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 how large 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 Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who studies airborne particles but who was 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 to wear over long periods of time, can be easily cleaned and allow for better communication because they don't muffle the voice or hide facial expressions. Valved masks, with one-way vents designed to allow breath to escape while blocking germs from entering, 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 allow large 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 the wearer, it does suggest that people who use them may also be more vulnerable to exposure than if they wore a regular mask. Valved masks are a particular concern " 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.
A medical worker performs a COVID-19 test at a test centre at Vnukovo airport outside Moscow, Russia using only a face shield. Image credit: AP Photo/Pavel Golovkin
To conduct the research, scientists from Florida Atlantic University used hollow foam heads fitted with various face coverings. They pumped a vaporized mixture of glycerin and distilled water 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 best masks allow some particles to escape. In videos of tests using the gold-standard mask, an N95, a puff of particles can be seen jetting out around the bridge of the nose, where the fit is poor. (Other studies have shown that N95 masks, which should filter 95 percent of small particles, lose about a third of their filtering potential if the fit is improper.) 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 practising social distancing.
But when the hollow heads were 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 Atlantic University 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 from the 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 valve was tested, a large cloud of particles also escaped through the valve, but the vent directed the plume downward. Standard N95 masks with valves, which are often used by construction workers and painters to prevent inhalation of dust and other particles, meet standards set by the Occupational Safety and Health Administration. Even so, valved respirator masks are not allowed in sterile medical environments because they allow the wearer'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 really work.
In a mask with a working 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 allow air in. "What we found was that everything escapes through that exhalation valve," said Manhar 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 appeared to be cosmetic. They didn't function because they were of poor quality, so the disks basically didn't move out and in as we'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 final word on face shields. A 2014 study has often been cited as evidence that face shields offer extra protection to the person wearing them, but even that study concluded that the benefit was limited. While the face shields protected the wearer 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 percent.
"Face shields can substantially reduce the short-term exposure of health care workers to large infectious aerosol particles, but smaller particles can remain airborne longer and flow around the face shield more easily to be inhaled," the researchers wrote, adding that for health workers, face shields should be worn in addition to masks, not as a substitute.
In Switzerland, health officials warned that a coronavirus outbreak in a hotel appeared to infect workers wearing face shields, while workers wearing traditional masks appeared to have been protected.
Marr said work in her own lab also shows that face shields offer almost no protection against aerosolized particles believed to play an important role in the spread of illness. "It provides maybe five percent protection if that," she said. "It's almost nothing for the particle sizes we're concerned about."
While face shields do block large splatters from a cough or sneeze, smaller particles get caught in air flows and never hit the plastic, slipping below it instead. "Air can't pass through the face shield " it has to bend and go around the shield," Marr said. "The aerosols are going to follow that airflow around the shield. It's not going to splat."
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. A clear plastic face shield might also be useful to a caregiver who needs to communicate with someone who is hearing impaired. 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 mask works very well," said Verma. "If it's comfortable, it can be worn for long periods of time. Definitely try to avoid shields only or masks with valves."
Tara Parker-Pope c.2020 The New York Times Company