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.

Copyright NPR 2021.