Study: A good cloth mask is a powerful weapon against the coronavirus

WASHINGTON — Face coverings, including masks made of cloth, are highly effective in protecting the people who wear them as well as those around them, according to a new study from Linsey Marr, a leading aerosol scientist at Virginia Tech University.

“Filtration works both ways,” Marr said. “If it works for source control,” she said during a media presentation on Monday — that is, if the mask filters out particles coming from the wearer’s mouth — “it’s going to work pretty well for exposure reduction to protect the wearer also.”

Testing 11 different types of face coverings (nine cloth masks made from coffee filters, cotton and other materials, a surgical mask and a face shield), Marr and co-author Jin Pan found that many of the materials they “challenged” with particles meant to simulate the coronavirus exhibited a 75 percent filtration capacity. A high-quality cloth mask consisting of three layers could allow filtration as high as 90 percent. Crucially, that accounts for incoming particles as well as outgoing ones.

The efficacy of surgical masks had been widely known. Their use was endorsed in a Wall Street Journal op-ed by former Food and Drug Administration head Scott Gottlieb published on Sunday evening. He encouraged people to wear N95 respirators or, failing that, surgical masks. “The level of protection depends on the type of the mask,” he wrote, cautioning people away from bandanas and other loosely-fitting face coverings.

Marr’s study suggests that cloth masks work better than some have claimed, in part because many virus-bearing particles are significantly larger than previously thought. Her study adds crucial new information about the relative benefits of cloth masks, which people find easier to wear — and more stylish — than their light-blue medical counterparts.

In particular, the study noted that a mask that contains an interior filter made from a common vacuum bag “achieved outstanding performance.”

The question of specific materials aside, the study counters the long-held misconception that masks do not protect those who wear them. Last week, some right-wing media outlets seized on a Danish study whose methodology has been criticized as slipshod and whose findings have been deemed inconclusive.

By contrast, Marr’s research involved testing different types of masks by putting them over the faces of mannequins. Those mannequins were then placed inside a plastic bin sprayed with a saline solution. Marr and her team recorded how many of those particles made it through the mask and into the mannequin’s mouth, which was fitted with a sensor.

Marr focused on particles measuring one to two microns in size. That is because the coronavirus tends to attach to larger droplets as they leave a person’s mouth or nasal passage and travel through the air. (People who work in hospitals and could face exposure to sub-micron particles do need to wear N95 respirators, so named for their 95 percent filtration capacity).