Going back to the office? What public health experts say about using the elevator.

As workers are expected to return to office buildings in coming weeks, the whoosh of opening elevators might herald an uncomfortable reunion: Welcome back to sharing very tight spaces with strangers.

Even before the novel coronavirus pandemic, special etiquette ruled the elevator's awkward confines: Face forward. Hold the door for stragglers. Chatterboxes unwelcome. Now, health agencies and medical experts are urging elevator riders to follow additional rules: Wear masks. Tap buttons with an object or knuckle. Avoid speaking when possible.

If riders follow those recommendations, "there's essentially no risk in an elevator," said infectious-disease physician Colleen Kraft, associate chief medical officer at Emory University Hospital in Atlanta.

"It is a short period of time," she said. "If you are wearing a mask and others are masked in the elevator, I don't think it poses any risk."

Joseph Allen, director of the Healthy Buildings program at the Harvard T.H. Chan School of Public Health, said it was understandable that elevators may cause anxiety. "It's a natural choke point in the building," he said. "We've been told to keep six feet away from others."

Intensity, frequency and duration are three factors that determine exposure to a hazard, Allen said. For most office workers, the frequency of elevator rides is low — perhaps a round-trip or two on a workday. And at a minute or less for most trips, the duration is short.

Occupancy restrictions and no-talking rules aim to reduce intensity by limiting the amount of virus released as droplets or aerosols. The elevator is "a place where risk can be managed quite effectively," Allen said.

Masks and face coverings, too, are essential. Kraft cited a phrase that has become common among infectious-disease experts: "My mask protects you, your mask protects me."

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New York plans to permit office workers to return to their buildings on Monday, Democratic Mayor Bill de Blasio told reporters. Coronavirus cases continue to decline there, unlike swaths of the South and Southwest, where cases have risen.

Although many white-collar employers are reluctant to order workers back this summer — Facebook, Google and other large tech companies will allow remote work to continue until 2021 — de Blasio estimated that 300,000 people will return to work in his city starting Monday.

New York's reopening guidelines mandate that elevators in office buildings stay below half-capacity. Employers "must advise workers and visitors to wear face coverings in common areas including elevators," according to the reopening plan. Elevator surfaces should be frequently cleaned and disinfected.

Ten people fit comfortably in each of the 62 elevators that shuttle up and down Manhattan's One World Trade Center, the tallest building in the United States. Those cars will be emptier than usual when workers return.

"We are limiting the number of passengers in our elevators to four. And face coverings are required," said Jordan Barowitz, a spokesman for the Durst Organization, the company that manages the 104-story skyscraper.

Limited research exists to describe elevators during pandemics past or present. When Lee Gray, an architectural history professor at the University of North Carolina at Charlotte who wrote a historical account of 19th-century elevators, hunted for examples from the 1918 influenza pandemic, he found little except for photographs that show that elevator operators wore masks.

In April, Richard Corsi, dean of engineering at Portland State University and an expert on indoor-air quality, shared on Twitter a crude simulation of an elevator ride. If an unmasked person rode an elevator up 10 floors while speaking on a phone and coughing once, Corsi's model predicted that infectious virus particles would linger as the elevator returned to the bottom floor.

But that's just one scenario, Corsi cautioned, from a not-yet-published calculation. We are our own source of ventilation.

"People drag air with them, especially if they're a group moving out," Corsi said. "To get a fair dose, there needs to be a pretty high concentration at pretty high level of stuff per liter of air," such as riding next to an infected person speaking or coughing for several minutes. "The key is to minimize the time you're in an elevator with somebody that's infected."

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A May report from a hospital in Wuhan, China, evaluated public spaces, including elevators, for the presence of the virus. Swabs of elevator buttons detected viral genetic material, though the study authors did not report cases of infection linked to those surfaces.

While people should be careful when touching surfaces they haven't cleaned, Kraft advised against wearing gloves. It's too easy to misuse them or be lulled into a false sense of security, she said. Instead, use a toothpick, a corner of shirt fabric or another barrier to touch buttons. Some companies are trying to make this easier — Isla Bella Beach Resort, a luxury hotel in Florida, has started handing out branded rubber-tipped styluses for guests to press elevator buttons.

There is no easy way to avoid using elevators in a tall building. Asking employees to walk in stairwells may be a fire hazard or physically onerous. Adding more elevators is costly. "I don't think that's economically viable," Gray said, because building owners would have to replace valuable tenant space with shafts. Some elevator companies are working on integrating elevators with apps, he said, which allow riders to summon a car via their phones.

To remain physically distanced, the Centers for Disease Control and Prevention recommends maintaining six-foot separations even while in an elevator. To Allen, that’s not practical — and may have a negative unintended effect.

"That really means that they're saying one person per elevator ride," he said. "In some of these big buildings, if we have one person ride an elevator, we are going to have hundreds, if not thousands of people in the lobby. And that creates a greater exposure." Instead, Allen suggests people file into the elevator in a checkerboard pattern, perhaps guided by stencils on the floor, to stay somewhat separated while avoiding crowds in the lobby.

"Everyone has a role to play here," Allen said. That includes building owners, who can post signs about properly using elevators; employers, who can stagger shifts to avoid rush-hour surges; and elevator riders, who can direct sneezes into an elbow or turn to face the wall.

“If everyone does their part,” he said, “we can reduce risk collectively.”