Long streams of virus-laden drops can be traced behind infected people

Drops generated by coughing from a walking individual are scattered differently in a narrow corridor and in an open space. In narrow corridors, the drops are concentrated in a small bubble and are left on. Credit: Xiaolei Yang

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

Long streams of virus-laden drops can be tracked behind infected people walking through a narrow corridor, affecting safe social distancing guidelines.

Computational simulations were used to accurately predict airflow and droplet dispersion patterns in situations where COVID-19 could be spread. In the diary Fluid physics, by AIP Publishing, the results show the importance of the shape of space in modeling how the virus-laden droplets move through the air.

Simulations are used to determine the flow patterns behind an individual going into different shaped spaces. The results reveal a higher risk of child transmission in some cases, such as behind people moving fast in a long, narrow hallway.

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

Open Space cough drops

Drops generated by coughing from a walking individual are scattered differently in a narrow corridor and in an open space. In an open space, the drops are scattered in a wide range attached to the person. Credit: Xiaolei Yang

If a person walks down a corridor coughs, their breath expels the drops that move around and behind their body, forming an awakening in the way a boat forms an awakening in the water as it travels. The investigation revealed the existence of a “recirculation bubble” directly behind the person’s torso and a long awakening that flowed behind them at about waist height.

“The flow patterns we found are closely related to the shape of the human body,” said author Xiaolei Yang. “At 2 meters downstream, the awakening is almost negligible at the height of the mouth and at the height of the foot, but it is still visible at the height of the waist.”

Once airflow patterns were determined, the investigation modeled the scattering of a cloud of droplets ejected from the simulated person’s mouth. The shape of the space surrounding the moving person is especially important for this part of the calculation.

Two types of dispersion modes have been found. In a way, the cloud of drops detaches from the person on the move and floats far behind that individual, creating a floating bubble of virus-laden drops. In the other way, the cloud is attached to the back of the person, following behind them like a tail as it moves through space.

The cloud of drops

Either way, the cloud of drops hovers about half the height of the infected person before reaching the ground, indicating a higher risk for children to inhale the drops. Credit: Xiaolei Yang

“For detached mode, the droplet concentration is much higher than for attached mode, five seconds after coughing,” Yang said. “This is a great challenge in determining a safe social distance in places like a very narrow corridor, where a person can inhale viral drops even if the patient is far in front of him.”

The danger is especially great for children, because in both ways, the cloud of drops floats at a distance above the ground, which is about half the height of the infected person – in other words, at the level of the children’s mouth.

Reference: “Effects of space size on the dispersion of cough droplets” by Zhaobin Li, Hongping Wang, Xinlei Zhang, Ting Wu and Xiaolei Yang, 15 December 2020, Fluid physics.
DOI: 10.1063 / 5.0034874