A new visualization shows why face shields and masks with exhalation valves may not be the best barriers for preventing the spread of 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, which is detailed in a study published Tuesday (Sept. 1) in the journal Physics of Fluids.

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, Live Science previously reported.

The simulations in the new study "indicate that face shields and masks with exhale valves may not be as effective as regular face masks in restricting the spread of aerosolized 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 Centers 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, Live Science previously reported.

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, said in a statement.

"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."

(Siddhartha Verma, Manhar Dhanak, John Frankenfield)

In the new study, the researchers simulated coughing by connecting a mannequin's head to a fog machine – which creates a vapor 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 traveled 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 aerosolized 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."

Originally published on Live Science.