Six Foot COVID-19 Rule Is “Arbitrary” – Social Distancing Is Not Effective Mitigation on Its Own

Visualization of the spread of droplets when 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 found that airborne transmission of COVID-19 is highly random and suggests that the two-meter rule was a number chosen from a “continuum” of risks, rather than a concrete measure of safety.

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

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

The results, published in the journal Physics of Fluids, suggest that social distancing in itself is not an effective mitigating measure, and underscore the continued importance of vaccination, ventilation and masks as we enter the winter months in the Northern Hemisphere.

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

“I remember hearing a lot about how COVID-19 spread through doorknobs in early 2020, and I thought to myself that if it did, the virus must leave an infected person and land on the surface or enter through liquid into spread the air. mechanical processes,” said Professor Epaminondas Mastorakos from Cambridge’s Department of Engineering, who led the study.

Mastorakos is an expert in fluid mechanics: the way fluids, including exhaled breath, behave in different environments. Over the course of the pandemic, he and his colleagues have developed several models for the spread of COVID-19.

“Part of the way this disease spreads is virology: how much virus you have in your body, how many virus particles you expel when you speak or cough,” says first author Dr. Shrey Trivedi, also from the Engineering Department. “But another part of it is fluid mechanics: what happens to the droplets once they’re expelled, and that’s where we come in. As fluid mechanics specialists, we’re like the bridge from the sender’s virology to the receiver’s virology and we can help with risk assessment.”

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

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

The researchers found that there is no sharp cut when the droplets spread beyond two meters. When a person is coughing and not wearing a mask, most of the larger droplets will fall on nearby surfaces. However, smaller droplets, which float in the air, can quickly and easily spread well above two meters. How far and how fast these aerosols spread depends 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 cough. “Every time we cough, we can expel a different amount of fluid, so if a person is infected with COVID-19, they can expel many or very few virus particles, and because of the turbulence, they spread differently with each cough,” Trivedi said. .

“Even if I blow out the same number of droplets every time I cough, because the flow is turbulent, there are fluctuations,” Mastorakos said. “When I cough, fluctuations in speed, temperature and humidity mean that the amount a person 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’s not a sign of safety, given the large number of variables associated with an airborne virus. Vaccination, ventilation and masks – while not 100% effective – are vital to contain the virus.

“We are all desperately looking for the back end of this pandemic, but we strongly recommend that people continue to wear masks in indoor areas 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 is continuing this research with similar simulations for spaces such as lecture halls that can help assess risk as people spend more time indoors.

Reference: November 23, 2021, Physics of Fluids.
DOI: 10.1063/5.0070528