Long streams of virus-laden droplets can remain behind infected individuals

The cough-generated droplets from a walking individual otherwise disperse in a narrow hallway and open space. In narrow corridors, the droplets are concentrated in a small bubble and remain farther behind. Credit: Xiaolei Yang

Walking fast in narrow corridors may increase COVID-19 transmission risk.

Long streams of virus-laden droplets can follow infected individuals walking down narrow corridors, impacting guidelines for safe social distances.

Computer simulations have been used to accurately predict airflow and droplet dispersion patterns in situations where COVID-19 may spread. In the diary Physics of Fluids, by AIP Publishing, the results demonstrate the importance of space shape in modeling how virus-laden droplets move through the air.

The simulations are used to determine flow patterns behind a walking individual in spaces of different shapes. The results show a higher transmission risk for children in some cases, such as chasing fast-moving people in a long narrow corridor.

Previous studies using this simulation technique have helped scientists understand the influence of objects, such as glass barriers, windows, air conditioners, and toilets, on airflow patterns and virus spread. Previous simulations usually assumed a large, open interior space, but did not take into account the effect of nearby walls, as might exist in a narrow hallway.

Cough-generated droplets of open space

The cough-generated droplets from a walking individual otherwise disperse in a narrow hallway and open space. In an open space, the drops are spread over a wide range to the person. Credit: Xiaolei Yang

When a person walking in a hallway coughs, his breath ejects droplets that travel around and behind his body and form a wake like a boat forms a wake in the water while sailing. The investigation revealed the existence of a “re-circulation bubble” directly behind the person’s torso and a long wake that flowed out behind him at approximately hip height.

“The flow patterns we found are strongly related to the shape of the human body,” said author Xiaolei Yang. “At 2 meters downstream, the wake is almost negligible at foot and leg height, but is still visible at hip height.”

After the airflow patterns were determined, the study modeled the distribution of a cloud of droplets expelled from the simulated person’s mouth. Especially the shape of the space around the moving person is crucial for this part of the calculation.

Two types of spreading modes have been found. In one mode, the cloud of droplets detaches from the moving person and floats far behind that person, creating a floating bubble of virus-laden droplets. In the other mode, the cloud is tied to the person’s back and follows him like a tail as he moves through space.

Cloud of drops

In both modes, the drop cloud floats about half the height of the infected person before reaching the ground, indicating a greater risk for children to inhale the droplets. Credit: Xiaolei Yang

“For the loose mode, the droplet concentration is much higher than for the linked mode, five seconds after a cough,” said Yang. “This presents a major challenge in determining a safe social distance in places such as a very narrow hallway, where a person can inhale viral droplets, even if the patient is far in front of him or her.”

The danger is especially great for children, since in both modes the cloud of droplets floats at a distance above the ground about half the height of the infected person, in other words, at mouth level for children.

Reference: “Effects of space sizes on the spread of cough-generated droplets from a walking person” by Zhaobin Li, Hongping Wang, Xinlei Zhang, Ting Wu and Xiaolei Yang, December 15, 2020, Physics of Fluids.
DOI: 10.1063 / 5.0034874

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