Face shields not effective replacements for cloth masks, experts say
A new study from Florida Atlantic University has modeled how effective face shields and face masks with exhaust-valves are in blocking the dispersal of aerosol-sized droplets. The research suggests the growing use of these alternative face coverings may not be helpful in curbing the spread of COVID-19, and validate experts calling for the general public to primarily use cloth or surgical masks for protection.
It’s fair to say face masks have become one of the most contentious public health issues during this ongoing global pandemic. By now, it is clear widespread face mask use is recommended in most parts of the world. But, what is not clear, is what kinds of facial coverings are most effective.
Due to ongoing supply chain considerations, surgical-grade and N95 masks are generally reserved for health care workers or those in high-risk environments. Triple-layer cloth masks are often suggested as effective enough for most people in general day-to-day life. However, lots of other facial coverings are being adopted. In some cases these are due to reasonable medical concerns inhibiting one from wearing a mask, but many wearing alternative face coverings do so simply because of comfort issues.
"There is an increasing trend of people substituting regular cloth or surgical masks with clear plastic face shields as well as using masks that are equipped with exhalation valves," says Siddhartha Verma, co-author on the new study. "A driving factor for this increased adoption is better comfort compared to regular masks.”
This new study employed a technique called flow visualization to model the effect of different facial coverings on droplet spread. The primary focus of this research was to look at two commonly used, but potentially problematic, facial coverings – large plastic face shields and face masks with exhaust valves.
It is important to note these kinds of droplet visualization studies do not directly correlate with COVID-19 transmission in real-world scenarios. These studies simply model the flow of droplets around different facial coverings in laboratory environments. Having said that, the findings are still relevant if one is trying to use a facial covering to both protect oneself from exposure to environmental contagions, and protect others from aerosol droplets they may be exhaling.
The study’s findings suggest face shields may be effective in protecting a wearer from large droplet projections, such as a cough or a sneeze, but that was about it. Manhar Dhanak, co-author on the study, notes face shields do allow for a volume of aerosolized droplets to easily spread around the sides and bottom of the visor.
"From this latest study, we were able to observe that face shields are able to block the initial forward motion of the exhaled jet, however, aerosolized droplets expelled with the jet are able to move around the visor with relative ease," says Dhanak. "Over time, these droplets can disperse over a wide area in both lateral and longitudinal directions, albeit with decreasing droplet concentration."
Looking at the results from the tests with the valved face mask, the conclusions are even more concerning. These valved masks may offer reasonable protection to the wearer, but the modeling showed the air a person was exhaling essentially being thrust, unfiltered, back out into the environment.
“… masks with exhalation ports include a one-way valve which restricts airflow when breathing in, but allows free outflow of air,” explains Verma. “The inhaled air gets filtered through the mask material, but the exhaled breath passes through the valve unfiltered."
Catherine Bennett, chair in epidemiology at Deakin University, suggests these two kinds of facial coverings should not be considered interchangeable with triple-layer cloth or surgical face masks. In terms of plastic face shields, Bennett points out these are primarily designed to be worn in conjunction with surgical face masks to add extra eye protection.
“But if shields are worn without other PPE, the wearer and their close contacts are not as well protected as there is too much opportunity for droplets to work around the open edges of the shield,” says Bennett, who did not work on this new study. “[Plastic face shields] are better than nothing and may be required in settings where being able to see facial expressions and mouth movement is important, for example when talking with the hearing impaired, but I wouldn’t recommend these be used in place of a mask.”
And Bennett is unequivocal when talking about masks with exhaust valves, pointing out these masks are fundamentally developed to be used in construction environments to protect a wearer from breathing in dust or other contaminants in the air. These types of masks stop small particles getting in but allow for unimpeded exhalations.
“So these masks will not reduce the spread of the virus from people who are symptomatic and out and about unaware they have the virus and are capable of transmitting it,” Bennett adds. “Similarly, the person you are chatting to or on the bus next to may also have the virus but not yet have symptoms. I’d want them to be wearing a mask without a valve to be sure they don’t spread it to me.”
Abrar Chughtai, a public health and infection control expert from the University of New South Wales, echoes Bennett’s concerns, suggesting valved face masks will not prevent spread of infection and should not be used by the general public for this purpose. He also agrees face shields are not an effective alternative to masks.
“Leakage around the chin is a problem with face shields, so face shields shouldn’t be used as an alternative to surgical masks,” says Chughtai. “They can be used with masks or respirators in health care settings to provide addition eye protection.”
Emeritus professor Mark Wahlqvist calls the new US research “elegant” and a valuable “contribution to the public discourse.” Wahlqvist suggests paying attention to countries such as Taiwan that have long-standing cultural mask-wearing behaviors, as these regions are offering clear indications widespread mask-wearing can be effective in controlling viral spread. And, this new research helps clarify what kinds of facial coverings we should be encouraging.
“[The new study] supports what we know from current evidence about the utility of masks in prevention for oneself and others,” says Wahlqvist. “Where mask use is a cultural norm as most notably and successfully in Taiwan, the pandemic is well-controlled. There, well-constructed plain or cloth masks have been effective in the community.”
The new study was published in the journal Physics of Fluids.
Source: Florida Atlantic University
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