New Study Confirms Face Shields and Valve Masks Can’t Be Trusted

Visualizations reveal these substitutes don’t measure up

Frustrated with the discomfort of face masks, some people have been using face shields, which have been promoted by some health experts to help prevent the spread of Covid-19. Other people are turning to masks with exhalation valves that make breathing out easier. Now, a new study based on a laboratory simulation brings into laser focus how ineffective both alternatives are at containing the coronavirus.

Face shields were touted as a possible alternative to masks in an April opinion piece in the Journal of the American Medical Association (JAMA) and in subsequent popular articles.

“We were a little surprised that they would say that,” says Siddhartha Verma, PhD, assistant professor in mechanical engineering at Florida Atlantic University and a member of the visualization research team. “And that was our motivation to show people what actually happens.”

The shield’s main job is to block splatters of fluid from the patient and can protect the wearer’s eyes, but they have not been shown to be very effective at protecting other people when worn by an infected person.

Verma and his colleagues used faux coughs and sneezes from health test dummies to demonstrate how things go. Droplets splatter predictably against the inside of a face shield (which is arguably gross). But smaller droplets called aerosols, which are known to stay airborne for extended periods, especially in poorly ventilated indoor spaces, escape under the shield and to the sides.

The study, detailed on September 1 in the journal Physics of Fluids, captured the emanations in images and video but did not quantify them, so a direct comparison between the effectiveness of face masks isn’t possible.

The idea in promoting shields is that doctors and nurses use them, they’re more comfortable, and people won’t be compelled to yank them down to talk. But medical workers don’t use just the face shields — they use them in conjunction with masks, Verma explains. The shield’s main job is to block splatters of fluid from the patient and can protect the wearer’s eyes, but they have not been shown to be very effective at protecting other people when worn by an infected person. Also, it is not known how much protection a shield might offer the wearer against any wafting coronavirus particles.

Proponents of face shields have cited a 2014 study that found the number of respiratory droplets from a cough can indeed be reduced by a shield. But the risk of a nearby person inhaling droplets is reduced by only 23%, that study concluded. “Face shields provide a useful adjunct to respiratory protection for workers caring for patients with respiratory infections,” the scientists concluded then. “However, they cannot be used as a substitute” for masks.

Masks with exhalation ports, if worn by someone who is infected, are likewise not very effective at protecting others, Verma and his colleagues showed in the new simulations. (The U.S. Centers for Disease Control and Prevention concurs.)

That’s no surprise. Masks with valves, which make exhalation easier and reduce moisture buildup, are meant to filter incoming dust, chemicals, or other particles, but they’re not designed to filter what goes out.

“A few people in the medical community are using these mask types,” Verma tells Elemental. “It was a bit surprising.”

The upshot: Face masks (both surgical and N95), and to a lesser degree homemade cloth face coverings, help reduce the spread of the coronavirus. If properly worn, they are one effective layer of protection, primarily to protect others, not so much the wearer, and are to be combined with avoiding large crowds, keeping your distance, and washing your hands.