Comfort at the cost of less protection
Study finds face shields and valved masks allow particles to escape
Face shields and valved masks— two options many people find more comfortable than cloth face coverings— appear to be less effective at blocking viral particles than regular masks, a new study shows.
The Centers for Disease Control and Prevention had already stated that clear plastic face shields and masks equipped with vents or valves are not recommended, because of concerns that they don’t adequately block viral particles. But the new research, which uses lasers to illuminate the path of coughs, offers a striking visual demonstration of howlarge plumes of particles can escape from behind a face shield or vented mask.
“I think these visualizations are really powerful for helping the general public to see and understand what’s happening,” said LinseyMarr, a professor of civil and environmental engineering at Virginia Tech who studies airborne particles but whowas not involved in the research.
The news will be disappointing to those looking for an alternative to regular face masks. Teachers and students, in particular, often prefer face shields because they are more comfortable towear over long periods of time, can be easily cleaned and allowfor better communication because they don’t muffle the voice or hide facial expressions. Valved masks, with one-way vents designed to allowbreath to escape while blocking germs fromentering, can feel more breathable and prevent the mask from getting moist as quickly.
But the new research, published in the journal Physics of Fluids, shows that face shields and vented masks allowlarge plumes of particles to escape, putting those around you at risk. And while the research did not specifically look at the level of protection the shields and masks offer thewearer, it does suggest that people who use them may also be more vulnerable to exposure than if theywore a regular mask. Valved masks are a particular concern as some of the nonmedical vented masks the researchers used had faulty valves, suggesting that some people may be walking around with open valves— essentially large holes— in their masks.
To conduct the research, scientists fromFlorida AtlanticUniversity used hollowfoam heads fitted with various face coverings. They pumped a vaporized mixture of glycerin and distilledwater through the heads to simulate a cough or sneeze, and used lasers to illuminate the path of the plume.
In two separate studies, the researchers showed that even the bestmasks allowsome particles to escape. In videos of tests using the gold-standard mask, anN95, a puff of particles can be seen jetting out around the bridge of the nose, where the fit is poor. That said, the researchers found that N95 masks, cloth masks and papery medical-style masks all block a significant amount of particles and appear to offer adequate protection for the typical person in the community who is practicing social distancing.
But when the hollow headswere fitted with clear plastic face shields or valved masks, the results were less encouraging. While the face shields did a good job blocking the initial splatter from the simulated cough, the laser illumination showed that plumes of
aerosolized particles swirled out from under the shield.
“Masks act as filters and actually capture the droplets and any other particles we expel,” said Siddhartha Verma, assistant professor in the department of ocean and mechanical engineering at Florida AtlanticUniversity and the study’s lead author. “Shields are not able to do that. If the droplets are large they will be stopped by the plastic shield. But if they are aerosol sized, 10 microns or smaller, they’ll just escape fromthe sides or the bottom of the shield. Everything that is expelled will very likely get distributed in the room.”
When a standard N95 mask with a valvewas tested, a large cloud of particles also escaped through the valve, but the vent directed the plume downward. Standard N95 masks with valves meet
standards set by the Occupational Safety andHealth Administration. Even so, valved respirator masks are not allowed in sterile medical environments because they allowthewearer’s germs to escape. During the pandemic, a number of knockoff versions of valved masks have appeared on the market, and while the valves look authentic, they don’t reallywork.
In amask with aworking valve, the pressure inside the masks forces a small disk to open and let air out, but when you inhale the disk fits tightly against the hole and doesn’t allowair in. “Whatwe foundwas that everything escapes through that exhalation valve,” saidManhar R. Dhanak, a study co-author and chairman of the university’s department of ocean and mechanical engineering. “And in some of the non-N95 commercial masks, the valve just ap
peared to be cosmetic.
They didn’t function because theywere of poor quality, so the disks basically didn’tmove out and in aswe’d expect them to.”
While the study allowed the researchers to compare the relative effectiveness of various masks and face shields, the methods used did not quantify the volume or size of the particles that escaped.
The research is unlikely to be the finalword on face shields. A 2014 study has often been cited as evidence that face shields offer extra protection to the personwearing them, but even that study concluded that the benefitwas limited. While the face shields protected thewearer from large cough splatters, they were less effective against smaller coughs and aerosols. And after the cough, as larger particles settled to the ground and aerosols dispersed around
the room, the face shield reduced aerosol inhalation by only 23%.
For some people, a face shield may still be the best option. For instance, a child with developmental disabilities may be more inclined to use a face shield than a mask. Although the findings suggest that a cloth or surgical mask offers more protection, experts say that any face covering is better than nothing at all and that face shields will keep some portion of large coughs and sneezes from splattering on the people around them.
For most people, a cloth mask of at least two layers, which covers the face from the nose to under the chin, is the best option.
“A good homemade maskworks verywell,” said Verma. “If it’s comfortable, it can beworn for long periods of time. Definitely try to avoid shields only or masks with valves.”