In brief

Computer simulations have shown that cough droplets can remain in flight for eight seconds, traveling more than six meters in the process.

© A*STAR’s Institute of High Performance Computing (IHPC)

Mapping cough droplets in flight

4 Jan 2021

Computational models show that there’s much more to COVID-19 transmission than the airborne-or-droplet binary.

Is COVID-19 airborne? As with almost everything in science, the answer may not be that straightforward. Instead of trying to classify COVID-19 as airborne or not, new research suggests it may be more helpful to understand what exactly happens to a droplet once it exits the body.

“There is a prevailing, but somewhat misguided view that airborne droplet transmission can be cleanly segregated into two modes,” said Fong Yew Leong, a Scientist at A*STAR’s Institute of High Performance Computing (IHPC).

According to this view, large droplets are heavy and succumb easily to the pull of gravity. Smaller ones, typically under 5–10 µm in diameter, are much lighter in comparison and can ride the wind, bringing their viral motherlode to distant areas. In this depiction, droplets are simplified as either airborne or not. Diseases, in turn, are transmitted either one way or the other.

To clear the air, Leong and the IHPC team used their computational modeling prowess and the National Supercomputing Centre (NSCC)’s facility to recreate a spray of droplets from a cough. In contrast to the prevailing theory, they found that a droplet’s fate is not only determined by its size. As it soars through the air, the droplet loses much of its water to evaporation and shrinks. The smaller it becomes, the lighter it gets, and the longer it can stay airborne.

Their simulations have shown, for instance, that a large droplet 100 µm in diameter could remain in flight for around eight seconds and reach a distance of over six meters away. Alarmingly, even a mammoth droplet spanning 1,000 µm can travel up to 1.3 meters.

Clearly, such figures have implications for the design of safe management measures to contain the COVID-19 outbreak. The findings demonstrate that it is still important to wear a mask, observe social distancing, and be mindful of personal hygiene so that droplets that settle on the clothes have no chance of making it inside the body, said study first author Hongying Li, a scientist on the IHPC team.

“These findings are greatly dependent on the environmental conditions,” Leong added. On a particularly arid day, for example, evaporation ramps up, prolonging a droplet’s hang time. Variables such as the speed and direction of the wind can also greatly affect a droplet’s ultimate fate.

“Since these variables can fluctuate throughout the day, we aim to undertake more studies under different environmental conditions,” Li said. Together with A*STAR’s Institute of Materials Research and Engineering (IMRE), the team has been working with partners to study different sites and environmental settings.

The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing (IHPC).

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Li, H., Leong, F.Y., Xu, G., Ge, Z., Kang, C.W., Lim, K.H. Dispersion of evaporating cough droplets in tropical outdoor environment. Physics of Fluids 32, 113301 (2020) | article

About the Researchers

Fong Yew Leong received his PhD in 2008 from the National University of Singapore under the Singapore-MIT Alliance program where he investigated the transport of aerosols in pulmonary airways. He is currently a Scientist at A*STAR’s Institute of High Performance Computing (IHPC) where his interests revolve around fluid mechanics, as well as interfacial and transport phenomena. He conducts theoretical and computational modeling studies for multi-disciplinary problems at the interface of physics, chemistry and biology.
Hongying Li received her PhD degree in 2011 from Nanyang Technology University, Singapore, joining A*STAR’s Institute of High Performance Computing (IHPC) Fluid Dynamics Department as a scientist upon graduation. Her research interest is in the field of computational modelling for multiphase flow and heat transfer as well as thermal management. Apart from basic research, Li has successfully applied her research experience and know-how to many industry projects, assisting industry partners on many fronts including products and solutions development.

This article was made for A*STAR Research by Wildtype Media Group