Study shows why face shields don’t work as well as face masks

Some people choose alternatives to masks for comfort. A study shows the difference in effectiveness.

mask experiment visualization

A visualization of how face shields fail to stop the spread of potentally COVID infused water droplets.

Credit: Siddhartha Verma, Manhar Dhanak, John Frankenfield
  • A new study provides a visualization of why face shields are ineffective at stopping the spread of COVID-19.
  • Using a mannequin that could simulate coughing, the authors demonstrated how water droplets slide around shields.
  • The authors conclude that shields are not an effective replacement for masks.

Face masks are everywhere these days, and for very good reason. They are proven to help reduce the chance of spreading COVID-19 by limiting how far water droplets people exhale can go, and most establishments won't let you in without one. Despite this, complaints about wearing them abound.

A common and all too human one is that they prevent people from seeing your facial expressions, a key element of human communication. Others protest that masks are uncomfortable, and people who need glasses are quick to tell you that masks often cause glasses to fog. Many have tried using face shields, large plastic screens, in place of a proper mask to avoid these issues.

While the CDC already stated that face shields are not recommended as a replacement for face masks, a new study offers a visualization of the price you pay in protection in exchange or an ounce of comfort.

A cool, if slightly terrifying, visualization. 

The straightforwardly named "Visualizing droplet dispersal for face shields and masks with exhalation valves" was published in the journal "Physics of Fluids" and led by Dr. Siddhartha Verma of Florida Atlantic University. In it, the researchers explain that while face shields are very good at blocking the forward motion of larger droplets of water, the large open space in their design allows for smaller droplets to pass them and disperse throughout the room, reducing their potential benefits.

The authors attached a face shield to a slightly modified mannequin that could simulate coughing to demonstrate this. Small droplets of water and glycerin, comparable in size to the lower end of estimates of what is needed for a virus to travel, were blown through the mannequin's mouth and highlighted with laser sheets as they traveled throughout the room.

As illustrated below, small droplets that stop moving forward do not immediately drop to the floor, but instead, they float toward the gap at the bottom of the shield. Following air currents, the droplets eventually made their way around the face shield and began to spread. Given enough time, they'll spread up to a few feet away.

Dr. Verma explained the weaknesses of face shields to the New York Times:

"Masks act as filters and actually capture the droplets and any other particles we expel. Shields are not able to do that. If the droplets are large they will be stopped by the plastic shield. But if they are aerosol sized, 10 microns or smaller, they'll just escape from the sides or the bottom of the shield. Everything that is expelled will very likely get distributed in the room."

The authors note that the water droplets' concentration is reduced by wearing a face shield, meaning that fewer droplets are spread than would be spread by a person without any protection. The benefits of this are limited, though, and a previous study carried out by the authors of this test also shows how much more effective proper face masks are than face shields. Another study from 2014 gives face shields a mere 23 percent efficiency at reducing the inhalation of such droplets.

It should be no surprise, then, that the study concludes that face masks are preferable to face shields when it comes to slowing the spread of COVID-19.

The study also considered face masks with exhaust values, as shown in the final test in the above video. Face masks with exhaust values allow unfiltered droplets to go through the valve. Why they don't do much to prevent droplets from spreading everywhere is obvious. Just as with face shields, the droplets that are initially unable to move forward eventually manage to get to the same place through dispersion.

Why you should wear a proper facemask, revisited.

The shortcomings of face shields and other mask alternatives are not shared by the thing they are meant to replace, the basic, well-made face mask.

As explained above, face masks work to keep others around you from getting your germs by keeping the water droplets you exhale, which may contain viruses, from spreading. They have also been shown to reduce the number of droplets from other people's breath that reach your face, potentially preventing you from getting sick. Given that face masks lack a large hole in them, as shields or masks with valves do, they allow far fewer droplets to escape than the competition.

The study considered the differences between a cheaply made face mask and a well-made one, with the cheap one proving much less effective. Even the best masks have some degree of leakage, so maintaining social distancing of at least two meters (about six feet) is still necessary.

No protective mask is perfect, and no set of rules offers complete safety. However, some objects and procedures work better than others at keeping people safe. As this study shows, face shields, masks with exhaustion valves, and cheaply made masks don't work as well as a well-made face mask.

A landslide is imminent and so is its tsunami

An open letter predicts that a massive wall of rock is about to plunge into Barry Arm Fjord in Alaska.

Image source: Christian Zimmerman/USGS/Big Think
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  • A remote area visited by tourists and cruises, and home to fishing villages, is about to be visited by a devastating tsunami.
  • A wall of rock exposed by a receding glacier is about crash into the waters below.
  • Glaciers hold such areas together — and when they're gone, bad stuff can be left behind.

The Barry Glacier gives its name to Alaska's Barry Arm Fjord, and a new open letter forecasts trouble ahead.

