As many people use clear plastic face shields and masks with exhalation valves instead of cloth or surgical masks because they’re more comfortable, researchers at Florida Atlantic University warn that they may not be as effective in stopping the spread of the COVID-19 coronavirus.
Face shields blocked the initial forward motion of simulated jets of a cough or sneeze in their study, the researchers said. However, the expelled droplets can move around the visor with relative ease and spread out over a large area depending on light ambient disturbances.
Also, many droplets can pass through the exhalation valves of masks equipped with them, making such masks ineffective in stopping the spread of the virus if the person wearing the mask is infected, the researchers said.
“As students return to schools and universities, some have wondered if it is better to use face shields, as they are more comfortable and easier to wear for longer periods of time,” said author Siddhartha Verma, PhD, assistant professor in the Department of Ocean and Mechanical Engineering.
“But what if these shields are not as effective? You would be essentially putting everyone in a tight space with droplets accumulating over time, which could potentially lead to infections,” said Verma.
The researchers used a hollow manikin head and simulated a cough or sneeze with a pressure impulse from a manual pump. Tracers composed of droplets of distilled water and glycerin were expelled through the mouth opening, and laser sheets visualized the spatial and temporal development of the ejected flow.
“We focused on the smaller droplets, since they can stay suspended for very long times and might contain enough virus particles to transmit COVID-19,” said Verma.
The researchers said that to minimize the community spread of COVID-19, the use of high-quality cloth or surgical masks that are of a plain design instead of face shields and masks equipped with exhalation valves may be preferrable.
“Even the very best masks have some degree of leakage. It’s still important to maintain physical distance while wearing them to mitigate transmission,” said Verma.
The study, “Visualizing Droplet Dispersal for Face Shields and Masks with Exhalation Valves,” was published by Physics of Fluids.