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"We focused primarily on nonmedical-grade masks that are recommended for use by the wider public," said lead author Siddhartha Verma. He's an assistant professor at Florida Atlantic University's Department of Ocean and Mechanical Engineering.
Using a mannequin head, air pumps and lasers, Verma and his team tracked emissions from a mechanically generated cough to see how well bandana-style coverings, folded cotton handkerchiefs, and/or stitched masks managed to contain them. Those are the types of face coverings that are widely available to the general public.
As a means for blocking cough droplets, thin, single-layer bandana-style coverings were almost worthless, the experiment found. But lined cotton masks with two layers of material stitched together were the most effective.
The findings are reported in the June 30 issue of Physics of Fluids.
For the study, the researchers placed a mannequin head at 5 feet, 8 inches, the height of an average man. It was outfitted with a manual air pump that roughly approximated the force and volume expelled by a typical cough.
Finally, a smoke machine laced the expelled air with distilled water and glycerin -- a setup that allowed lasers to see any microscopic droplets as they flew out the mannequin's three-quarter-inch mouth opening.
Minus any mask at all, unobstructed "coughs" generated droplets that traveled up to 12 feet over 50 seconds -- twice the 6-foot social distancing guideline that is a pillar of coronavirus control measures from the U.S. Centers for Disease Control and Prevention.
A handkerchief folded according to instructions from the U.S. Surgeon General did stop some of the coughed emissions. But there was "notable leakage," the tests found. And a single-layer bandana did an even poorer job.
But a homemade mask made of two layers of cotton (with 70 threads per inch) stopped emissions "almost completely" and with little leakage. (A pharmacy-bought nonsterile cone-style mask was better than a handkerchief or bandana, but not as effective as the stitched cotton mask).
"I'm not surprised at all," said Pete Jobst, director of college facilities at the Virginia-Maryland College of Veterinary Medicine in Blacksburg, Va.
But, he said, people should know that no consumer-stitched cloth mask will do what a medical-grade mask will do.
Why? Because unlike the professionally made personal protective equipment (PPE) that front-line hospital workers rely on to stay safe, "the cloth mask is not a respirator," Jobst said. PPEs are designed to protect the wearer. Consumer masks only protect other people, Jobst said.
But some consumer cloth masks do a better job than others, said Amy Price, a senior research scientist with the Anesthesia, Informatics and Media (AIM) Lab at Stanford University in Palo Alto, Calif.
Consumer masks are important, she said, because they keep virus particles that you might be releasing inside the cloth, and by doing so, you "create a mask-driven kind of herd immunity, based on everyone wearing a mask and everyone providing protection for each other."
With that goal in mind, Price and Stanford colleague Dr. Larry Chu, director of the AIM Lab, have a three-step formula for making an effective cloth mask.
First, "to stop the virus from spreading, you need to have an outer layer of cotton-polyester that's fairly tightly woven and repels moisture," Price said. "Basically, you don't want to see through it, because if you can see through it, the virus can see through it, too."
But you also need to breathe. So choose a material that is both tight and breathable.
Then add a second layer: Place a nonwoven material between the outside fabric and your mouth and nose. Price said that could be a lightweight craft-store polyester, or even two or three sheets of facial tissue. "The idea is that you almost create an obstacle course for any viral particles. And when you're back home, you throw this layer away," she said.
Finally, cover the nonwoven layer with a third, inner layer of soft cotton that can wick away moisture. That will help keep the mask comfortable, because bacteria and sweat can build up on the inside.
"The mask that's the most important one is the one that you're actually going to wear," Price said.
But remember, even a bandana is better than nothing -- if that's all you have.
"We know we can do better, without it costing very much more, so why not?" Price said. "I mean, if you have the opportunity to get a flu shot that's 1% effective or one that's 70% effective, which would you choose?"
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