‘It’s not N95 or bust’

Va. Tech researcher finds three-layer cloth masks are 74% effective at protecting mask wearers, others

Published November 23, 2020 by Sydney Lake

With the protective face mask market growing to meet the demand of both the general public and first responders, it’s a challenge for consumers to choose the most effective mask to purchase to stay safe amid the COVID-19 pandemic.

Linsey Marr, the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech, might be able to help with that decision. She found through research — based on testing different materials — that a three-layer mask is 74% efficient in filtering out particulate matter, including the COVID-19 virus, protecting both the wearer and those with whom they may come into contact.

Regular cloth masks made from common fabrics including cotton and acrylic are about 40% effective at filtering out virus particles, according to her research.

“Masks were originally advertised for source control to reduce the amount of viruses … but it turns out that masks … offer similar protection to the wearer,” Marr says. “If it’s able to filter out 80% of what’s going outward from my mouth, it also filters out somewhere around 80% of what I would be breathing in from the air around me.”

Expanding beyond testing used to develop standards for N95 masks that can block 0.3-micron particles, Marr’s study was set up to represent conditions closer to a real-life experience of a mask wearer in the general public. The COVID-19 virus is about 0.1 microns in size, but “doesn’t come out of us naked,” Marr says, as it’s carried in larger respiratory droplets (aerosols) that contain salts and proteins. This means an aerosol could be up to 100,000 times larger in mass than the virus itself, leading Marr’s team to study particulate matter of up to 100 microns.

To test inward (protecting the wearer) and outward (protecting others) efficacy, Marr’s team mounted two mannequins on opposite sides of a chamber to mimic people talking closely. A medical nebulizer generated droplets from the exhaling mannequin’s mouth while a vacuum line pulled air containing emitted particles to the inhaling mannequin.

Materials tested included bandanas, thin cotton, thin acrylic, microfiber, vacuum bags; coffee filters, a MERV 14 filter and cotton.

Based on the findings from her study, Marr suggests using a three-layer mask with a structured outside layer that can fit close to the face with a wire over the nose bridge, a middle layer with filter material as well as a layer to protect the filter itself.

“It’s not N95 or bust,” Marr says of her findings. “Cloth masks fall somewhere in between. They do help reduce the amount of virus that’s going to be in the air and reduce the amount that the wearer breathes in. And really anything we can do to reduce that is going to be helpful.”

Of the tested materials, the vacuum bag performed best, with an outward protection efficiency and inward protection efficiency measured at greater than 50% and 75% respectively for particles 0.5 microns or larger. 

“No one intervention is 100% effective,” Marr says. “Distancing helps. Masks help. Ventilation helps. Hand washing helps. Maybe each of those things only helps 50%, but when you combine them all together you’ve got a greater than 90% risk reduction.”

Marr’s official mask-making recommendation? “Make it three layers, with a soft, flexible, tightly woven fabric layer up against the mouth; a layer made of material designed to filter particles, like a vacuum bag or MERV 14 filter; and finally, another soft and tightly woven outer layer of fabric. The face covering should fit snugly against the wearer’s face with no gaps that could affect its performance.”

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