May 19, 2020

expert reaction to paper studying mechanisms of coughing and airborne transmission of viruses

A study, published in Physics of Fluids, reports on the mechanisms of coughing and airborne transmission of viruses.

Dr Philippe Trinh, Lecturer in Applied Mathematics at the University of Bath (with input from Dr Kit Yates and Professor Paul Milewski, both from the Department of Mathematical Sciences), said:

“The publication by Dbouk and Drikakis, titled “On coughing and airborne droplet transmission to humans”, and published today (19 May 2020) performs a computational fluid dynamics simulation of the coughing process, essentially in order to determine the extent or distance in which saliva droplets are transmitted. This is published in a world-leading peer-reviewed journal in fluid dynamics. The authors are clear about the models, parameters, and assumptions they have used—however, we should acknowledge that, like many computational experiments that reach this degree of sophistication, reproducing the results and checking the assumptions requires some effort.

“In essence, their conclusions are that in typical environmental conditions with no wind, the cough droplets will spread below the UK governmentally recommended distance of 2m for social distancing. However, as confirmed by this study (and numerous other studies) the propagation of droplets is highly variable to the environmental conditions. Thus according to their computations, at wind speed of 4 to 15km/h, saliva droplets can disperse and travel up to 6m in length, forming a much less concentrated cloud.

“Although the conclusions seem to be backed up by their computer simulations, it is important to acknowledge that this is an incredibly complicated process to model. There are many components that go into the computations, from the specific parameters used to the actual modelling of dynamics. Despite this, I believe that the main conclusion that should be drawn from this analysis is that the UK-recommended distance of 2m is conservative under quiet conditions; changes in the air flow (like wind or air conditioning) and other environmental properties (e.g. temperature, humidity) can cause that figure to rise quite dramatically. This is in-line with similar investigations over a range of models and setups.

“Currently, there is a great deal of research around the world on the question of airborne transmission of coronavirus, and there are many issues that remain important to clarify when establishing guidelines on easing lockdown. For example, the authors have highlighted the open question of how indoor ventilation systems can affect droplet and viral spread, which remains a key question related to the re-opening and operation of businesses. 

“An important distinction needs to be made, which is that it remains unclear to what extent saliva droplets or viral particulates in the air will contribute to the probability of infection. This study provides guidelines on the mechanical aspects of droplet transmission, but it forms only one piece of the puzzle: the connection with viral transmission remains very unclear.”

Dr Julian Tang, Associate Professor of Respiratory Sciences at the University of Leicester, said:

“The findings from this simulation/ modelling paper are not that surprising – if the wind is blowing in the right direction then droplets from exhaled breath (whether from breathing, talking, coughing, laughing, singing) – can reach other people. 

“But in reality, there will be a lot of dilution in the process, especially being outdoors – so exactly how many droplets carrying how much virus will differ in each scenario.

“If the dilution factor is great enough, the actual number of viable viruses reaching someone else may not be enough to cause infection or disease.

“But it does show that the 2 m separation rule will not prevent all transmission events – but will likely prevent more than a 1 m separation – or no separation at all.

“Note that the reduction of transmission risk is all incremental – masking, social distancing, lockdown – but it all adds up to reducing the transmission of the virus through the population.”

Dr Simon Clarke, Associate Professor in Cellular Microbiology, University of Reading, said:

“The fact that droplets from a cough can travel for more than 2m is already understood, but this new study helps to provide more insight into the physical mechanisms at work as droplets travel through the air.

“This is a reminder that the 2m rule is recommended, not because staying 2m away from all other people provides you with a force field against infection, but because it is a reasonable distance to stay away from people to reduce risk of infection. While 2m is better than 1m, 10m or 100m is even better, although the protective effect is not proportional to the distance.

“The most important point to take away from this paper is not that we need to change guidelines on social distancing, but that coughing is one of the best ways to spread infected droplets if you’re ill. So if you have a cough, stay at home until you’re better – and if you cough unexpectedly when you are out and about, cough into your elbow. Then go home, and stay there.”

“On Coughing and Airborne Droplet Transmission to Humans” by Talib Dbouk et al. was published in Physics of Fluids at 16:00 UK time on Tuesday 19 May.

All our previous output on this subject can be seen at this weblink: www.sciencemediacentre.org/tag/covid-19

Declared interests

None received.