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

The droplets generated by a coughing individual are distributed differently in a narrow corridor and an open space. In narrow corridors, the droplets concentrate in a small bubble and continue to linger. Photo credit: Xiaolei Yang

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

Long streams of virus-laden droplets can chase infected individuals walking down a narrow corridor, affecting safe social distancing guidelines.

Computer simulations have been used to accurately predict airflow and droplet distribution patterns in situations where COVID-19 could spread. In the magazine Physics of liquidsThe results from AIP Publishing show how important the shape of space is in modeling the movement of virus-laden droplets 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 risk of transmission for children in some cases, e.g. B. Behind fast moving people in a long, narrow hallway.

Previous research using this simulation technique has helped scientists understand the impact of objects such as glass barriers, windows, air conditioning, and toilets on airflow patterns and virus spread. The previous simulations typically assumed a large, open interior space, but did not take into account the effects of nearby walls, such as might exist in a narrow corridor.

Coughing created droplets of open space

The droplets of a walking individual generated by coughing are distributed differently in a narrow corridor and an open space. In an open space, the droplets are distributed in a large area that is bound to the person. Photo credit: Xiaolei Yang

When a person walking in a corridor coughs, their breath expels droplets that move around and behind their body, forming a trail like a boat makes a trail in the water as it moves. The investigation found that there was a “recirculation bubble” just behind the person’s torso and a long wake streamed out at about waist level behind them.

“The flow patterns we found are closely related to the shape of the human body,” said author Xiaolei Yang. “Two meters downstream, the wake is almost negligible at mouth and leg height, but is still visible at waist level.”

Once the airflow patterns were determined, the study modeled the spread of a cloud of droplets ejected from the mouth of the simulated person. The shape of the space surrounding the moving person is particularly critical to this part of the calculation.

Two types of propagation modes were found. In one mode, the cloud of droplets detaches from the moving person and hovers far behind that person, creating a floating bubble of virus-laden droplets. In the other mode, the cloud is attached to the person’s back and follows them like a tail as they move through space.

Cloud of droplets

In either mode, the cloud of droplets hovers about halfway up the infected person before it hits the ground, which indicates a higher risk for children to inhale the droplets. Photo credit: Xiaolei Yang

“In detached mode, the droplet concentration five seconds after a cough is much higher than in attached mode,” said Yang. “This is a major challenge in determining a safe social distance in places like a very narrow corridor where a person can inhale viral droplets even if the patient is far in front of them.”

The danger is particularly great for children, as in both modes the cloud of droplets hovers at a distance above the ground that is about half the size of the infected person – in other words, at mouth level for children.

Reference: “Effects of Room Sizes on the Propagation 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 liquids.
DOI: 10.1063 / 5.0034874