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FILE — A woman protected by a face shield against COVID-19 prays at the Basilica de los Angeles in Cartago, Costa Rica.

FILE — An employee of Murray's Bar in Dublin, Ireland, wears a face shield as a precautionary measure against spreading COVID-19.

Science is clear on the efficacy of face masks — it's proven to slow the spread of COVID-19. But what of face shields?

You’ve probably seen someone wearing one in your neighborhood. It’s basically an individually sized salad bar sneeze guard attached to a headband.

The advantages of a transparent plastic face shield over a face mask aren’t trivial. Breathing is easier while wearing a shield. Speech isn’t muffled like it can be through a mask. Eyeglasses are less likely to fog up. People wearing a full-face visor instead of a mask tend not to touch their mouth, nose and eyes as much.

You can see if a person is smiling at you through a face shield. With a face mask, you can’t know for sure if it’s grin or a scowl under the fabric.

Psychologists and speech therapists find that communicating with patients wearing a face shield is easier than with those wearing a mask. Hearing-impaired people can read the lips of people protected by a shield.

Doctors, nurses, dental workers and other front-line health workers wear shields, but mostly as an added layer of protection in conjunction with masks. They're essential for intubation, which often causes to patients to expectorate violently. The shields also extend the longevity of the masks by blocking droplet contamination.

If someone coughs in your face, a mask won’t protect your eyes, but a shield will. And unlike face coverings designed for single use, such as surgical masks and N95 masks, plastic shields can be washed down and reused indefinitely.

Despite the benefits, the Centers for Disease Control and Prevention is clear on its guidance: Face shields are not recommended for “normal everyday activities or as a substitute for cloth face coverings.”

For one thing, it’s unknown how well shields, which have openings at the sides and bottom, protect others from droplets expelled by the wearer. Furthermore, the level of protection a shield affords to the person wearing it has not been thoroughly studied.

In a recent coronavirus outbreak at hotel in Graubünden, Switzerland, only those workers who were wearing a face shield were exposed.

"It has been shown that only those employees who had plastic visors were infected. There was not a single infection among employees with a mask," Rudolf Leuthold, director of the cantonal health department in Graubünden, told The Local.

Nevertheless, face shields have their advocates.

A Cincinnati physician says teachers and students should wear full face shields instead of cloth masks when classes resume in the fall.

"These [holds up his fingers] are the 10 most deadly weapons, which is why you do not want to touch your ‘t-zone,' your eyes, nose and mouth," internist Dr. Will Sawyer told Fox19. "It’s self-inoculation."

Sawyer maintains people can't help touching face masks to constantly adjust them, but a face shield forces them to keep their hands away from their face. However, the CDC says COVID-19 is primarily transmitted via respiratory droplets by coughing and sneezing, speaking and even breathing. Hand-to-mouth/nose/eye transmission is believed to be much less common, although it does happen. Whether you wear a mask or a shield, frequent hand washing is strongly encouraged.

School districts in Ohio, Pennsylvania and other states are including face shields among the protective gear they’re ordering in anticipation of reopening in the fall. In the Bay Area, for example, the Palo Alto Unified teachers union wants the district to outfit teachers with both face masks and shields.

Face shield worn during a one-hour walk in 13mph sustained winds with 35mph gusts. No problem - actually no watery eyes, which was nice. Ball cap did blow off once #faceshields pic.twitter.com/zScHp0Kvom

Dr. Eli N. Perencevich, an infectious disease physician and professor of internal medicine and epidemiology at the University of Iowa Carver College of Medicine, co-authored an essay in the Journal of American Medical Association arguing for greater use of face shields by the general public. The opinion piece was published April 29.

Perencevich referred to a 2014 simulation study in which face shields were shown to “reduce immediate viral exposure by 96% when worn by a simulated health care worker within 18 inches of a cough.”

But the author of the study, William Lindsley, a research biomedical engineer at the National Institute for Occupational Safety and Health, told NBC News people should stick to cloth face coverings.

"A cloth mask or a medical mask is going to do a better job of protecting you against the smaller particles than a face shield would," Lindsley said. "A face shield is good against the really big stuff [droplets] that you can kind of see. But as the stuff gets smaller and smaller, it's just easy for that to go around the face shield and be inhaled."

There is mounting scientific evidence that the coronavirus can be spread as an aerosol, mainly indoors, suggesting that viral particles could be inhaled through the openings at the bottom and on the sides of a face shield.

Swiss Federal Office of Public Health spokesman Daniel Dauwalder told The Local visors or shields are “no substitute for face masks,” but they could serve as a "complementary form of protection."

California and New York both require people to wear face coverings in public. But while New York allows anyone to substitute a plastic shield for a mask, California's Department of Health's guidance states that shields be worn only by those who have a medical condition that prevents them from wearing a mask. It also stipulates that the shield should have a drape attached to its bottom edge.

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Mike Moffitt is an SFGATE Reporter. Email: moffitt@sfgate.com. Twitter: @Mike_at_SFGate

Reporter Mike Moffitt has been writing and editing stories for newspapers and news websites for more than 25 years. Before joining the SFGate team, he worked at the San Francisco Examiner, Arizona Republic and Phoenix Gazette. He covers news, politics, science, sports, outdoors and Bay Area history.