Do Face Shields Protect Against COVID-19? A Mask and Shield Explainer

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As teachers and students cope with in-person learning during the pandemic, many educators are looking for clearer ways to connect with their students behind masks.

The Centers for Disease Control and Prevention and the American Academy of Pediatrics both recommend all adults and children over age 2 wear a face mask in addition to social distancing, unless they have breathing difficulties or would be unable to remove the mask themselves. More than three-fifths of states now require people to wear masks in public, though some of these, such as Ohio, Texas, and Virginia, do not require children under 10 to wear masks.

In addition, many states and school districts have required universal mask-wearing as a condition for in-person classes. Some, like South Carolina, have bought mask and other personal protective gear for their schools.

Widescale mask wearing has been shown to reduce the spread of the virus, particularly in conjunction with social distancing and regular hand and surface cleaning. But there are drawbacks in an educational setting.

“Covering the lower half of the face reduces the ability to communicate, interpret, and mimic the expressions of those with whom we interact,” said Manfred Spitzer, the medical director of the Psychiatric University Hospital in Ulm, Germany, and the founder of the Transfer Center for Neurosciences and Learning, in a recent study in the journal Trends in Educational Neuroscience. “Positive emotions become less recognizable, and negative emotions are amplified … [reducing] bonding between teachers and learners, group cohesion, and learning—of which emotions are a major driver.”

In contrast to opaque masks, transparent face shields allow students to see a teacher’s full range of expressions—but do they really protect against COVID-19?

How does COVID-19 spread in the air?

The coronavirus that causes COVID-19 spreads through both large droplets and tiny aerosols that enter the air when someone who is infected speaks, sneezes, or coughs. The Centers for Disease Control and Prevention considers someone at higher risk of becoming infected if they come within six feet of someone with COVID-19 for a total of 15 minutes over the course of 24 hours. A brief, close argument can expose someone to more viral particles than people speaking softly, and studies so far suggest children produce similar amounts of the virus as adults do, though preadolescents appear to be significantly less likely to transfer the infection.

Virus droplets are seen dispersing when someone wearing a face mask coughs or sneezes. As with the first set of images above, lasers highlight the emerging spray a) before the cough; b)after a half-second; c)after 3.8 seconds, and d) after 16.6 seconds.
Virus droplets are seen dispersing when someone wearing a face mask coughs or sneezes. As with the first set of images above, lasers highlight the emerging spray a) before the cough; b)after a half-second; c)after 3.8 seconds, and d) after 16.6 seconds.
—Source: Physics of Fluids

"You know, kids will be kids; they will not use their inside voice; they may yell and they may talk very loudly," said Tina Tan, a pediatric infectious disease doctor at Lurie Children's Hospital in Chicago and a coronavirus expert with the Infectious Disease Society of America. "We know that when you do that, if you're infected, you actually generate smaller particles by which the virus can be transmitted. And these smaller particles stay in the air for longer periods of time and travel for further distances.

The aerosolized virus can also drift up in response to body heat, according to Jose-Luis Jimenez, a chemistry professor and aerosols expert at the University of Colorado, who developed an online transmission estimator to gauge how far the virus may spread under different conditions or using different kinds of masks.

What does a mask do?

Masks provide layers of fabric or other materials to filter airborne droplets and aerosols that carry the coronavirus particles. A dense cotton weave—or the random shred of multi-sized fibers in an N95 mask—tend to trap both larger and smaller particles more efficiently.

To be most effective, masks need to be worn by all people who are within six feet of one another, and particularly those in enclosed spaces such as classrooms. Even if a mask only effectively traps half of virus particles as an infected person exhales, if their neighbor sits father away and also wears a mask, the chance of viral particles reaching them is significantly cut down. One recent study in the journal Infectious Disease Modeling found, for example, that if 4 in 5 people in New York wore masks that can screen half of viral particles (a common standard for properly worn surgical and cloth masks), it would reduce the projected death rate from COVID by as much as 45 percent over two months.

Can I use a face shield instead of a face mask?

No. Face shields are meant to be worn with, not instead of a face mask; they are often used along with masks, goggles, and head coverings. That’s because masks are meant to fit as tightly as possible to someone’s face, preventing droplets and aerosols from escaping around the edges. They need to fit snugly from just under the chin to the bridge of the nose.

"When you look at somebody that wears a mask, there actually is a seal that forms," Tan said. "If you have a mask that's very loose and hangs on a person, it's not going to be very effective because the particles can actually fall through those cracks between the mask and your face. It's similar with a face shield that might cover the front but doesn't go around the sides."

For the same reason, face masks with valves are not as effective at preventing the spread of the coronavirus; the valves make it easier for people to exhale, but that reduces the protection the mask gives to other people. A study in the journal Physics of Fluids this July shows how easily the virus can travel far from a face shield or from a mask worn improperly or with an exhalation valve.

That can make a big difference when it comes to spreading the virus. For example, a study of the seasonal flu found face shields stopped a majority of large droplets after a cough by someone with the flu but did little to block smaller aerosolized particles of the virus. Within a half hour of someone coughing, these smaller particles dispersed through the room, and the face shield blocked only about 23 percent of them.

The CDC does not recommend anyone use a face shield alone and is still evaluating how effective face shields are against the coronavirus when used with other protective gear. Homemade cloth masks containing at least two layers of fabric are considered more effective than wearing nothing, but the CDC does not consider homemade masks as real protective gear. They do not protect as well as a medical mask.

Whether face masks are homemade or medical, teachers or students should check to make sure there are no gaps between the mask and skin on the nose, chin, or cheek. While health-care workers use commercial testing kits to ensure their masks fit properly, at least one study found that people could make an accurate test at home using an essential oil diffuser and a two-gallon bag.

What should teachers do to help students see their faces?

This can put teachers, particularly those of young children and English-language learners, in a difficult position.

Spitzer noted that teachers may need to increase body language and voice expression to make up for smiles and other normal expressions that are likely to be covered by masks.

Tan noted that some have recommended teachers use face shields in conjunction with regular cloth or surgical masks and remove the face mask during parts of a lesson that require students to watch the teacher’s mouth to help students follow their expressions better. The face shield can still help prevent larger droplets from getting on a teacher’s face or eyes if a student coughs or sneezes while they are close. But Tan cautioned that repeatedly lowering and raising a face mask during the day increases teachers’ exposure to the virus.

In August, the U.S. Food and Drug Administration also cleared a transparent plastic surgical mask for use in hospitals and schools during the pandemic. There have not yet been experimental studies comparing the relative effectiveness of transparent versus cloth or fiber face masks in reducing COVID-19 transmission. However, Allysa Dittmar, the president of ClearMask, the company which makes the mask, said it could be useful for “those who can benefit from improved visual communication, such as children, older adults, deaf and hard of hearing people, and those who do not speak the same language.”

Vol. 40, Issue 13, Pages 4-5

Published in Print: November 18, 2020, as Do Face Shields Protect Against COVID-19? A Mask and Shield Explainer
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