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Face shields and exhalation valve masks have become a popular alternative for people seeking protection against Covid-19, the disease caused by the new coronavirus, but US research has cast doubt on their effectiveness.
Researchers at Florida Atlantic University examined the performance of face shields and exhalation valves in preventing the spread of aerosol-sized droplets, one of the main forms of transmission of Covid-19, with disturbing results.
Visualizations from a mask equipped with an exhalation port indicated that a large number of droplets passed through the valve unfiltered.
The scientists said they did the research to help the public understand the effectiveness of face shields and masks equipped with exhalation valves, increasingly popular substitutes for surgical or regular cloth masks because people find them more comfortable.
Face shields reduce moisture and fogging when worn with glasses and are easier to inhale. They also protect the eyes from splashes and aerosols of infected droplets, are easily cleaned and disinfected, and allow visual communication for the hearing impaired.
Unfortunately, the smallest aerosol droplets can penetrate under the bottom of the shield and from the sides of the visor. The researchers found that, during an exposure lasting between one minute and half an hour, the shield was only 23 percent effective in reducing inhalation of droplets.
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One-way exhalation valves, on the other hand, do not filter the user’s breath, potentially exposing other people to infected droplets.
“Overall, the images presented here indicate that face shields and masks with exhalation valves may not be as effective as normal face masks in restricting the spread of aerosol droplets,” the study concluded, adding that the guidelines The US Centers for Disease Control (CDC) discourage the use of face shields as the sole means of virus prevention.
The CDC also cautions that masks equipped with exhalation valves should not be used when a sterile environment is required.
Gao Xiaodong, a hospital infection specialist at Shanghai’s Zhongshan Hospital, said he agreed with the US researchers that both face shields and exhalation valve masks had limitations in stopping the spread of the virus.
Gao said that face shields came in different sizes and that many people did not follow the instructions on how to use them correctly.
As for the exhalation valve masks, initially invented for use during smog, Gao called them “selfish” as they provided no protection to anyone except the wearer. “Hospital staff do not wear this type of mask and no patient can use them either,” he said.
This article Covid-19 research shows the downsides of face shields and valve masks that first appeared in the South China Morning Post
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