June 11, 2020

expert comments about 2 metre social distancing

There has been discussion in the media about the 2m social distancing rule, specifically regarding the benefits or disadvantages compared to a smaller distance. 

Prof Jonathan Reid, Professor of Physical Chemistry, University of Bristol, said:

What do we know from scientific evidence about the importance of physical distancing in reducing viral transmission?

“Physical distancing is a way of reducing the exposure of an individual to the large droplets generated when someone sneezes, coughs or even speaks. These large droplets do not remain airborne for long, they settle out falling to the ground in under 10-20 seconds. However, they are initially carried forward by the momentum of the cloud when someone sneezes, coughs or speaks at anywhere from a speed of 5 m/s to 30 m/s.

“It has long been established that the droplets can be carried over a distance of 1-2 m, with the larger droplets falling out in around 1 m and the smaller droplets (similar to the diameter of a human hair) in around 2 m. So, the further you stand away from someone, the smaller the fraction of droplets you are exposed to when someone sneezes or coughs in your direction, and the less likely you are to come into contact with the virus.

Is there a strict cut-off in terms of effectiveness?

“No, it is just a matter of reducing risk with increased physical distance. The further you stand away from someone, the fewer droplets you will be exposed to when someone sneezes or coughs in your direction. 1 m only prevents you from being exposed to the largest of droplets, 2 m reduces your exposure but doesn’t make it zero risk. In fact, droplets smaller than the diameter of a human hair can be carried over more than 2 m, and the very smallest droplets, often referred to as aerosols, can remain airborne for minutes or hours in a room and travel over larger distances. The distances travelled also depend on the temperature and humidity, and if there are air currents, e.g. a breeze outside or a fan inside. So, the guidelines on physical distances are not strict cut-offs, they are just guidance and recognise that the distance the droplets are carried depends on a number of factors.

“At the moment, we don’t know how much virus these droplets carry and how long the virus survives for when it is in these droplets and aerosols. We also don’t know how much virus someone needs to be exposed to to get COVID-19 and this also then depends on how long they may be near someone. So assessing risk is difficult and, just as with face masks, it makes sense to be cautious until we have more information if the goal is to minimise the spread of the virus, a larger distance will lead to reduced risk on a population scale.”

Dr Shaun Fitzgerald FREng, Royal Academy of Engineering Visiting Professor at the University of Cambridge, said:

“If one wants to compare the differences of 2m vs 1m for example, it would be helpful to consider a number of other factors too which lead to changes in risk including duration of proximity, orientation and mitigation measures such as face coverings.

“As an engineer, it would absolutely make sense to me that if you can’t maintain 2m distancing at all times, then consider other factors which influence risk, and introduce some other measures. Your risk to acquire an infection is a function not just of distance; it is also a function of how long you are in sufficiently close proximity to somebody.

“The 2m guidance rule in the way it’s been set out has been really helpful, because there is elegance in the simplicity of a message. Just saying 2m is something we can all relate to, we can all understand.”

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

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