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.
Dr. 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 5 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,” Dr. Marr said. “The aerosols are going to follow that air flow 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 face shield combined with a mask would offer additional protection and may be useful to those who are routinely in contact with other people indoors.
“A good homemade mask works very well,” said Dr. Verma. “If it’s comfortable, it can be worn for long periods of time. Definitely try to avoid shields only or masks with valves.”