Where Did 6 Feet Come From?

I have written several articles on the coronavirus and on masks and healthcare issues. A series of links have been provided at the bottom of this article for your convenience. This article will, however address a different aspect of the virus or on healthcare issues in general.

Researchers say that social distancing guidelines should differ depending on where you are and what you (and those around you) are doing.

Review shows eight of ten studies find that respiratory droplets can travel beyond six feet. One concludes that a sneeze can send aerosols as far as 26-feet inside a room.

If you live in the United States, chances are you’ve heard guidance from the Centers for Disease Control and Prevention about social distancing. Per the organization’s website, individuals should stay at least six feet (two meters, or one tall person) away from one another—a form of social distancing—to help prevent the spread of the novel coronavirus that causes Covid-19.

But the CDC isn’t the only word on social distancing. The World Health Organization recommends that people stay at least half that distance apart—three feet, (one meter, or about a toddler’s height). Meanwhile, an opinion published in late March in the Journal of the American Medical Association by a particle fluid dynamicist at the Massachusetts Institutes of Technology suggested that people might do well to stay more than 27 feet apart (8.2 meters, or several tall people) to avoid infecting one another.

These conflicting recommendations are understandable, considering that SARS-CoV-2 didn’t exist (to the best of our knowledge) six months ago. Scientists are scrambling to figure out the details of how it spreads from person to person—but there isn’t a single study that can definitively describe all the ways the virus transmits between hosts. For now, these guides for social distancing are based on data collected on other pathogens—some of which may be outdated.

The WHO’s three-foot recommendation originates with work done in the 1930s done by William Wells, a Harvard researcher who studied tuberculosis. He found that droplets—bits of spit, mucus, and sputum (aka phlegm) emitted when we breathe, cough, or sneeze—tend to land within three feet of where they’re expelled.

Those droplets—a term researchers use to refer to biggish particles, ones that are more than five microns in diameter—can last on surfaces for a few hours to days, depending on the temperature. That’s why other precautions like hand washing and disinfecting are so important to prevent the spread of disease.

The three-foot cutoff for droplets has stuck around for nearly a century, and to be fair, scientists haven’t had reason to doubt its validity. Other viral outbreaks, like the flu, SARS, and MERS, which are also transmitted through droplets, seemed to behave similarly—at least enough so that authorities didn’t feel the need to update their guidance.

Rules vs. guidelines

We take comfort in the certainty of rules that scientists and public health officials give us—particularly with those that are meant to keep us safe. But as with most things in biology, the more scientists learn more about the ways that viruses spread, the clearer it is that these rules are more like guidelines.about:blank

When the SARS pandemic hit in 2003, for example, scientists found some evidence that the three-foot cutoff may not be enough. Researchers looked at the prevalence of SARS infections within a single flight, and concluded that droplets of the virus could actually travel between passengers six feet apart—not three.

The study, which looked at just over 100 people and was published in the New England Journal of Medicine, was allegedly the basis for the CDC updating their message to say that people should stay six feet apart to prevent transmission, according to a recent episode of Radiolab. Quartz tried to find the origins of the six-foot guideline with the CDC, but after multiple attempts over two weeks, the agency failed to comment.

Now, there’s anecdotal evidence that the six-foot cutoff may not be enough, either. “It is possible that special circumstances might lead to increased or decreased risks,” Giorgia Sulis, an infectious disease physician and epidemiologist at McGill University, told Quartz in an email. But with limited time to have studied SARS-CoV-2, none of those special situations have been investigated in detail.about:blank

One thing that could dramatically influence recommendations for social distancing is if SARS-CoV-2 could not merely be transmitted via droplets, but in even smaller particles called aerosols. Unlike heavier droplets which fall more or less to the ground, aerosols evaporate in the air, where they linger. This gives them a chance to spread out even farther, and increases the likelihood that someone inhales, swallows, or touches them (and then touches their nose or mouth).

Certain behaviors might promote aerosol formation—like being intubated to be put on a ventilator, or disconnected from it. Both the WHO and CDC note that Covid-19 may be transmitted through aerosols in health care settings. And anecdotal reports suggest it could happen outside the hospital: For example, there have been reports of people who were presymptomatic and singing within each other’s company spreading the virus to one another.

It’s certainly plausible that Covid-19 could be airborne. Already, scientists know that some infections, like measles, colds, chickenpox, and noroviruses are airborne. And previous work has suggested that everyone emits some potentially infectious material when speaking or singing. Lydia Bourouiba, the scientist who studies fluid dynamics at MIT, has done work that shows that coughs and sneezes can be projected with such force that they emit both droplets and aerosols capable of traveling between 23 and 27 feet. But none of this has been demonstrated in a lab with SARS-CoV-2 in particular.