Thanks to global warming, the glacier has been retreating, so far removing two-thirds of its support for a steep mile-long slope, or scarp, containing perhaps 500 million cubic meters of material. (Think the Hoover Dam times several hundred.) The slope has been moving slowly since 1957, but scientists say it's become an avalanche waiting to happen, maybe within the next year, and likely within 20. When it does come crashing down into the fjord, it could set in motion a frightening tsunami overwhelming the fjord's normally peaceful waters .

"It could happen anytime, but the risk just goes way up as this glacier recedes," says hydrologist Anna Liljedahl of Woods Hole, one of the signatories to the letter.

The Barry Arm Fjord

Camping on the fjord's Black Sand Beach

Image source: Matt Zimmerman

The Barry Arm Fjord is a stretch of water between the Harriman Fjord and the Port Wills Fjord, located at the northwest corner of the well-known Prince William Sound. It's a beautiful area, home to a few hundred people supporting the local fishing industry, and it's also a popular destination for tourists — its Black Sand Beach is one of Alaska's most scenic — and cruise ships.

Not Alaska’s first watery rodeo, but likely the biggest

Image source:

There have been at least two similar events in the state's recent history, though not on such a massive scale. On July 9, 1958, an earthquake nearby caused 40 million cubic yards of rock to suddenly slide 2,000 feet down into Lituya Bay, producing a tsunami whose peak waves reportedly reached 1,720 feet in height. By the time the wall of water reached the mouth of the bay, it was still 75 feet high. At Taan Fjord in 2015, a landslide caused a tsunami that crested at 600 feet. Both of these events thankfully occurred in sparsely populated areas, so few fatalities occurred.

The Barry Arm event will be larger than either of these by far.

"This is an enormous slope — the mass that could fail weighs over a billion tonnes," said geologist Dave Petley, speaking to Earther. "The internal structure of that rock mass, which will determine whether it collapses, is very complex. At the moment we don't know enough about it to be able to forecast its future behavior."

Outside of Alaska, on the west coast of Greenland, a landslide-produced tsunami towered 300 feet high, obliterating a fishing village in its path.

What the letter predicts for Barry Arm Fjord

Moving slowly at first...

Image source:

"The effects would be especially severe near where the landslide enters the water at the head of Barry Arm. Additionally, areas of shallow water, or low-lying land near the shore, would be in danger even further from the source. A minor failure may not produce significant impacts beyond the inner parts of the fiord, while a complete failure could be destructive throughout Barry Arm, Harriman Fiord, and parts of Port Wells. Our initial results show complex impacts further from the landslide than Barry Arm, with over 30 foot waves in some distant bays, including Whittier."

The discovery of the impeding landslide began with an observation by the sister of geologist Hig Higman of Ground Truth, an organization in Seldovia, Alaska. Artist Valisa Higman was vacationing in the area and sent her brother some photos of worrying fractures she noticed in the slope, taken while she was on a boat cruising the fjord.

Higman confirmed his sister's hunch via available satellite imagery and, digging deeper, found that between 2009 and 2015 the slope had moved 600 feet downhill, leaving a prominent scar.

Ohio State's Chunli Dai unearthed a connection between the movement and the receding of the Barry Glacier. Comparison of the Barry Arm slope with other similar areas, combined with computer modeling of the possible resulting tsunamis, led to the publication of the group's letter.

While the full group of signatories from 14 organizations and institutions has only been working on the situation for a month, the implications were immediately clear. The signers include experts from Ohio State University, the University of Southern California, and the Anchorage and Fairbanks campuses of the University of Alaska.

Once informed of the open letter's contents, the Alaska's Department of Natural Resources immediately released a warning that "an increasingly likely landslide could generate a wave with devastating effects on fishermen and recreationalists."

How do you prepare for something like this?

Image source:

The obvious question is what can be done to prepare for the landslide and tsunami? For one thing, there's more to understand about the upcoming event, and the researchers lay out their plan in the letter:

"To inform and refine hazard mitigation efforts, we would like to pursue several lines of investigation: Detect changes in the slope that might forewarn of a landslide, better understand what could trigger a landslide, and refine tsunami model projections. By mapping the landslide and nearby terrain, both above and below sea level, we can more accurately determine the basic physical dimensions of the landslide. This can be paired with GPS and seismic measurements made over time to see how the slope responds to changes in the glacier and to events like rainstorms and earthquakes. Field and satellite data can support near-real time hazard monitoring, while computer models of landslide and tsunami scenarios can help identify specific places that are most at risk."

In the letter, the authors reached out to those living in and visiting the area, asking, "What specific questions are most important to you?" and "What could be done to reduce the danger to people who want to visit or work in Barry Arm?" They also invited locals to let them know about any changes, including even small rock-falls and landslides.

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