Research is unlikely to be the last word on face protection. A 2014 study has often been cited as evidence that face shields provided additional protection for the person wearing them, but even that study concluded that the benefits were limited. While the face shields protected the wearer from large splashes of cough, they were less effective against minor coughs and aerosols. And after coughing, the face shield only reduced aerosol inhalation by 23 percent as larger particles settled on the floor and aerosols were dispersed around the room.
"Face shields can significantly reduce short-term exposure of health care workers to large infectious aerosol particles, but smaller particles can stay in the air longer and flow more easily around the face shield to be breathed," the researchers wrote, adding for health care workers Face protection should be worn in addition to masks, not as a substitute.
In Switzerland, health officials warned that a coronavirus outbreak at a hotel appeared to infect workers with face shields, while workers appeared to be protected with traditional masks.
Dr. Marr said the work in her own laboratory also shows that face shields offer almost no protection against aerosolized particles, which are believed to play an important role in the spread of disease. "It offers maybe 5 percent protection if that is," she said. "It's almost nothing for the particle sizes that we're concerned about."
While face shields protect large splashes from coughing or sneezing, smaller particles get caught in air currents and never hit the plastic, but instead slide under it. "Air can't go through the face shield – it has to bend and go around the shield," said Dr. Marr. “The aerosols will follow this air flow around the shield. It won't ripple. "
For some people, a face shield may still be the best option. For example, a child with developmental disabilities may be more likely to use a face shield than a mask. A clear plastic face shield can also be useful for a caregiver who needs to communicate with a hearing impaired person. Although the results suggest that a drape or surgical mask may offer more protection, experts say that any face covering is better than nothing, and that face shields prevent some of the big cough and sneeze from splashing on people around them.
For most people, a two-layer fabric mask that covers the face from nose to chin is the best option. A face shield in combination with a mask provides additional protection and can be useful for people who routinely come into contact with other people indoors.
"A good homemade mask works very well," said Dr. Verma. "If it is comfortable, it can be worn for a long period of time. In any case, avoid just shields or masks with valves."