Study raises questions about efficacy of gaiters, bandanas, knit masks in stopping spread of virus

Duke University researchers tested more than a dozen different face coverings.Emma Fischer/Duke University

Duke University researchers have developed a simple, inexpensive method to test how well masks stop people from spewing droplets that could spread the coronavirus — and their initial tests have raised questions about the virus-blocking efficacy of neck fleeces and bandanas.

“Some mask alternatives, such as neck fleece or bandanas, offer very little protection,” researchers said in the study, which was published Friday in Science Advances.

One problem in both cases was simply the thinness of the fabric, said Professor Warren Warren, a professor of physics, chemistry, and radiology who was one of the co-authors of the study.

He said a general rule of thumb is: ‘If you have a mask and you can see through it in the light and you can blow through it, it’s probably not a very good mask.” A knitted mask also performed poorly in the study.

11coronamasks - Dr. Martin Fischer, Ph.D. developed a simple, low-cost technique to visualize the effect of face masks on droplet emissions during normal wear. Illuminated through a sheet of laser light, droplets from speaking with no mask are seen on the top right compared to different masks in the three other boxes. Shawn Rocco/Duke Health Shawn Rocco

The study also said that “speaking through some masks (particularly the neck fleece) seemed to disperse the largest droplets into a multitude of smaller droplets, which explains the apparent increase in droplet count relative to no mask in that case. Considering that smaller particles are airborne longer than large droplets (larger droplets sink faster), the use of such a mask might be counterproductive.”

The researchers were trying to determine whether masks blocked droplets from being released into the air, not whether they blocked the virus from coming in. Public health officials say people should wear masks to prevent the spread of the virus, and mask orders are in force in dozens of states.

“I think most people really want to do the right thing” and prevent the spread of the virus, Warren said. “If you’re really going to be trying to do that … you have to think a little bit more about what sort of style of mask you’re wearing.”

The study also offered some good news, finding that a paper surgical mask that was tested worked almost as well as a N95 mask in blocking the outflow of droplets, and a number of homemade cloth masks also did a good job.

“What we’re really seeing is that most of the homemade solutions, if you fit them right so there aren’t big gaps, they do a pretty decent job, working just about as well as the disposable medical masks,” Warren said.

The researchers studied mask efficacy by asking subjects wearing masks to speak the words “Stay healthy, people,” in the direction of an expanded laser beam inside a dark enclosure. Droplets in the laser beam then scattered light, which was recorded with a cellphone camera. A computer algorithm was used to count the droplets, researchers said in the study.

The researchers said the equipment needed was “commonly available,” inexpensive, and “the experimental setup is simple and can easily be built and operated by non-experts.”

The researchers noted that they were conducting “proof-of-principle experiments” that only involved a small number of speakers, but suggested their method could serve as a base of future studies.

“It’s worth understanding that this is not a clinical trial with 10,000 people in every possible circumstance you can do,” Warren said. But he said researchers wanted to get some information out, rather than “get a paper out in the fall of 2021,” when it could be too late to be of any help.

The researchers suggested that the measurement method could be used to guide “mask selection and purchase decisions.” They said their method had already been used to guide a mask-purchasing decision for the “Cover Durham” initiative in Durham, N.C.

Jill Crittenden, a researcher at MIT who is member of the mask expert group n95decon.org, said, “I think that’s great that they want as many people to be testing as possible.”

The study shows that “masks vary a lot in how much they protect other people,” she said.

She also said she found some of the findings shocking, including the finding about the neck fleece. “You could imagine some really large droplets essentially go through a sieve and become smaller particles, which are known to travel farther, so that’s worse than nothing,” she said.

She also emphasized that the “fit of the mask is important.” The study looked at droplets that were being expelled in a forward direction, but, “if you have huge gaps on the side, for example, it could be blowing out the side of your face,” she said.