En español | You don’t need a fancy medical-grade mask to deter the spread of the coronavirus, but some face coverings work better than others, according to a new study published in the journal Science Advances.
Researchers at Duke University tested the effectiveness of 14 commonly available masks and found that surgical masks and N95 respirators without valves worked the best at blocking respiratory droplets from projecting into the air when a person talks. A variety of double-layer polypropylene and cotton masks also reduced a significant amount of spray from normal speech.
Least effective, however, were a bandanna, a knitted mask and a neck fleece (also called a gaiter mask); the last may be worse than wearing no mask at all. The researchers found that the neck fleece they tested — a thin single layer of polyester and elastane (also known as spandex or Lycra) — actually dispersed more spray into the air, not less. They assume that the material broke down larger respiratory droplets into smaller particles.
5 Most Effective Masks
Fitted N95 (no exhalation valve)
- 3-layer surgical mask
- Cotton-polypropylene-cotton mask
- 2-layer polypropylene mask
- 2-layer cotton pleated-style mask *
*Researchers tested three 2-layer cotton pleated-style masks. See the full study for photos and specific design details of all masks tested.
3 Least Effective Masks
- Gaiter-type neck fleece
- Double-layer bandanna
- Knitted mask
Source: Duke University
“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 wrote.
Martin Fischer, director of the Advanced Light Imaging and Spectroscopy facility in the Duke Department of Physics and coauthor of the study, said in an online video interview that the research was not a systemic study of mask materials but, rather, was done to establish a simple way to test the effectiveness of masks.
“Overall, the majority of those masks we tested work just fine,” he said.
As to the gaiter, Fischer noted that they tested only one they had on hand and it was thin enough to see through when held up to the light. Gaiters made of thicker materials or folded over would likely produce different results.
“We haven’t tested them. I’m convinced the results would be different. They might be better,” Fischer said.
Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, who studies aerosols, was skeptical of the Duke testing method and conducted her own test on two types of gaiters, following National Institute for Occupational Safety and Health (NIOSH) standards.
“We tested neck gaiters with more conventional methods, and I am pleased to see that they perform similarly to cloth masks and very well if doubled over,” Marr wrote on Twitter.
Face masks are most effective when worn by all
In recent months public health experts have stressed the importance of wearing face masks as a way to deter the spread of the coronavirus, especially by asymptomatic carriers, or people who don’t know they have the virus because they never develop symptoms. Face coverings act as a barrier by helping to keep respiratory particles from escaping an infected individual and landing on another person.
“Just speaking lets these particles get out, and these particles can carry viruses,” including the virus that causes COVID-19, explains Eric Westman, M.D., an associate professor of medicine at Duke University School of Medicine and a coauthor of the study. And knowing which masks are best at blocking these particles can help guide consumer decisions that affect the spread of the virus, he adds. The researchers designed a setup consisting of a box, a laser, a lens and a cellphone camera to test the masks.
Want to know how your mask fares? You don’t need a team of Duke scientists and a high-powered laser to find out. A quick and easy way to tell if a mask is good at blocking particles is to pull it tightly and hold it up to the sky. “If you can see daylight through the mask, it’s not so good. Or if you can take it and blow really hard, and [your air] just goes right through, that’s not blocking much,” Westman says.
His advice to the general public, and especially to those concerned about their risk of severe illness from a coronavirus infection, is to “wear the best one that you can.” If a standard three-layer surgical mask is available to you — and some are now showing up on store shelves — go for it. But know that your double-layer DIY cotton mask is also fine in the community setting. Just be sure not to wear an N95 mask with a valve, Westman says. These masks release exhaled air, which defeats the purpose of wearing the mask to protect others.
Westman says more research is needed to test mask effectiveness, but in the meantime, the study shows that low-cost options do work.
“If everyone wore a mask, we could stop up to 99 percent of these droplets before they reach someone else,” Westman said in a Duke University news release. “In the absence of a vaccine or antiviral medicine, it’s the one proven way to protect others as well as yourself.”
Editor’s Note: This article was updated to clarify the most effective and least effective masks, according to the Duke University study.