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Explained: Here’s why face shields are no match for sneeze droplets

If someone wore a face shield but no mask, and if someone infected with Covid-19 sneezed in the vicinity, would the face shield offer enough protection on its own?

Written by Kabir Firaque | New Delhi |
Updated: December 21, 2020 3:06:50 pm
A face shield for sale on a street in New York. (The New York Times: Marian Carrasquero)

The face shield is an intriguing part of healthcare workers’ personal protective equipment (PPE): Although it is meant to protect the face from airborne particles that may contain the novel coronavirus, it has a prominent opening at the bottom edge. Indeed, that is why medical staff often wear a mask beneath the shield. If someone wore a face shield but no mask, and if someone infected with Covid-19 sneezed in the vicinity, would the face shield offer enough protection on its own?

It would not. In a study published in the Physics of Fluids, journal of the American Institute of Physics, researchers from Japan’s Fukuoka University examined the flow of air around a face shield when someone nearby sneezes. They found that sneeze droplets travel very quickly to the edges of the face shield, and may be inhaled by the wearer if she happens to breathe in at that very moment.

The researchers are now working on creating face shields safe enough to be worn on their own.

The science…

When fluid is ejected instantaneously from a circular orifice, it generates a “vortex ring” — a doughnut-shaped vortex. “It might be easier to understand it as a bubble ring made by dolphins,” lead researcher Fujio Akagi said, by email.

Vortex rings have the ability to transport microscopic particles. Akagi and colleagues carried out simulations of an infected person sneezing and ejecting a vortex ring from 1 m in front of a person wearing a face shield. The leading vortex ring reached the top of the face shield; trailing vortex rings reached the lower end as they broke down. Sneeze droplets could reach the shield within a half-second to a second, and enter. “In this case, if the time when the droplets enter the inner surface of the shield is close to the time of the user’s inhalation of the breath, the apparent probability that the user will inhale the droplets is high,” Akagi said. 📣 Follow Express Explained on Telegram

… and the shield

Face shields are effective in preventing infection in the eyes, Akagi explained. “Don’t misunderstand us — wearing a face shield can help prevent infection… what we are saying is that we have scientifically shown that there is an increased risk of infection if only a face shield is worn,”Akagi said.

Although medical staff wearing a face shield also wear a mask, usually a surgical mask with a high filtering effect, such masks have the disadvantage of being difficult to breathe through. “We thought if the face shield is approved as sufficiently preventive, the medical mask can be replaced with a standard mask… For this reason, we have tried to clarify the flow around the face shield,” Akagi said.

Akagi described another situation: medical personnel treating deaf patients. To explain to the patients how the mouth moves, the medical staff need to remove the mask altogether. “If staff are only allowed to wear a face shield, this problem can be solved,” Akagi said.

The team is now developing improved shields, so that medical workers will be able to prevent infection using only a face shield and a regular mask — or, ideally, only a face shield.

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