks around loose edges — similar to the seal on an N95 respirator.

The findings come at a time when the Centers for Disease Control and Prevention is recommending that Americans wear cloth face coverings in public to help reduce the transmission of the coronavirus, but without offering much guidance on the best practices for making such coverings.

The research has not yet been peer-reviewed, but it was posted Wednesday on the scientific preprint site medRxiv and on the university's website in the interest of sharing information quickly in the midst of a pandemic. Scientists who reviewed the study at NPR's request praised it as vitally needed work.

"I think it's really a very important study," says Ben Cowling, a professor of infectious disease epidemiology at the University of Hong Kong who has studied the efficacy of face masks. "We need better information on what kind of homemade masks, what kind of fabric masks, are the best and how we can improve or upgrade basic masks to make them better."

The CDC guidelines on cloth face coverings are intended to protect other people from the wearer, since evidence shows that people can spread the coronavirus before they're even showing symptoms of infection. However, the new research shows that with the added nylon layer, homemade masks may also offer lots more benefit for the wearer.

"Cloth masks," Cowling says, "most likely provide some protection, maybe not as good as surgical masks" — which are constructed with nonwoven fabrics made from plastics. "But if we can upgrade [cloth masks] with nylon wrapping around the outside or some other special components, then perhaps we can get a cloth mask which is just as good or even better than a surgical mask."

"It's a good design feature that they've come up with," says Raina MacIntyre, a biosecurity researcher at the University of New South Wales in Australia and the author of one of the few studies comparing the effectiveness of cloth face coverings with surgical masks. The current shortage of medical-grade masks, she notes, is spurring a new wave of research into creating more effective homemade masks. "There's some really good solutions out there, some really promising ideas. And this looks like one of them."

As part of the research, Fernandez and her colleagues solicited homemade masks from volunteers who were making them to donate to Boston-area hospitals. To test the various masks' filtration capabilities, they used an instrument called a PortaCount — which is normally used to fit-test the filtering capabilities of medical-grade masks like N95 respirators — to measure the ability to block out particles ranging from 20 nanometers to 1,000 nanometers. (The coronavirus that causes COVID-19 is approximately 60 to 140 nanometers in diameter — far too small to be visible to the human eye.)

The device measured the number of particles immediately outside and inside each mask while someone was wearing it. A surgical mask was also tested as a baseline measurement; it blocked out 75% of the particles on average, which is in line with other testing that has suggested that surgical masks filter out between 60% and 80% of small particles in a lab setting.

Then, the researchers added to the masks a nylon stocking overlayer made by cutting a ring of material, about 8 to 10 inches top to bottom, from one leg on a pair of pantyhose. "I would recommend perhaps a queen-sized [pair of pantyhose] just to make breathing easier," Fernandez says. The wearer puts the ring over their head like a headband, then pulls it down on top of the cloth mask, creating a tight fit to the face. This forces particles that might have otherwise gone around the loose edges of the mask and been inhaled to instead go through the mask, which can filter them out, Fernandez explains.

When worn alone, the homemade masks' abilities to filter varied widely, with some blocking fewer than 30% of particles. But adding the pantyhose layer boosted all the masks' performance by anywhere from 15% to 50%, the study found. Tights should also work, as long as they offer a snug fit, says Fernandez, who plans to include tights in future testing.

Fernandez says the idea to try stockings came from a colleague at Northeastern who had previously studied how to make effective homemade masks in the early 1980s, in the wake of the 1979 Three Mile Island nuclear accident in Pennsylvania. "And what they found in the '80s was that if you just put a section of pantyhose over your face and stuffed anything in there, that would do a pretty good job of keeping the fallout particles out," she says.

The homemade cloth masks that performed best in the testing with the nylon layer were all made of a tightly woven cotton, the kind used for quilting, and they all contained a filter of some kind — either organic cotton batting or what's known as interfacing, a lightweight, gauzy textile used to stiffen fabrics, such as shirt collars.

MacIntyre notes that more research needs to be done, such as how many washings the delicate nylon hosiery can withstand before it loses effectiveness. But in principle, she says, it makes sense that people who are donning homemade masks start snipping away at pantyhose and adding it as an extra layer now. Cowling agrees that it "could be an important aspect of everybody wearing face masks."

As research into homemade masks grows, "we'd like to see recommendations from the CDC or elsewhere in the world, from other public health authorities, on what are the best ways to do it," he says.

That said, Cowling stresses that masks alone won't be enough to manage the transmission of the coronavirus as the world begins to contemplate how to emerge from lockdowns. But in combination with other strategies, such as enhanced testing, contact tracing for confirmed cases and continued social distancing measures, masks could help keep transmission of the virus at a low level.

While getting a pair of nylons is pretty easy (for now), questions remain in the public's mind about the best material for a homemade mask. Here are some tips from mask researchers:

Use a thick-weave cotton: In general, thicker, high-grade cotton masks tend to do a better job of filtering out small particles, says Dr. Scott Segal, a professor and chair of anesthesiology at Wake Forest School of Medicine who has been putting various cloth masks to the test since March. His rule of thumb: Hold up the fabric to a bright light or to the sun. If "you can see the light outlining the individual fibers in the fabric, it's probably not a good filter. And if you can't, it's probably going to filter better." Thin T-shirt material didn't do a great job in his testing, though "probably anything is better than nothing," he says. Thicker, heavier-weight T-shirts would probably be better filters, he adds.

Layer your fabric: Cloth masks made from multiple layers seem to do a better job than single-layer ones, says Yang Wang, an assistant professor of environmental engineering at Missouri University of Science and Technology who studies how fine particles like aerosols are transmitted and has been testing how various household items hold up as mask materials. A single-layer mask made from a 400-thread-count pillowcase had a filtration efficiency of around 10%, but if you bumped it up to four layers of cloth, the efficiency went up to around 20%. "It's not ideal, but by using more layers, you can bump up the filtration efficiency," Wang says — just make sure not to use so many layers that you can't breathe.

Editor's note: The original photo on this story has been replaced to show the researcher wearing a mask without a grommet, which was only inserted in the mask for testing. Face masks should be worn with the pleats opening downward, not upward, as in the previous image.

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