The Most Effective COVID-19 Face Masks

You've probably witnessed or purchased an array of face masks—multi-layer surgical masks, home-sewn cloth coverings, repurposed bandanas, and fleece neck gaiters, to name a few. It has been proven that wearing a mask is one of the ways that helps to control the community spread of COVID-19. However, not all face mask options stack up when it comes to protecting other people from your (potentially infectious) respiratory droplets.

To test the options for protection, researchers at Duke University in Durham, North Carolina, devised a simple experiment that examined the efficacy of commonly available masks. By estimating the number of droplets transmitted through each item worn during normal speech and measuring respiratory droplets produced without a face covering, they were able to prove which COVID-19 face masks were better than others.

While there was plenty of research on N95 respirators and surgical masks used in healthcare settings, "there's very little science around the non-medical masks," Eric Westman, MD, an associate professor of medicine at Duke and a co-author of the study, told Health. As a result, this study was done to learn which face masks best protected essential workers and other vulnerable populations in the community.

Dr. Westman and colleagues at Duke tested 14 different face masks, and a swath of polypropylene, in an experiment they designed. 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.

For each mask they tested, they recorded approximately 40 seconds of video. The first 10 seconds was the baseline where nothing was spoken into the black box. For the next 10 seconds, the person wearing the mask being tested repeated the phrase "Stay healthy, people" five times. Then, a 20-second observation period followed. These steps were repeated 10 times for each mask being tested, for the study, and also for the no-mask control trial. A computer algorithm counted the number of respiratory particles in each video.

The most noteworthy finding was that the fitted N95 mask proved to be the best mask overall, with a droplet transmission of below 0.1%, as noted for this study in Science Advances. In other words, explained Dr. Westman, "those masks blocked transmission of about 99% of respiratory droplets." However, the US Food and Drug Administration recommends the general public not to use the tight-fitting N95 masks, the type that healthcare workers use, when there are shortages of these masks.

Dr. Westman and colleagues found that the next best option for protection was a three-layer surgical mask. These blocked upwards of 95% of transmission, according to Dr. Westman. But several two-layer cotton masks also performed well. "The cotton ones were almost as good as the surgical and the N95s," he told Health.

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," stated Dr. Westman, adding that the double-layer bandana only blocked 50% of the speaker's respiratory droplets, and tests of a single-layer fleece gaiter suggested 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, Dr. Westman explained to Health.

The 14 masks used in the study were photographed and the researchers listed how each mask performed. The propylene face covering was not included, however, here is the ranking list of each mask performed, from best to worst, with the corresponding photograph number in parentheses:

1. Fitted N95 mask with no exhalation valve (#14)
2. Three-layer surgical mask (#1)
3. Cotton-polypropylene-cotton mask (#5)
4. Two-layer polypropylene apron mask (#4)
5. Swath of polypropylene mask material (not shown)
6. Two-layer pleated cotton mask (#13)
7. Two-layer pleated cotton mask (#7)
8. N95 mask with exhalation valve (#2)
9. Two-layer "Olson style" cotton mask (#8)
10. One-layer Maxima AT (air textured) mask (#6)
11. One-layer pleated cotton mask (#10)
12. Two-layer pleated cotton mask (#9)
13. Knitted mask (#3)
14. Double-layer bandana (#12)
15. No mask
16. Gaiter-type neck fleece (#11)

The research team did not attempt to test all mask designs or potential uses. So, it is not known what would happen if someone coughed or sneezed and it was unclear whether the laser captured all of the speaker's transmissions or whether the cell phone camera was sensitive enough to detect tiny droplets.

"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," co-author Martin Fischer, PhD, associate research professor in Duke's Department of Chemistry, who set up the testing apparatus, stated in a prepared statement.

While the Duke study is far from the last word on the efficacy of face masks, the study reinforced the role that masks play in reducing the spread of COVID-19. As Dr. Westman pointed 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 stated, 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," added 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.