THURSDAY, Dec. 17, 2020 — You might want to think twice before you enter a hallway with strangers during the pandemic: Researchers report that following a fast-walking person with COVID-19 down a narrow corridor could increase your risk of infection, even if you keep your distance.

That’s because that person can leave long streams of virus-laden droplets behind them, according to a study published Dec. 15 in the journal Physics of Fluids.

The risk is especially high for children.

The findings are from computer simulations that assessed airflow and droplet dispersal patterns behind walkers in different locations.

Previous simulations have focused on large, open, indoor spaces, but haven’t considered the effect of nearby walls, such as those in a narrow corridor.

If a person walking in a corridor coughs, they expel droplets that go around and behind their body, forming a wake in the same way a moving boat leaves a wake in water, the researchers explained.

And coughing people have a “re-circulation bubble” directly behind their torso and a long wake of droplets streaming out behind them at about waist height.

“The flow patterns we found are strongly related to the shape of the human body,” said study co-author Xiaolei Yang, of the Chinese Academy of Sciences in Beijing. “At 2 meters downstream, the wake is almost negligible at mouth height and leg height but is still visible at waist height.”

The researchers identified two ways droplets spread.

In one, the cloud of droplets leaves the moving person and lingers far behind them, creating a drifting virus-laden cloud.

In the other mode, the droplet cloud remains attached to the person, trailing behind them like a tail.

Here is a photo of the two types of spread:

“For the detached mode, the droplet concentration is much higher than for the attached mode, five seconds after a cough,” Yang said in a journal news release. “This poses a great challenge in determining a safe social distance in places like a very narrow corridor, where a person may inhale viral droplets even if the patient is far in front of him or her.”

Children are at special risk in both scenarios, because droplet clouds linger above the ground at about half the height of an infected adult — or at a child’s mouth/nose level.

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