In a recently published study, the researchers unveiled a simple method to evaluate the effectiveness of various types of masks, analyzing more than a dozen different facial coverings ranging from hospital-grade N95 respirators to bandanas. Of the 14 masks and other coverings tested, the study found that some easily accessible cotton cloth masks are about as effective as standard surgical masks, while popular alternatives such as neck gaiters may be worse than not wearing a mask at all.
“You can really see the mask is doing something,” said one of the study’s co-authors, Warren S. Warren, a professor of physics, chemistry, radiology and biomedical engineering at Duke. “There’s a lot of controversy and people say, ‘Well, masks don’t do anything.’ Well, the answer is some don’t, but most do.”
The search for a way to determine the effectiveness of different masks began with a request from a professor at Duke’s medical school who was working to provide at-risk and underserved populations in Durham, N.C., with the critical face coverings, according to a news release from the university. Faced with so many varieties of masks all claiming to have virus-blocking capabilities, the professor sought help — in the university’s physics department.
Enter Martin Fischer, a chemist and physicist.
Using a simple contraption that harnesses the power of a laser, which can be easily purchased online for less than $200, and a cell phone camera, Fischer created a device that allowed his team to track individual particles released from a person’s mouth when they are speaking. The rest of the setup includes a box that can be made out of cardboard and a lens.
Testing the face coverings was equally uncomplicated, according to the study published Friday in Science Advances, a peer-reviewed journal.
Speakers said the same phrase into the box without a mask and then repeated the process while wearing one. Each face covering was tested 10 times. Inside the device, the airborne particles passed through a sheet of light created by the laser hitting the lens and produced visible flashes that were recorded by the phone’s camera.
“Even very small particles can do this kind of [light] scattering,” Warren said. “We were able to use the scattering, and then tracking individual particles from frame to frame in the movie, to actually count the number of particles that got emitted.”
A fitted N95 mask, which is used most commonly by hospital workers, was the most effective, Warren said, noting that the mask allowed “no droplets at all” to come out. Meanwhile, a breathable neck gaiter, well-liked by runners for its lightweight fabric, ranked worse than the no-mask control group.
“These neck gaiters are extremely common in a lot of places because they’re very convenient to wear,” Warren said. “But the exact reason why they’re so convenient, which is that they don’t restrict air, is the reason why they’re not doing much of a job helping people.”
The high droplet count could be linked to the neck gaiter’s fabric breaking up bigger particles into many little ones that are more likely to hang around in the air longer, Fischer said in the video. This effect makes wearing them possibly “counterproductive,” he added.
“It’s not the case that any mask is better than nothing,” he said. “There are some masks that actually hurt rather than do good.”
Other types of face coverings that may fall into that category are bandanas and knitted masks, the study found. An N95 mask with an exhalation valve also failed to measure up.
“Those relief valves are fantastic if what you want to do is protect yourself from the outside world because air doesn’t come in through them,” Warren said. “If what you’re trying to do in this pandemic is protect the outside world from you, it completely defeats the purpose.”
Still, he stressed that people without access to medical-grade masks shouldn’t worry.
“We’re not as a society going to be having everybody wear disposable N95 face masks,” he said. “It’s not affordable, and it’s not reasonable.” The researchers specifically made note of the effectiveness of common cotton cloth masks, finding that several of the ones tested performed about as well as surgical masks, which come in second to the N95. Experts with the World Health Organization have recommended that fabric masks should ideally have three layers.
Although the study was “not a clinical trial” that involved testing “10,000 patients and seven different languages and all possible conditions,” Warren said its general conclusions still stand.
“We’re very careful not to over-claim here,” he said. “We are not going to try to say our evidence is that this is the thread count you should use on the sheet for the two-ply cotton mask that you’re making.
“But the broad take-home picture — that masks do work in cutting down transmission and that some masks that you can easily get are better than others — potentially has value in protecting everybody and getting us out of this awful situation,” he added.
Warren said he and his fellow researchers are now focused on producing a step-by-step instruction guide for creating the testing device. The team has already been approached by people from foreign countries, including Africa, who have expressed interest, he said.
“It is quite possible for people with a modest amount of scientific training to use this quite safely and quite effectively,” Warren said, noting that he does not recommend that the average person go out and try to construct the device themselves. “The idea is that you could have community centers, groups that are helping to test out different designs. Particularly as we’re trying to provide face masks to a large number of people who don’t have them, you want to be providing ones that work.”