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Sunday, March 3, 2024

Scientists Put Masks to the Test—With a Cell Phone and a Laser

Eric Westman needed a laser. And someone who knew how to use it.

It was April, and the pandemic coronavirus SARS-CoV-2 was still burning through New York City. Five hundred miles away, in Durham, North Carolina, Westman had been hard at work for weeks, raising money to pay out-of-work costume designers and sewing hobbyists who’d organized online to make hundreds of cloth masks out of hard-to-find fabric and elastics donated by a local ballet company. A physician and obesity researcher at Duke University, Westman had teamed up with other doctors and community organizers to distribute the masks for free to nursing homes, jails, homeless encampments, and to other places with vulnerable populations in the Research Triangle. They focused on reaching people who couldn’t easily socially distance, like farm workers, bus drivers, and grocery store employees, especially in Black and Latinx neighborhoods, which were expected to be hit the hardest.

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Their work had caught the eye of local government officials, who backed their efforts by giving them funds to buy off-the-shelf cloth masks. Before Westman’s team made any bulk purchases, though, they wanted to test each mask’s claims of virus-impeding powers. A common rule of thumb is the so-called sunshine test—if light gets through the cloth, the weave isn’t tight enough to stop infectious particles. But that didn’t seem scientific enough. Researchers at the National Institutes of Health had recently shown they could use lasers to visualize the stream of droplets people produce while speaking, whether or not they are wearing a face mask. Westman wondered if anyone at Duke could do something similar. “We were about to spend tens of thousands of dollars of taxpayer money on these masks,” he says. “We wanted a more systematic way of evaluating them than just holding them up to the sun.”

As cloth masks have become a must-wear accessory in Covid-era America, there remains scant information about how well various designs and materials stop the spread of viral particles. Researchers in high-tech aerosol labs are actively working on this question. But without established standards for fiber-count and filtration performance, people are still mostly winging it when it comes to figuring out which mask is right for them. That could soon change. Westman’s quest for answers led to the publication last week of the first blueprint for a low-cost testing device which promises to aid efforts to make masks that work, and to weed out ones that don’t.

In May, Westman started calling and emailing his contacts at Duke, looking for anyone who could help. His request eventually reached Martin Fischer, a chemist and physicist who specializes in using ultrafast light-pulsing technologies to peer beneath the surfaces of objects, from human skin to 14th-century paintings. In other words, he’s a laser expert.

Fischer had some ideas about how to get Westman’s team the data they wanted. He’d need a box, a laser, a video camera (he settled for a cell phone), and a pixel-to-particle conversion algorithm. Getting that stuff would be easy. But he’d also need another person to help him operate his imagined contraption—one to speak into it, and one to record them doing it. Duke officials had enacted strict restrictions against people from different households commingling on campus. So Fischer entreated his daughter Emma, a Duke neuroscience student, to help him out. She agreed, and the school granted them special permission to get into Fischer’s lab.

Over the course of a single weekend this spring, the duo hacked together a simple device for recording and measuring how many particles escape from a person’s mouth when they are speaking. It worked like this: Step one, lab lights off. Then in the dark, one person put their face flush up to a funnel attached to the cardboard box and repeated a stock phrase five times in a row. (Fischer the elder chose “Stay healthy, people.”) As he spoke, the plume of respiratory particles—tiny spheres of mucus and other mouth, nose, and lung gunk—escaping from his lips was channeled through the funnel and into the enclosed box. Inside, those droplets encountered a band of green light created by a laser the duo had positioned to shine through a slit on one side of the box. Every time the particles crossed the beam, they lit up in a fireworks display of bright green flashes. The cell phone’s camera, staged opposite to the funnel, captured the show. Even for someone who works with lasers all the time, Fischer was shocked at the amount of ejecta produced by his own speech: ”It was like Christmas in there.”

Thanks to the reliable laws of physics that describe the relationship between the size of a particle and the amount of light it gives off when it gets scattered by a laser beam into the lens of a camera, Fischer was able to back-calculate the size of the smallest particle they could detect: half a micron. Knowing that, he was able to quickly write a bit of computer code that opened the video footage, tracked individuals particles from frame to frame, and quantified the number of detectable particles emitted. In the end, this produced a picture of how many particles built up in the box during about 35 seconds of talking. That was the control.

Next, he repeated the experiment wearing 14 different masks, including N95s—valved and unvalved—surgical masks, a bandana, a spandex-blend neck gaiter, and cotton masks of varying designs. Then his team, which included a handful of physics and engineering colleagues collaborating remotely, compared the ratio of droplets produced while wearing each one to the no-mask control and ranked each one accordingly. These results were published Friday in Science Advances.

