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NEWS : Covid-19 Resource Center article

Mask’s fit may determine how well it protects against COVID-19
Researchers found that wearing a medical procedure mask snugly beneath a cloth mask nearly doubled the effectiveness of filtering out particles the size of SARS-CoV-2, compared with wearing just one mask.

They said the elimination of any gaps — rather than simply adding layers of materials — is the reason this approach works better.

The CDC has previously recommended double masking to protect against COVID-19. However, the study that led to that conclusion was “limited” in the types and combinations of masks that were tested, according to Emily Sickbert-Bennett, PhD, associate professor of infectious diseases at the UNC School of Medicine, and colleagues.

"Medical procedure masks are designed to have very good filtration potential based on their material, but the way they fit our faces isn't perfect,” Sickbert-Bennett said in a press release.

The researchers asked three volunteers — including two men with shaven faces — to perform several common job-related movements inside a 10-foot by 10-foot stainless steel exposure chamber with an air temperature of 22°C to 24°C and humidity of 42% to 52%. The air in the chamber contained aerosols from small salt particles. The volunteers wore a combination of medical procedure and cloth masks, which were fitted with a device that measured the concentration of particles entering the breathing space beneath the masks. This allowed the researchers to compare the particle concentration under the masks with that in the chamber, giving them the fitted filtration efficiency (FFE).

The researchers wrote in JAMA Internal Medicine that the mean FFE when one procedure mask was worn ranged from 43% to 62%. On average, wearing a second procedure mask improved mean FFE from 55% when one mask was worn to 66% when two masks were worn. A single cloth mask performed less efficiently (mean FFE range = 41%-44%) compared with a medical procedure mask.

Although wearing a medical procedure mask (mean FFE = 61%) over the cloth mask modestly increased FFE (mean range = 55%-60%) “the overall performance was no different than wearing the procedure mask by itself,” the researchers said. However, wearing the procedure mask under the cloth mask yielded “marked improvements” in overall FFE (mean range = 66%-81%).

“The improvement in the FFE of medical procedure masks when doubled or when worn underneath reusable cloth face coverings is consistent with minimizing leaks between the mask and facial skin, including the bridge of the nose,” the researchers wrote.

Therefore, “the best kind of double-masking is when you and the person you are interacting with are each correctly wearing a very snug-fitting mask,”