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Infectious Diseases

Updated 07 April 2020

Why physical distancing is important - but an expert thinks the '2m rule' may not be enough

So whats's the optimal distance for physical distancing? Recommendations say 1-2 metres, and an infectious disease professor backs this up, saying that larger droplets seem to reach that distance through coughing or sneezing - but an MIT expert suggests these "clouds" can travel further.

In an effort to mitigate the spread of the new coronavirus, physical distancing has been initiated by many countries around the world. However, while the World Health Organization (WHO) and medical experts advise maintaining a distance of 1–2m from others, a researcher from the Massachusetts Institute of Technology (MIT) believes it isn't enough.

Lydia Bourouiba, an associate professor at MIT, has researched the dynamics of exhalations (such as coughs and sneezes) for years at The Fluid Dynamics of Disease Transmission Laboratory.

Her research found that exhalations cause "gaseous clouds" that can travel up to 8m. And more recent research, also carried out by Bourouiba, states that peak exhalation speeds are up to 30m per second.

More than this, experts are also trying to determine whether this virus spreads through both large droplets and smaller droplets. According to Bourouiba's research, when a person coughs or sneezes, it emits these gaseous clouds that can carry droplets of different sizes. Sneezing and coughing into your elbow, she added, does not entirely avoid this cloud.

Small vs. large droplets

Bourouiba refers to research focused on tuberculosis transmission in the 1930s by William F. Wells, who classified respiratory droplet emissions into "large" and "small" droplets.

Wells’ analysis showed that large respiratory droplets settle faster than they evaporate, and contaminate the immediate vicinity of the infected individual, whereas in the case of small droplets, they evaporate faster than they settle.

Drawing on this, Bourouiba argues that current guidelines are based on large droplets as the method of transmission of the new coronavirus, and are also based on the idea that these large droplets can only travel a certain distance.

To accept that these droplets simply "hit a virtual wall and stop there and after that we are safe", is flawed, Bourouiba told USA Today, as there is no evidence to suggest this.

However, Dr Paul Pottinger, an infectious disease professor at the University of Washington School of Medicine, told USA Today: “The smaller the germ particles, the lower the risk that they might infect somebody who would breathe them in or get them stuck in their nose or their mouth,” adding that the larger respiratory droplets, such as spit and snot, “that almost look like rain” is the bigger threat.

Physical distancing in the time of the Covid-19 pandemic

With the rise in the number of people becoming infected with the Covid-19 virus, health experts from around the world are emphasising the need for people to keep their distance from each other. But what exactly this distance should be has become a matter of debate.

In a paper published in the Journal of the American Medical Association last week, Bourouiba said that although the Centers for Disease Control and Prevention (CDC) recommends a 2m separation between people, these distances are mostly based on "estimates of range", and have not considered the potential presence of a "high-momentum cloud carrying the droplets long distances".

“Given the turbulent puff cloud dynamic model, recommendations for separations of 3–6ft (1–2m) may underestimate the distance, timescale, and persistence over which the cloud and its pathogenic payload travel, thus generating an underappreciated potential exposure range for a health care worker,” she wrote.

In her paper, she also mentions that peak exhalation speeds can reach 10 to 30m per second, and cautions that surgical and N95 masks that are currently used by healthcare workers are not tested for these characteristics of respiratory emissions that may occur.

As a result, there is an urgency in revising guidelines on the needs for protective equipment that are currently given by the WHO and the CDC, Bourouiba told USA Today.

Another study that was carried out by a team of Chinese government epidemiologists and published in the journal Practical Preventive Medicine, found that the virus can travel up to 4.5m, also challenging the safe distance advised by health authorities around the world.

Their research was based on a bus passenger who infected fellow travellers who sat a mere 4.5m away from him. This took place in January 2020. However, the study was retracted on 10 March.

However, Pottinger also believes that if Bourouiba’s claims of the virus being effective at distances of up to 8m is true, more people would be falling ill: “If this really travelled very efficiently by air, we wouldn’t be having this conversation. Everybody would know it’s true because everybody would be infected,” he said.

But Bourouiba is pushing for recommendations to be made based on current science, and especially stressed the urgency of policies stating that high-grade respirators should be made available for healthcare workers.

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Image: Getty