To understand exactly how effective both face masks and face shields can be at protecting you and others from contracting the virus, Rand McClain, DO, chief medical officer of Live Cell Research, a company dedicated to enhancing health and quality of life through biological innovation, says you'll need to think of things on a teeny-tiny germ level. In fact, it may help to imagine that you've suddenly been bestowed with a pair of super-glasses that can physically see SARS-CoV-2 (aka the novel coronavirus that causes the disease COVID-19) in the air.
When someone's wearing a mask without a shield, they have one less buffer between them and a carrier of the virus, says Dr. McClain. This is important because medical professionals believe the virus travels from one person to another in two forms: as droplets (large liquid bodies that get exhaled through talking or coughing) or aerosols (smaller and more numerous sprays that travel more easily). The mask-wearer, therefore, will likely have the droplets land directly on their mask while the aerosols continue to float through the air.
The problem is, a mask won't completely deter those droplets and aerosols from breaking through the cloth—especially if you're spending a prolonged time period with someone who has the virus. "The masks aren't 100 percent effective," says Dr. McClina. "We've seen studies showing that a 10 to 30 percent increase in protection is available with a standard bandana cotton mask." Medical-grade masks, meanwhile, circulate out between 60 to 80 percent of particles. (Read: They are not a virus catch-all.)
Face shields, on the other hand, provide an extra layer of protection (kind of like a bulletproof vest or sunglasses) against the droplets. That's why Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, recently suggested Americans wear a face shield or goggles in addition to their face masks if they have them. While it's not currently universally recommended, doing so can help keep you and others even safer. "You really want perfect protection of the mucosal surfaces. You have mucosa in the nose, mucosa in the mouth, but you also have mucosa in the eye," he said during an Instagram Live with ABC News. "Theoretically, you should protect all the mucosa surfaces."
So, how does a face shield work, exactly? "Just like you would expect with a shield, you're going to block those droplets that are streaming directly towards you," explains Dr. McClain. "They'll hit the face mask and land—and that's that." The path for the aerosolized particles is a bit more complicated, though. "Some aerosolized particles could obviously just make contact with a mask and that's the end of the road for them. With the aerosolized particles, though—because they're light enough to float in the air—they could be drifting underneath, toward the mask. Someone inhales and you'd see the air pathway drop underneath the shield and then travel up toward the person within the shield," Dr. McClain says.
In other words, the shield makes it extra-tricky for the aerosols to migrate where you don't want them to be (on your face)—particularly if you're also wearing a mask beneath it. If a metaphor is helpful, you can think of masks as moisturizers with SPF (which aren't always effective at protecting your skin against sun damage) and shield as tried-and-true sunscreen (which is more effective at guarding your against UV, but has confounding factors like duration spent in the sun).
"The question, as usual with this virus, is 'Is the juice worth the squeeze?' In an environment where people are coughing around you and aren't wearing masks and you're exposed a lot—like in health care setting—it makes sense," says Dr. McClain. Yes: It may be overboard to wear a shield and a mask in a sparsely-populated park but for essential workers—the cashiers, delivery people, and medical workers who are facing constant exposure to the virus—every bit of protection counts.
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