Face shields, masks with valves less effective than surgical masks

As mask-wearing continues amid the novel coronavirus (COVID-19), people are increasingly using clear plastic face shields and masks with exhalation valves because they can be more comfortable. However, researchers from the American Institute of Physics found they were less effective than regular cloth or surgical masks in a new study published in the journal Physics of Fluids.

For the study, 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.

The visualizations showed face shields block the initial forward motion of a simulated jet of a cough or a sneeze, but the expelled droplets can move around the visor with relative ease and spread out over a large area depending on light ambient disturbances. The visualizations for a mask equipped with an exhalation port illustrate a large number of droplets pass through the exhale valve unfiltered, which make it ineffective in stopping the spread the COVID-19 virus if the person wearing the mask is infected.

The research suggests that to minimize the community spread of COVID-19, it may be preferable to use high-quality cloth or surgical masks that are of a plain design instead of face shields and masks equipped with exhale valves.

"Even the very best masks have some degree of leakage,” said Siddhartha Verma, PhD, one of the study authors, in a statement. “It's still important to maintain physical distance while wearing them to mitigate transmission.”

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