• The Financial District


A new visualization shows why face shields and masks with exhalation valves may not be the best barriers for preventing the spread of COVID-19, Rachael Rettner wrote for LiveScience on September 1, 2020. 

Although face shields initially block droplets from a simulated cough, small droplets can easily move around the sides of the visor and eventually spread over a large area, according to the visualization, which is detailed in a study published Sept. 1 in the journal Physics of Fluids. For masks with exhalation valves, a stream of droplets passes, unfiltered, through the valve, meaning the mask would in theory do little to hinder the spread of potentially infectious droplets. The simulations in the new study "indicate that face shields and masks with exhale valves may not be as effective as regular face masks in restricting the spread of aerosolized droplets," the authors wrote. 

However, the Centers for Disease Control and Prevention (CDC) does not recommend either of these as alternatives to cloth masks. Masks with one-way valves, which are intended for use in construction work, allow users to breathe in filtered air and exhale warm, moist (and unfiltered) air through the valve, Live Science previously reported. But because respiratory droplets from the wearer are expelled into the air, the CDC says people should not wear these masks to prevent COVID-19 spread. The CDC also does not recommend face shields as a substitute for cloth masks because evidence is lacking to show their effectiveness, the agency says. 

In the new study, the researchers simulated coughing by connecting a mannequin's head to a fog machine — which creates a vapor from water and glycerin — and using a pump to expel the vapor through the mannequin's mouth. They then visualized the vapor droplets using a "laser sheet" created by passing a green laser pointer through a cylindrical rod. In this setup, simulated cough droplets appear as a glowing green vapor flowing from the mannequin's mouth. For the face shield simulation, the shield initially deflected droplets toward the ground after a cough. But small droplets remained suspended at the bottom of the shield and then floated around the sides, eventually spreading about 3 feet (0.9 meters) to the front and sides of the mannequin. In some cases, the droplets spread backward, behind the mannequin, instead of forward.

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