Study shows how masks with valves and face shields allow spread of virus

People wearing plastic face shields or masks fitted with a valve can spray invisible droplets over a very wide area when they sneeze or cough, making the devices ineffective at preventing the spread of coronavirus when used on their own, a simulation model shows.

In a report published Tuesday in the US journal Physics of Fluids, researchers at Florida Atlantic University used vertical and horizontal laser sheets to track tiny droplets of distilled water and glycerin as they spread from a hollow mannequin head fitted with a plastic face shield or a mask with a breathing valve on it.

The face shield initially blocks the passage of the droplets as they move forwards, but "the expelled droplets can move around the visor with relative ease and spread out over a large area," the researchers said.
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