The two-meter COVID-19 rule is ‘random measurement’ of safety

The two-meter COVID-19 rule is 'random measurement' of safety

Visualization of dispersal of droplets by coughing. The drops are color-coded by size. Red = large, green = medium, blue = small, purple = very small. Credit: Shrey Trivedi et al, University of Cambridge

A new study has shown that the airborne transmission of COVID-19 is very random and suggests that the two-meter rule was a number chosen from a risk continuum, rather than any concrete measurement of safety.

A team of engineers from the University of Cambridge used computer modeling to quantify how droplets disperse when people cough. They found that in the absence of masks, a person with COVID-19 can infect another person two meters away, even when outdoors.

The team also found that individual coughs vary widely and that the ‘safe’ distance could have been set to anywhere between one to three or more meters, depending on the risk tolerance of a given public health authority.

The results, published in the journal Physics of liquids, suggests that social distancing is not an effective mitigation measure per se, and emphasizes the continuing importance of vaccination, ventilation and masks as we enter the winter months of the Northern Hemisphere.

Despite the focus on hand washing and surface cleaning in the early days of the pandemic, it has been clear for almost two years that COVID-19 is spreading through airborne transmission. Infected people can spread the virus through coughing, talking, or even breathing when they emit larger droplets that eventually settle or smaller aerosols that can float in the air.







Visualization of dispersal of droplets by coughing. The drops are color-coded by size. Red = large, green = medium, blue = small, purple = very small. Credit: Shrey Trivedi et al, University of Cambridge

“I remember hearing a lot about how COVID-19 spread through door handles in early 2020, and I thought to myself that if that was the case, then the virus should leave an infected person and land on the surface. or dispersed in the air through fluid mechanical processes, “said Professor Epaminondas Mastorakos of Cambridge’s Department of Engineering, who led the research.

Mastorakos is an expert in fluid mechanics: the way fluids, including exhaled breath, behave in different environments. During the pandemic, he and his colleagues have developed various models for how COVID-19 spreads.

“Part of the way this disease is spread is virology: how much virus you have in your body, how many virus particles you expel when you talk or cough,” said lead author Dr. Shrey Trivedi, also from the Department of Engineering. “But another part of it is fluid mechanics: what happens to the droplets once they are expelled, and this is where we come in. As specialists in fluid mechanics, we are like the bridge from the virology from the emitter to the receiver’s virology. And we can assist with risk assessment. “

In the current study, Cambridge researchers set out to ‘measure’ this bridge through a series of simulations. For example, if a person coughed and emitted a thousand drops, how many would reach another person in the same space, and how large would those drops be as a function of time and space?

The simulations used refined computational models that solve the equations for turbulent flow, along with detailed descriptions of droplet motion and evaporation.

The two-meter COVID-19 rule is 'random measurement' of safety

Visualization of dispersal of droplets by coughing. The drops are color-coded by size. Red = large, green = medium, blue = small, purple = very small. Credit: Shrey Trivedi et al, University of Cambridge

The researchers found that there is no sharp cut once the droplets are spread out over two meters. When a person coughs and does not wear a mask, most of the larger drops will fall on nearby surfaces. However, smaller droplets, floating in the air, can spread quickly and easily well over two meters. How far and how fast these aerosols disperse will depend on the quality of the ventilation in the room.

In addition to the variables surrounding mask wearing and ventilation, there is also a high degree of variability in individual coughs. “Every time we cough, we can emit a different amount of fluid, so if a person is infected with COVID-19, they can emit many virus particles or very few, and because of the turbulence, they spread differently for each cough,” Trivedi said.

“Even though I exhale the same number of drops every time I cough because the flow is turbulent, there are fluctuations,” Mastorakos said. “If I cough, fluctuations in speed, temperature and humidity mean that the amount someone gets at the two-meter mark can be very different each time.”

The researchers say that while the two-meter rule is an effective and easy-to-remember message for the public, it is not a safety mark given the large number of variables associated with an airborne virus. Vaccination, ventilation and masks – although not 100% effective – are essential to control the virus.

“We are all desperate to see the reverse of this pandemic, but we strongly recommend that people continue to wear masks in indoor spaces such as offices, classrooms and shops,” Mastorakos said. “There is no good reason to expose yourself to this risk as long as the virus is with us.”

The research team continues this research with similar simulations for rooms as lecture rooms that can help assess the risk as people spend more time indoors.


Free online tool calculates the risk of COVID-19 transmission in poorly ventilated areas


More information:
Estimates of the stochasticity of droplet spread by cough, Physics of liquids (2021). DOI: 10.1063 / 5.0070528

Provided by the University of Cambridge

Citation: The two-meter COVID-19 rule is ‘random measurement’ of safety (2021, 23 November) retrieved 24 November 2021 from https://phys.org/news/2021-11-two-meter-covid-arbitrary -safety. html

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