A new visualization shows why face protection and Masks with exhalation valves may not be the best barriers to preventing the spread of COVID-19.
Although face shields initially protect droplets from a simulated cough, small droplets can easily move around the sides of the visor and eventually spread over a large area, as can be seen from the visualization posted in a diary posted on Tuesday (September 1) Physics of liquids.
In masks with exhalation valves, an unfiltered stream of droplets flows through the valve, which means that in theory the mask would do little to prevent the spread of potentially infectious droplets.
In contrast, the researchers previously showed that some cotton face masks reduce the spread of droplets to just inches from the face during a simulated cough. Live Science previously reported.
The simulations in the new study “show that face shields and masks with exhalation valves may not be as effective as regular face masks in limiting the spread of aerosol droplets,”
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Face masks have become a part of everyday life during the COVID-19 pandemic. However, some people turn to plastic face shields or masks with exhalation values because they find these alternatives more comfortable for longer periods of time. Face shields also have the benefit of allowing users to show facial expressions.
However, the Centers for Disease Control and Prevention (CDC) do not recommend any of these alternatives as alternatives to cloth masks. One-way valve masks designed for construction work allow users to inhale filtered air and exhale warm, moist (and unfiltered) air through the valve. Live Science previously reported. However, because the wearer’s breath droplets are expelled into the air, these masks should not be worn to prevent the spread of COVID-19, according to the CDC.
The CDC also doesn’t recommend face shields as a substitute for cloth masks, as there is no evidence of their effectiveness, the agency said.
“As students return to schools and universities, some have wondered if it was better to use face shields because they are more comfortable and easier to wear for longer periods of time,” said Siddhartha Verma, assistant professor at Florida Atlantic University for technology and IT, said in a statement. “But what if those shields weren’t as effective? They’d essentially house everyone in a very small space, with droplets building up over time that could potentially lead to infection.”
In the new study, the researchers simulated coughing by connecting a mannequin’s head to a smoke machine – which uses water and glycerin to create a vapor – and using a pump to expel the vapor through the mannequin’s mouth. They then visualized the vapor droplets with a “laser sheet” that was created by moving a green laser pointer through a cylindrical rod. In this setup, simulated cough drops appear as glowing green vapor that pours out of the mannequin’s mouth.
For the face protection simulation, the shield first directed droplets to the ground after a cough. But small droplets stuck to the bottom of the sign and then floated around the sides, eventually spreading about three feet forward and down the sides of the mannequin. In some cases, the droplets spread behind the mannequin backwards rather than forwards.
In the case of the mask with valve, a jet of droplets flowed through the valve in front of the mask when coughing. At first this jet of droplets moved towards the ground, but eventually the droplets spread over a wide area.
The researchers also tested two different brands of commercially available surgical masks. Both masks were not recommended for medical use by the manufacturers. Although the masks looked similar, one brand was effective in preventing aerosol droplets from spreading forward while the other allowed large numbers of droplets to leak through the mask.
“This indicates that even with commercially available masks that appear to be superficially similar, there can be significant differences in the quality and type of materials used to make the masks,” the authors say.
Because the study is a simulation, it does not provide any data on the exact conditions that would cause an infection to spread. For example, with SARS-CoV-2, the virus that causes COVID-19, it is unclear how long the virus remains infectious in the air and how far infectious particles can travel, or how much virus is needed to make a person sick .
The authors also found that “even the best masks have some degree of leakage,” Verma said. “It is still important to maintain physical distance while wearing [masks] Mitigate transmission. ”
Originally published on Live Science.