A US simulation study highlights the limitations of face shields and masks with exhalation valves — a popular alternative to face masks because of comfort — against COVID-19.
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.
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.
In contrast, the researchers previously showed that some cotton face masks reduce the spread of droplets to only a few inches from the face during a simulated cough.
The simulations in the study "indicate that face shields and masks with exhale valves may not be as effective as regular face masks in restricting the spread of aerosolised droplets," the authors wrote.
Face masks have become a part of everyday life during the COVID-19 pandemic. But some people are turning to plastic face shields or masks with exhalation values because they find these alternatives more comfortable to wear for long periods of time. Face shields also have the advantage of allowing users to show facial expressions.
However, the US Centres 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. 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.
"As students return to schools and universities, some have wondered if it is better to use face shields, as they are more comfortable and easier to wear for longer periods of time," study lead author Siddhartha Verma, an assistant professor at Florida Atlantic University's College of Engineering and Computer Science. "But what if these shields are not as effective? You would be essentially putting everyone in a tight space with droplets accumulating over time, which could potentially lead to infections."
In the study, the researchers simulated coughing by connecting a mannequin's head to a fog machine – which creates a vapour 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.
For the mask with a valve, a jet of droplets passed through the valve in the front of the masks during coughing. Initially, this jet of droplets travelled toward the ground, but eventually the droplets dispersed over a wide area.
The researchers also tested two different brands of commercially available surgical masks. Both of these masks were not recommended for medical use by the manufacturers. Although the masks looked similar, one brand was effective at stopping the forward spread of aerosolised droplets, while the other allowed a high number of droplets to leak through the mask.
"This indicates that even among commercially available masks which may appear to be similar superficially, there can be significant underlying differences in the quality and type of materials used for manufacturing the masks," the authors said.
Since the study was a simulation, it doesn't provide data on the exact conditions that would result in the spread of an infection. For example, with SARS-CoV-2, the virus that causes COVID-19, it's unclear exactly 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 noted that "even the very best masks have some degree of leakage," Verma said. So "it's still important to maintain physical distance while wearing [masks] to mitigate transmission."
Several places across the world are experiencing a steep surge in COVID-19 infections. Face masks have become increasingly accepted as one of the most effective means for combating the spread of the disease when used in combination with social-distancing and frequent hand-washing. However, there is an increasing trend of people substituting regular cloth or surgical masks with clear plastic face shields and with masks equipped with exhalation valves. One of the factors driving this increased adoption is improved comfort compared to regular masks. However, there is a possibility that widespread public use of these alternatives to regular masks could have an adverse effect on mitigation efforts. To help increase public awareness regarding the effectiveness of these alternative options, we use qualitative visualizations to examine the performance of face shields and exhalation valves in impeding the spread of aerosol-sized droplets. The visualizations indicate that although face shields block the initial forward motion of the jet, the expelled droplets can move around the visor with relative ease and spread out over a large area depending on light ambient disturbances. Visualizations for a mask equipped with an exhalation port indicate that a large number of droplets pass through the exhale valve unfiltered, which significantly reduces its effectiveness as a means of source control. Our observations suggest that to minimize the community spread of COVID-19, it may be preferable to use high quality cloth or surgical masks that are of a plain design, instead of face shields and masks equipped with exhale valves.
Siddhartha Verma, Manhar Dhanak, John Frankenfield
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Physics of Fluids abstract