Specific environments could change the virus’ transmission dynamics, too. “Walking in the open air, regardless of weather conditions, is likely much less concerning compared to interaction in closed spaces where air circulation is more limited,” Sulis says.

Time spent in the presence of a potential source could also affect transmission. So could viral load. “The longer the exposure time, the higher the chances of getting infected,” says Sulis, although again, infection risks are “limited” for those who interact from afar. And if a person is carrying a larger number of virion particles (viruses that haven’t found their way into one of our cells), they’d have more opportunities to spread those particles to others.

Without specific studies on SARS-CoV-2, all scientists and public health officials have are vague guidelines—which are at odds with the public’s desire for hard and fast answers. It’s possible that there is a specific, safe distance to remain apart to stop the spread of Covid-19. And it could be six feet when passing someone on foot outside on a blustery day, but farther when sitting at a distance from them to have a conversation. There simply isn’t enough data to be sure.

Which is likely why, even though six feet is a good guideline, it’s not wise to walk right up to it, literally speaking. Given the devastation of this pandemic, it’s best to give your neighbors as wide a berth as possible. Or, stay inside and look out the window instead.

Despite what studies show six feet seems to be the new standard of measurement in our world. Whether we’re sitting, walking, or running, we’re all urged to practice social distancing. There’s even a country song sporting the phrase in its title. Some research claims that six feet simply isn’t far enough away. So just how accurate is this coronavirus outbreak rule of thumb?

In a new report in The BJM, scientists from the Massachusetts Institute of Technology and the University of Oxford argue that the six-foot rule is part of oversimplified, overly rigid, and outdated science.

They contend that six feet isn’t always enough to keep people safe during the COVID-19 pandemic.

The history of droplet emission science

Researchers say modern social distancing guidelines are based on scientific studies from over 100 years ago. The six-foot rule was originally proposed in 1897 by Carl Flügge, a German bacteriologist who studied the movement patterns of fluid droplets expelled from the mouth when speaking, coughing, or sneezing. In his research, he found that visible fluid droplets containing pathogens travel about six feet. It was a hypothesis only. He tried testing it using glass plates. But it wasn’t four decades later that technology advanced enough to confirm the idea. It took high-speed photography to be able to visualize the emission of droplets.

In the 1940’s, a study of a bacterium known as haemolytic streptococci confirmed Flügge’s original findings. That study shows that most bacteria-containing droplets stay close to their source. In the early 1940s, scientists finally got their first glimpses of people’s sneezes hurtling through the air in real time, at a capture rate of 30,000 frames a second, confirming that indeed, most of the stuff we throw into the air when we sneeze, cough, or yell tends to settle down to the ground within about a wingspan or so (say, 3 to 6 feet). It also finds that a minority (10%) of people produce droplets that travel as far as 9.5 feet away.

When ‘the dogma was born’

Back then, scientists maintained that most of the infectious gunk people expel (say, about 90% of their pathogens) travel less than 6 feet away.

Their study measurements were never meant to be taken as hard-and-fast rules about how far we should stand from other people during a pandemic, though.

Nevertheless, these 3-to-6-feet rules of thumb have become easy-to-follow protocols for keeping potentially sick people at arm’s length during the coronavirus outbreak.

“The dogma was born,” the professor Lidia Morawska, a leading aerosol scientist in Australia, said of the 80-year-old 6-foot rule. “Like any dogma, it’s extremely difficult to change people’s minds and change the dogmas.” 

But as the coronavirus pandemic drags on for months on end, Morawska and other leading air and virus scientists and engineers are starting to lead a charge toward dismantling the old 6-foot rule and taking a more nuanced approach to managing the novel coronavirus’ spread.

Instead of always being on super-high alert, or assuming that a distance of 6 feet (or wearing masks, or washing hands) keeps us 100% safe all the time, they say, we should be learning how better to assess the situations we’re in every day, letting our guard down now and again when it’s relatively safe and moving back onto high alert when it’s appropriate. 

Can we start thinking differently?

On Tuesday, hoping to inject a little more of such empowerment into the ways people protect themselves from the virus’ spread, researchers from Oxford and MIT released a new traffic-light system that they hope will help people live life to its fullest while still being careful enough during the pandemic. 

