Face masks are about as effective at protecting the wearer from COVID-19 exposure as they are at protecting others, a new study from Virginia Tech researchers finds.
Masks were originally advertised to reduce the amount of virus a person exhaled, noted Linsey Marr, a professor of civil and environmental engineering and the study’s lead author.
“If I’m sick, you know, I put on the mask and then it reduces the amount that I released into the air around me,” Marr said Monday. “It turns out masks work essentially the same way and offer very similar protections to the wearer.”
Marr’s niche expertise in the airborne transmission of viruses brought her international attention last spring as evidence mounted that the novel coronavirus spreads through tiny droplets exhaled from the nose and mouth.
Marr’s team first began studying cloth masks in March, when N-95 masks were in short supply and health officials called on the public to make their own face coverings.
“I realized it would be really important for the general public to wear masks because of pre-symptomatic or asymptomatic transmission,” of the virus, she said. “It was up to the public to make their own cloth masks.”
The team tested 10 different kinds of face mask materials — including a microfiber used for cleaning eyeglasses, a thick bandana, a kid’s T-shirt and a pillow case — as well as a plastic face shield.
Manikin heads were fitted with the face coverings. Researchers used a misting machine and a vacuum machine on two different heads to simulate breathing in and exhaling. They then calculated how many various-sized particles from a salt spray were blocked by the masks.
Most of the fabrics could block at least 50% of particles the size of two microns — a rough estimate of a respiratory droplet coming out of a masked mouth — with a vacuum bag, microfiber cloth and surgical mask nearing 90% efficacy.
Researchers also tested two versions of homemade masks recommended by the Centers for Disease Control. They used a 200-thread-count pillowcase and rubber bands for the non-sewn version and a cotton T-shirt for a sewn mask.
While these homemade masks were not as effective at stopping particles less than one micron, they achieved about 50% efficacy for two-micron particles and 75% efficacy for particles greater than five microns (a human red blood cell is about seven microns in size).
“One criticism of masks is, ‘Oh, they don’t work,” Marr said. “It’s not N-95 or bust.”
Cloth masks, in addition to physical distancing, good ventilation and handwashing combine to reduce the risk of contracting and spreading the virus.
“Anything we can do to reduce that is going to be helpful,” Marr said.
The face shield fared the worse by far, based on the study results.
“It has a big gap between the face shield and the face, so the air can easily go through, bypass the gaps,” said Jin Pan, a doctoral student who co-authored the study. “It’s not fully blocked, so everything, the aerosols expelled from people, and the aerosols coming from other people, can get through.”
Marr noted that such shields can be valuable for eye protection, but “we cannot rely on a face shield itself.”
Pan said she was surprised that researchers did not see a huge disparity between how the masks blocked particles coming in versus coming out.
“That difference is not so significant,” she said. “This is the most unexpected result.”
Based on the findings, the team recommends people wear a three-layer mask, consisting of outer layers of tightly woven fabric with an inside layer of a material designed for filtration, such as a vaccum bag or air filter.
That combination should result in an efficiency of 70% for the most penetrating particle size and more than 90% for particles larger than one micron, if the mask fits well, according to the study.
The study, “Inward and outward effectiveness of cloth masks, a surgical mask, and a face shield,” was published Friday on medRxiv, a preprint server for manuscripts about health sciences. The study has not yet been peer reviewed.
The pandemic has accelerated the pace of scientific research, with many COVID-19 related papers becoming distributed in the public sphere before peer review.
Martin Fischer, a chemistry professor at Duke University, said in an email that Marr’s study complements his own research on evaluating the efficacy of face coverings.
“While using a different approach, the results are in essence very similar: there is variability in mask performance based on material and fit, and thicker/denser masks with a better fit tend to perform better,” he wrote. “I think the Marr group’s study is providing important and much needed information about efficacy of a variety of mask types.”