Where does the six-foot guideline for social distancing come from?

How close is too close?
How close is too close?
Image: Reuters/Tyrone Siu
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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 For now, these guides for social distancing are based on data collected on other pathogens—some of which may be outdated.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.

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.

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.