Despite the general and persisting belief that six feet is a safe distance in most circumstances, recent studies continue to prove this belief wrong. In a recent review of 10 studies on droplet emissions, eight of the 10 studies find that respiratory droplets can travel farther than six feet.

One of those studies even reveals small droplets from a sneeze indoors can travel between 20 to 26 feet!

Large droplets and small droplets both carry pathogens

The original six foot rule is based on old scientific methods, which only allows scientists to visualize large respiratory droplets. Modern technology gives scientists the ability to visualize tiny droplets (aerosols) that can’t be seen with the naked eye.

While large droplets do tend to fall within six feet of their source, smaller droplets can travel much farther and can hang around in the air for longer periods of time. Moreover, small droplets can travel different distances depending on ambient airflow, like a person talking.

Given that small droplets can carry pathogens, it is important to consider their movement dynamics when developing social distance guidelines. This is particularly true for SARS-CoV-2 (the virus responsible for COVID-19), which can attach to air particles and remain in the air for up to 16 hours, researchers say.

Safe social distancing depends on what you do and where you are

The MIT and Oxford scientists write that it is also important to consider exactly what activities people are engaging in when social distancing. Some activities cause droplets to travel farther and stay in the air longer than other activities. Singing, talking loudly, and breathing heavily all increase the travel distance of emissions.

Activities that cause people to breathe heavily are of particular concern. That’s because they can cause high momentum droplet expulsions that are similar to coughs. These activities include things like running and other sports. On the bright side, respiratory droplets become more diluted in outdoor environments, making outdoor settings much safer than indoor settings for sports and other athletic activities.

Moving forward

The scientists conclude by urging for more nuanced social distancing guidelines. Specifically, they argue that distancing guidelines should be based on graded levels of risk.

While six-foot distancing is likely adequate in low-risk settings, greater distancing measures should be considered in high risk settings. Those include indoor areas with poor ventilation, situations in which large groups of people are together for long periods of time, and in the absence of face covering.

The coronavirus doesn’t follow hard-and-fast rules about infection, and neither should you

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Environmental scientists like Morawska say when it comes to talking about how the coronavirus spreads, there’s no point in trying to distinguish a droplet from an aerosol or distinguish what happens at 3 feet away versus 10. What matters most is how much virus has a chance to get into your body, regardless of how it gets there.

“There are three modes of transmission, and all three modes of transmission have to be controlled,” she said.

Those three modes are people (the most common source of infection), surfaces, and the air. 

“These things happen at the same time, and therefore distinguishing what’s what is very difficult,” Morawska said.

Life is more dangerous, then, in places where people become animated, excited, or otherwise loud in close confines, with stale air.

“Breathing out, singing, coughing, and sneezing generate warm, moist, high-momentum gas clouds of exhaled air containing respiratory droplets,” Bourouiba and her coauthors wrote in the BMJ. 

In such instances, a distance of even 20 to 30 feet may not be enough to protect you from an infection.

Meatpacking plants are then understandably ripe for viral spread because “the combination of high levels of worker contagion, poor ventilation, cramped working conditions, background noise (which leads to shouting), and low compliance with mask wearing” all contribute to viral spread, Bourouiba and her coauthors noted.

The same issue pops up easily in bars, gyms, indoor music venues, churches, and clubs. 

We need to be able to adapt to environments with different levels of vigilance so we’re not constantly on max alert

coronavirus risk infographic
A different risk chart, similar to the one that Bourouiba and her coauthors invented for the BMJ, which weighs the relative risks of different activities during the pandemic, based on various criteria. 

Engineers, virologists, and environmental scientists all stress that learning how to live with the virus can be done.

“I think everybody understands what traffic lights are in the general public,” Bourouiba said. 

She says it’s important to have such guides in mind as we weigh how to reopen, gather, and socialize during the pandemic, keeping our environment, and our behaviors in it, top of mind.

“We need to be able to adapt so that we are not constantly at max alert,” she said. “Not just the 6-foot rule or the mask on its own,” she added.

The simple questions to ask include: How long is the contact? Are people wearing face masks? Is the setting well-ventilated? And will it be quiet or loud? 

“It’s not rocket science to work out what needs to be done to minimize the risk of infection transmission,” Morawska said.


studyfinds.org, “Social distancing six feet apart is based on ‘outdated science,’ scientists say,” By Brianna Sleezer;qz.com, “Where does the six-foot guideline for social distancing come from?” By Katherine Ellen Foley; businessinsider.com, “The 6-foot social-distancing rule is based on nearly 80-year-old science. Scientists at MIT and Oxford have created a traffic-light system to use instead.” By Hilary Brueck;

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