Lab study shows which face coverings are effective
When it comes to effectively limiting the spread of the coronavirus, not all masks are created equal.
Masks have been the subject of debate in the United States for months, as inconsistent mandating and skepticism among a segment of the population has led to uneven usage. Yet the tide may be turning. Currently 32 states (including Indiana) and the District of Columbia require wearing masks in public, after repeated studies show it is a powerful tool in the fight against COVID-19. One model projects that more than 30,000 lives could be saved in the U.S. by December if mask wearing was universally accepted.
The science behind that effectiveness is now mostly understood: Masks help prevent aerosol transmission of the virus by containing droplets emitted by an infected person during speaking, coughing or sneezing. These droplets otherwise evaporate into the air and leave behind nuclei that don’t always settle quickly, which can then be inhaled by a non-infected person, propagating the spread.
Below this high-level view is a great deal of nuance. The volume and actual words someone says play a factor, as does the type of mask.
To evaluate these variables, the University turned to David Leighton, professor of chemical and biomolecular engineering. Leighton and Mark McCready, professor of chemical and biomolecular engineering and senior associate dean for research and graduate studies, conducted a test with several types of masks and face coverings to determine which would block the emission of droplets most effectively. They measured the emissions using particle image velocimetry and laser sheet imaging. The methods allowed droplets to be visible and velocity measurements to be tabulated.
Leighton explained that droplets vary widely depending on the volume of the speech and the words being spoken. For example, droplets produced by vowel sounds or yelling at a sporting event — vocalizations that tend to originate from the back of the throat — are usually very small, but can remain suspended for longer periods of time. Consonant sounds — which tend to originate from the front of the mouth — usually produce larger droplets and at a higher volume. These sounds have a high degree of plosive pressure, wherein the air in the lungs is briefly blocked before being released with greater velocity.
The effectiveness of different types of face coverings in blocking these different volumes of droplet emissions varies. Leighton’s experiment showed the droplets speakers emitted while saying two words with high plosive pressure: “pop” and “fish.”
“The first question we had to address was the issue of face shields versus face masks,” Leighton said. “Can a face shield act as an adequate substitute for a face mask? And unfortunately, the answer is no.”
Leighton explained that face shields may only be useful to protect the wearer against large droplets — those about 100 microns in diameter. (A human hair is about 40 microns.) Droplets of that size are usually only emitted by a person sneezing in close proximity. Of course, that’s a rare occurrence. Face shields provided neither adequate blocking of droplets emitted by the wearer during speaking, nor of another individual in the wearer’s presence.
With face shields ruled out, the researchers turned to face masks and coverings. Leighton tested a single-layer fabric mask, a double-layer fabric mask and a gaiter — those scarf-like garments that can be pulled over the mouth and nose.
“You have to wear a mask. There’s just no way around it.” – David Leighton
“The single-layer material does not perform well in the ‘pop’ test,” Leighton said. The issue is simply one of thickness: One layer of fabric is not very effective for blocking droplet emission. A single-layer bandana, for example, only contained about 33 percent of the particles released, compared to wearing no mask.
The double-layer fabric mask performed quite well. In both the “pop” and “fish” tests, the masks contained about 90 percent of the particles released, compared to no mask.
As for the gaiter, the results were mixed. When dry, the gaiter provided some protection, though a fair amount of particles were released in the “pop” test. When the gaiter gets wet, however, the results are clearly negative. This is important because gaiters are often used in exercise.
“Say you’ve spilled water or you have some cumulative moisture,” Leighton explains. “The word ‘pop’ has sufficient pressure to literally cause an explosion of droplets from the mask, and it’s going to carry with it all the viral particles that have been trapped previously on the material.”
The clear takeaway for Leighton is the effectiveness of wearing properly fitting, double-layer masks. One way to conduct a rudimentary test of a mask’s effectiveness is to perform your own “pop” test, Leighton said. Put on the mask, put your hand close to your mouth, and say “pop.” A good mask should be able to limit the amount of air (and the droplets it contains) from coming through, and you wouldn’t feel much of an effect on your hand. Moreover, a good cloth mask can provide some protection against droplets being inhaled by the wearer.
“So the question is, are you protected by wearing a mask?” Leighton said. “Primarily, you’re protected by other people wearing a mask. If nobody is infected, nobody has to wear a mask. But the problem is, there are going to be infected people around us and most of those don’t know it. You don’t know. I don’t know.
“You have to wear a mask. There’s just no way around it.”