By far, the mask that was best at blocking a speaker’s exhaled particles was the fitted, unvalved N95, for which “we did not detect any particles at all,” says Fischer. The surgical mask performed similarly well, blocking almost all detectable speech particles, followed by cotton masks that contained a layer of polypropylene. Most other cotton masks fell into the middle of the pack, along with valved N95 masks, which are designed to protect the user from inhaled environmental threats like wildfire smoke, pollution, and pathogens—but because they contain an exhalation valve, do little to block potentially infectious particles from escaping. The bandana did next to nothing. But that wasn’t even the worst. The neck gaiter, made out of a lightweight, breathable fabric favored by runners and cyclists, let through even more particles than the control group—110 percent relative to wearing no mask at all.

If you’re wondering how that is even possible, you’re not alone. Fischer was similarly stumped. Then he went back and looked at the footage again of himself wearing the neck gaiter. “You can see that it’s not just that there are more particles, but that on average, the particles are much smaller,” he says. His team believes the stretchy, porous material is actually fracturing bigger, heavier droplets, splintering them into tinier particles that can more easily remain suspended in the air.

If that’s true, it would blow up the maxim that any mask is better than no mask, says Kimberly Prather, an environmental aerosol researcher at UC San Diego who was not involved in the study. But there’s another possible explanation: Maybe the extra particles aren’t all respiratory droplets. Instead, they could be fibers shedding off the material itself. This has been shown to happen before, and would be easy enough to test—but Fischer and his coauthors didn’t. “Splintering would be bad, but we don’t know for sure that’s what’s going on,” says Prather.

She also points out that the sample size for most of the mask testing is precisely one person. The study doesn’t capture all the variability in how people’s face shapes and speaking patterns might affect the effectiveness of different kinds of masks. So, while this project’s results are in line with other, larger, more rigorous studies, one shouldn’t read too much into the performance outcomes of individual masks based on this study alone, she says.

Still, Prather is impressed that the Duke team’s technique can detect particles down to half a micron. Most laser visualization methods are sensitive only to about 20 microns. “That’s a big deal, because this captures aerosols—the particles that come out during speech—not just bigger droplets emitted during coughing or sneezing,” she says. “Keeping it in perspective, I think it’ll be a great comparison tool to look at variability between people, more conditions. There’s a lot of different things you can do with the setup they’ve developed.”

Fischer and Westman also recognize the study’s shortcomings. “This was never going to be a definitive ranking of all masks under all types of conditions,” says Fischer. Doing that would require hundreds, or even thousands, more people testing lots more masks. “What we don’t want people taking away is: ‘This mask will work. This will not.’ It’s not a guide to masks. It is a demonstration of a new, simple methodology for quickly and somewhat crudely visualizing the effect of a mask,” he says.

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The publication has enough information for many researchers to reproduce the device. But Fischer’s team is now working on refining the design and creating step-by-step instructions so other people can create their own versions. For one thing, you don’t need to use a laser as powerful as the one in Fischer’s lab—a pump laser that costs upwards of $200,000. A simple 2-watt green laser, “basically a laser pointer on steroids” will get the job done and can be purchased online for about $100, he says. Other modifications, design specifications, and safety protocols will make up the blueprint, which they are planning to distribute later this year. Over the weekend he got an email from a North Carolina museum interested in building an interactive laser box exhibit so people could come in and see the effects of wearing a mask for themselves.

As for Westman’s team, Fischer’s contraption helped them rule out the masks that did little to limit one’s particle plume, and identify designs that appeared to block particles almost as well as the N95 masks that hospital administrators and elected officials were scrambling to acquire for their health care workers in the spring. “The most important thing for us was learning that we could feel confident that two-layer cotton masks would be a product that would work,” says Isaac Henrion, a Durham community organizer who Westman hired to help procure masks and assist with scaling the distribution effort. Both he and Westman declined to name specific suppliers they chose to work with—or not. But with data in hand, they started signing purchase agreements. To date, they've given away at least 125,000 reusable cloth masks for people in North Carolina.

Still, months after he started his face-covering crusade, Westman is still seeing patients every day who refuse to wear masks outside the doctor’s office. “It’s totally vexing,” he says. He had expected that by this point the public debate would be focused on which type of masks people should wear, not whether to wear one at all. “If people knew—if they could just see—how many particles come out of their mouth when they speak, maybe that would make a difference,” he says. “It should be obvious, but it seems like there are a lot of people who just don’t know.”

He’s hoping they can change that, one laser at a time.

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