Simulations by scientists from the Florida Atlantic University show that these items can help, but are nowhere near as effective as regular face masks.
Face shields and masks with valves may help contain some droplets that come out of people’s mouths and noses, but they are nowhere near as effective as regular, cloth, or disposable face masks.
This is what a study by the Florida Atlantic University, in the United States, recently published in the journal Physics of Fluids shows.
In the research, scientists used a mixture of distilled water and glycerin to simulate the droplets expelled when someone (a mannequin, in this case) coughs or sneezes.
Then, using laser sheets, the scientists tested the protective capacity of a face shield and an N95 mask equipped with exhalation valves.
“We were able to observe that face shields are able to block the initial forward motion of the exhaled jet, however, aerosolized droplets expelled with the jet are able to move around the visor with relative ease,” said Manhar Dhanak, Ph.D., professor and director of SeaTech, and co-author of the study. “Over time, these droplets can disperse over a wide area in both lateral and longitudinal directions, albeit with decreasing droplet concentration.”
Masks with valves protect more than face shields, but still allow a large number of droplets to be exhaled without any type of filter, which significantly reduces their effectiveness.
In addition, the researchers concluded that both are not as effective as regular masks for restricting the spread of smaller aerosol droplets.
According to the scientists, there is an increasing tendency for people to replace cloth or surgical masks with face shields or masks with exhalation valves.
“A driving factor for this increased adoption is better comfort compared to regular masks,” said Siddhartha Verma, Ph.D., assistant professor and lead author of the study.
Face shields have gaps that allow particles to spread, while masks with valves restrict airflow when breathing in, but not when breathing out. “The inhaled air gets filtered through the mask material, but the exhaled breath passes through the valve unfiltered,” Verma explained.