Agents of Shield

Coughing mannequin video shows why a face shield isn't enough

"It will protect against the spray, but not against the aerosol."

Provided you find a mask uncomfortable — and not everyone does – face shields might appear to be a good alternative. Clear and less restrictive, they sidestep some of the annoyances of masks, allowing your face to see the light of day.

But it's possible that comfort comes at a cost. Authors of a new scientific study want you to see if for yourself.

New research shows that face shields can catch the majority of droplets released during a cough. However, smaller aerosol-sized droplets remained suspended in the air, hovering near the bottom opening of the face shield.

After 10 seconds, those hovering droplets could spread three feet away — either in front or behind — the cougher, the paper notes. The droplets decrease in concentration, meaning that the farther out they get, the fewer of those droplets remain afloat.

The study was published Tuesday in Physics of Fluids.

A video showing the mannequin's cough and the spread of aerosolized droplets afterwards. Physics of Fluids.

Siddhartha Verma, the study's first author and a professor at Florida Atlantic University, says the experiment is supposed to be an illustration of a simple fact: it's possible that smaller aerosols can escape from face shields.

The fact that droplet concentration decreases as the droplets waft away underscores the importance of social distancing, he says. The fact that they escape at all reminds us that a face shield can't replace a face covering.

That said, a face shield can augment a face mask. A face shield can reduce the chance of encountering the virus through other avenues, like the eyes, as Deborah Birx, the White House coronavirus response coordinator, advised in July.

"It's okay to use both of them at the same time," Verma tells Inverse. "But only a face shield is not as effective as a single mask. It helps for [people] to see for themselves how the droplets actually spread."

Richard Flagan is a professor at CalTech who studies aerosols and was not involved with the study. He says the video is useful in that it illustrates the issues with face shields (and masks with valves, which the CDC also does not recommend).

"It will protect against the spray, but not against the aerosol," Flagan tells Inverse.

Droplets released over about 3 seconds while the mannequin wore a traditional surgical mask. (The cough occurred half a second before the top right photo was shot).Physics of Fluids

What does the video really show? – The study is based on a simulation. The scientists shot a mixture of water and glycerin (fake droplets) out of a mannequin's face to emulate a cough. The imaging methods in the study were intended to capture droplets within the one to ten-micron range, small enough for them to linger in the air.

Verma calls it a "simplified" but "reasonable" representation of a real cough.

Flagan points out that the study doesn't tell us much about the conditions were like in the room (the study doesn't report temperature or humidity) or the number of droplets that might escape. However, it does give us the major plot points of what might happen to small or even medium-sized droplets released during a cough.

The largest droplets are caught, but the smallest are not.

This study isn't definitive, but there is some emerging evidence that fits alongside it.

In July, staff members at a Swiss hotel were permitted to choose between a face covering or a face shield. When a Covid-19 outbreak hit the hotel, only those who wore face shields were infected, a health official told the Swiss news outlet, 20 Minutes.

"A face shield by itself is not going to protect the wearer very much. It would protect from the projected droplets but it won't protect them from all the small droplets that remain after someone coughs or sneezes," Flagan says.

"It won't protect people in the vicinity of the person that's wearing it."

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The movement of small aerosols captured in the study. The cough occurred half a second before the photo on the upper right was taken.

The backstory on face shields– The Centers for Disease Control and Prevention doesn't recommend the use of face shields over face masks, and cautions that we still don't know how much protection a face shield can offer. That said, you'd be forgiven for feeling confused about them.

Some state guidelines suggest that face shields are potential alternatives. In Minnesota, for instance, guidelines suggest that, though we still don't know if face shields work they may be an option in "situations where a face mask is problematic," but doesn't specify what those situations are.

"That was sort of the main motivation for us to do this study – to show people what actually happens."

Then there was a JAMA Viewpoint article published in April. That article (not a study) described an earlier study demonstrating that face shields had blocked viral exposure to the flu by 96 percent. The shield also blocked 68 percent of small aerosols.

That JAMA viewpoint article was interpreted to suggest that face shield were more effective than masks in some cases – like a South Florida local news report that suggest face shields "could provide more protection" than masks.

Importantly, the viewpoint that argued for the effectiveness of face shields was focused on the fact that those smaller aerosols that may escape weren't as risky as they may actually be. That may have been the thinking in April, but it's not the case today. The World Health Organization now notes that small aerosols, like those released while speaking, do play a role in spreading the virus — especially when inside.

With the role of smaller aerosols better defined, and emerging case studies suggesting that masks really do work, Verma's study is working off some solid footing. Still, it is far from definitive — Verma interprets his work as a visual reminder to wear a simple, well-constructed mask (but not one with a valve attached).

"That was sort of the main motivation for us to do this study – to show people what actually happens," he says.

Abstract: 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.

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