Americans are sporting a dizzying array of face masks—multi-layer surgical masks, home-sewn cloth coverings, repurposed bandanas, and fleece neck gaiters, to name a few. That's because wearing a mask is one of the fundamentals for controlling community spread of COVID-19. But how do different mask options stack up when it comes to protecting other people from your (potentially infectious) respiratory droplets?
To answer that question, researchers at Duke University in Durham, North Carolina, devised a simple experiment to examine the efficacy of commonly available masks by estimating the number of droplets transmitted though each item worn during normal speech. They also measured respiratory droplets produced without a face covering.
The study grew out of a practical question about which face masks could protect essential workers and other vulnerable populations in the community, notes Eric Westman, MD, an associate professor of medicine at Duke and one of the study coauthors. While there’s plenty of research on N95 respirators and surgical masks used in health care settings, “there’s very little science around the non-medical masks,” he tells Health.
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So Dr. Westman turned to his physicist colleagues at Duke, who created a way to put these face masks to the test. Here’s how it worked: A person wearing a mask spoke into the front of a black box. A laser beam projected through one side of the box formed a green “light sheet” through its center. A camera positioned at the back of the box captured on video any light scattered by respiratory particles passing through the speaker’s mask.
Researchers tested 14 different face masks and a swath of polypropylene. For each test, they recorded approximately 40 seconds of video. The first 10 seconds served as a baseline. During the next 10-second interval, the speaker repeated the phrase “Stay healthy, people” five times. A 20-second observation period followed. The team ran the drill 10 times for each mask and material in the study and for the no-mask control trial. A computer algorithm counted the number of respiratory particles in each video.
Overall, the experiment revealed “stark differences” in the masks’ effectiveness, the study authors point out in a proof-of-concept study appearing online August 7 in the journal Science Advances. The fitted 95 mask proved to be the best overall, with a droplet transmission of below 0.1%. In other words, explains Dr. Westman, those masks blocked transmission of about 99% of respiratory droplets. Tight-fitting N95 masks, the type that health care workers use, are not recommended for the general public due to persistent shortages, notes the US Food and Drug Administration.
protective-mask-efficacy , COVID-19 mask wearing: Different types of face masks protection for coronavirus prevention: reusable homemade cloth wear, disposable medical mask, n95 respirator.
Next best were three-layer surgical masks, which blocked upwards of 95% of transmission, according to Dr. Westman. Several two-layer cotton masks also performed well. “The cotton ones were almost as good as the surgical and the N95s,” he adds.
You might think any face mask would be better than no mask at all, but a few of the options offered very little protection. “The real twist that we didn’t foresee was that a bandana or a gaiter…didn’t really work at all,” observes Dr. Westman. The double-layer bandana only blocked 50% of the speaker’s respiratory droplets, he reports, while tests of a single-layer fleece gaiter suggest that it “might make matters worse.” The neck gaiter appeared to make the speaker’s respiratory particles smaller, creating a greater risk of airborne transmission, he explains.
Here’s a photo of all 14 masks used in the study (the propylene face covering was not included here), along with how the masks performed, from best to worst, with their corresponding photo numbers in parentheses:
protective-cloth-masks-efficacy , Pictures of face masks under investigation
“This was just a demonstration—more work is required to investigate variations in masks, speakers, and how people wear them—but it demonstrates that this sort of test could easily be conducted by businesses and others that are providing masks to their employees or patrons,” coauthor Martin Fischer, PhD, associate research professor in Duke’s Department of Chemistry, who set up the testing apparatus, said in a prepared statement.
While the Duke study is far from the last word on the efficacy of face masks, the study reinforces the role that masks play in reducing the spread of COVID-19. As Dr. Westman points out,“ Everyone emits particles when they speak and these particles are potentially infectious. According to the US Centers for Disease Control and Prevention, an estimated 50% of transmission occurs before people develop any symptoms. Wearing a mask is an easy way to reduce the risk of unknowingly spreading the infection, he says, but it’s not a panacea—people also need to observe social distancing rules and practice good hand hygiene. “Masks don’t take the place of these other measures,” adds Dr. Westman.
The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.
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