Coughing 'Can Spread SARS-CoV-2 Virus Beyond Two Metres'

Peter Russell

November 24, 2021

The two-metre rule introduced to help protect people from the SARS-CoV-2 virus might be less effective than commonly supposed, scientists have said.

The advice was issued by the Government soon after the COVID-19 pandemic took hold, although later eased to as little as 1 metre – subject to mitigations to reduce risk – to help the economy recover.

However, engineers at the University of Cambridge found that the two-metre rule did not represent a concrete measurement of safety but was instead chosen from a risk 'continuum'. They said that a 'safe' distance could have been set to anywhere between one and three, or more metres.

First author, Shrey Trivedi, from Cambridge’s Department of Engineering, who led the research, said: "One part of the way that this disease spreads is virology: how much virus you have in your body, how many viral particles you expel when you speak or cough."

"But another part of it is fluid mechanics: what happens to the droplets once they're expelled."

Airborne Transmission

The study, based on computer modelling and published in the journal  Physics of Fluids , found that when an infected person without a mask coughs, most of the larger droplets fall on nearby surfaces.

However, the scientists found that smaller droplets from an infected individual without a mask could spread well beyond two metres, even when outdoors.

Visualisation of trajectory of the droplets. Credit: University of Cambridge/Physics of Fluids

An additional factor behind infection risk was found to be the strength of cough and surrounding air conditions.

"Each time we cough, we may emit a different amount of liquid, so if a person is infected with COVID-19, they could be emitting lots of virus particles or very few, and because of the turbulence, they spread differently for every cough," explained Shrey Trivedi.

Environmental Conditions

According to Prof Epaminondas Mastorakos, also from the Department of Engineering, who led the research: "Even if I expel the same number of droplets every time I cough, because the flow is turbulent, there are fluctuations.

"If I’m coughing, fluctuations in velocity, temperature, and humidity mean that the amount someone gets at the two-metre mark can be very different each time."

Mitigating risk by vaccination, ventilation, and mask wearing were vital to help stop the SARS-CoV-2 virus spreading, the authors concluded. However, they acknowledged that the two-metre rule has been an easy to remember message for the public.

"We’re all desperate to see the back of this pandemic, but we strongly recommend that people keep wearing masks in indoor spaces such as offices, classrooms, and shops," said Prof Mastorakos.

With the onset of colder weather, the team are continuing their research with similar simulations for spaces such as lecture rooms that can help assess the risk as people spend more time indoors.

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