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The Fallacy of the 2-Metre Rule - Part 1

Published 02/01/2021

“People think they are protected indoors and they’re really not”


Six months into the Covid-19 pandemic, an American governor recounted a visit she’d made to a local business. Upon entering, she didn’t wear a mask, she told her TV audience, “because I was socially distant.”

Only when she ventured near employees did she slip on her mask. Her message: Stay 6 feet (2 metres) apart from others. “But if you can’t, have a mask ready, and put it on.”

For nearly a year, the 6-foot/2-metre rule has been invoked by officials worldwide and gained traction with the public.

And yet, scientists say, the rule is misleading — based on an erroneous understanding of aerosol science and insufficient to curb Covid transmission.

“It has been biggest disservice that public health groups have given,” Robert Schooley, M.D., an American infectious-disease specialist told a panel at an international workshop on Covid-19 transmission. “It never should have come out of anybody’s mouth.”

Early on, experts assumed SARS-CoV-2, like the first SARS coronavirus, was spread almost entirely via close contact. They assumed virus-laden droplets, launched by a sneeze or a cough, were too heavy to travel beyond 2 metres, so physical distancing would largely control transmission.

A mountain of research has debunked these notions.

We now know large droplets can journey beyond 2 metres and, at any rate, play just a partial role in the spread of Covid-19. Often, the disease is transmitted via far smaller particles, known as aerosols, that can sail across a large room and hover for hours, only to be inhaled by vulnerable people.

“Indoors, those distance rules don’t matter,” says Robyn Schofield, PhD, an atmospheric chemist at Melbourne University in Australia. “People think they are protected indoors and they’re really not.”

The evidence documenting long-range aerosol spread of Covid-19 has major implications for hospitals, restaurants, pharmacies, retail spaces, offices — any enclosed, high-traffic space.

In the short term, the science makes a strong case for face coverings. However, it’s impossible to eat, drink, or have your teeth cleaned while wearing a mask, and at some point, following the Covid-19 vaccine rollout, mask guidelines may disappear.

SARS-CoV-2, however, will remain in our midst, along with influenza and other airborne pathogens that continually endanger lives. In addition, new airborne viruses and strains of antibiotic-resistant bacteria are certain to emerge.

“Distance alone will never solve the aerosol problem,” says Jose-Luis Jimenez, Ph.D., a University of Colorado chemist. “If you are in the same room, you can get infected.”

That’s why building operators must augment their current infection-control precautions, upgrading ventilation, filtration, and air dis-infection technology.

The Dubious Origin of the 2-Metre Rule

Back when commercial flights were divided into smoking and non-smoking sections, it didn’t much matter where you sat; before long, the smell of smoke would permeate the plane.

In the same way, particles carrying SARS-CoV-2, which are similar in size to smoke particles, can permeate a room.

“High concentrations of smoke do not usually build up outdoors because the smoke is quickly diluted by the large volume of air. This is also true for respiratory droplets,” explains at team of mechanical engineers at Clarkson University in New York.

But indoors — where the odds of Covid transmission are 18.7 times greater than outdoors — the picture is quite different: Lack of ventilation “allows the concentration of small airborne respiratory droplets to build up over time, reaching all corners of a room.”

Would a 2-metre separation, even if bolstered by a plexiglass shield, block cigarette smoke?

“The answer is no,” William Ristenpart, PhD, a University of California chemical engineer, told the international Covid workshop panel. “[SARS-CoV-2] aerosol particles behave the same way.”

Yet restaurants and shops often operate as if coronavirus particles either vanish within 2 metres or can be stopped by plastic dividers. Restaurants place tables 2 metres apart, and offices do the same with desks. See-through barriers have been installed in karaoke bars and countless retail establishments.

Surprisingly, these precautions rely on a model of disease transmission that dates back nearly a century.

It was 1934 when William Wells, an American scientist, published a paper dividing infectious respiratory particles into two categories: large and small. Wells believed small droplets (less than about 10 microns) evaporate rapidly while large ones quickly succumb to gravity.

“He found that the farthest any droplets travelled before either settling or evaporating was about 6 feet,” the Clarkson team recounted.

But Wells didn’t have the high-speed video to deconstruct the explosive aftermath of a sneeze or a sigh.

Turns out, respiratory droplets come in a wide range of sizes, not just “small” and “large,” and they can drift far and wide.

Once expired, droplets travel in clusters, and the warm, moist cloud enveloping the droplets allows the tiniest among them to “evade evaporation” for long periods, explains Lydia Bourouiba, PhD, an expert in fluid dynamics at the Massachusetts Institute of Technology.

“The cloud is in fact moving forward, trapping its payload over wide ranges of distances,” says Bououiba, who films exhalations with cameras running at thousands of frames per second. “There is really no barrier of 1 to 2 meters.”

Even large droplets can travel beyond 2 metres. “You have to get up to a size of 30 microns for something that won’t travel more than 2 metres,” reports Linsey Marr, PhD, an environmental engineer at Virginia Tech university.

And that’s in stagnant air. Under high air velocity, Marr says, “a 30-micron droplet can travel 7 to 8 metres.”

A 5-micron aerosol takes about half an hour to drop to the floor and, under certain conditions, can travel over 100 metres, according to Marr. Air currents in a room play a big role in how far aerosols can journey.

Yet the 2-metre theory continues to influence, even dictate, public-health guidance.

“The idea of 6 feet is to keep you out of each other’s large droplets, but that doesn’t do anything for the background aerosols that might be in the room,” says Marr.

Read part two here.

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