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

May 20, 2020, 7:00 PM UTC

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One of the core disruptions of life during the coronavirus pandemic is the new necessity for social distancing—also and more accurately called “physical distancing.” Because the coronavirus can travel on liquid droplets breathed or coughed out by infected people, an array of health authorities recommend staying away from crowds and maintaining physical separation from others. It’s why restaurants, bars, stores, and other places where people mingle closely have faced such economically devastating restrictions. The U.S. Centers for Disease Control and Prevention (CDC) specifically recommends a six-foot buffer.

But where does that number come from? Why six feet—and not 10 or four?

There is disagreement at the very highest levels: The World Health Organization (WHO) recommends just three feet of physical distancing, while some emerging evidence suggests the coronavirus can spread over much greater distances. The varied messages illustrate the sometimes awkward dance between evolving, complex science and simple, straightforward public health guidelines—choreography that’s particularly tricky amid increasing distrust of health authorities.

The WHO’s three-foot guideline follows some of the earliest research into how diseases spread. In the 1930s, Harvard researcher William F. Wells measured how far large exhaled droplets traveled and arrived at the three-feet figure.

But more recent science suggests Wells’ research didn’t capture the whole picture. In 2003, a group of researchers found that SARS—an illness caused by another type of coronavirus—was transmitted to others as far as six feet away from an infected person while traveling on an airplane. Several experts have cited this study as the most likely source for the CDC’s six-foot guideline.

Other studies, however, suggest even more caution may be warranted. Lydia Bourouiba, an MIT researcher who works at the intersection of fluid dynamics and epidemiology, argued in a recent report that coughs and sneezes create complex “turbulent gas clouds” that might carry pathogens as far as 27 feet. A new University of Nebraska study specifically examining patients with COVID-19 found that the virus may travel not just on heavy, short-range droplets but also in much finer—and more mobile—aerosol clouds. New laboratory research released last week by the National Academy of Sciences found potentially infectious droplets can remain suspended in air for as long as 14 minutes.

The steady flow of new scientific findings is a challenge for agencies like the CDC, who want to communicate best practices to the public in the clearest and most consistent way possible.

“The CDC is making the best recommendation that they think they can make based on limited evidence,” says Janet Baseman, a professor of epidemiology at the University of Washington. But the evolution of such guidelines can actually be a threat to the effectiveness of public health messages. Shifting messages about the role of protective masks in controlling the coronavirus, for instance, may have undermined public trust. Across America, that distrust—which has also been fueled by mixed messages from politicians and media figures—is increasingly blossoming into overt defiance of public health guidelines.

Krys Johnson, an epidemiologist at Temple University, says that uncertainty and changing recommendations are inevitable when fighting a completely new disease, making transparency especially important.

“It’s better to communicate something and state that you don’t know certain things than to wait until you know everything,” says Johnson, who previously helped battle the Zika outbreak in Florida that began in 2016. “Otherwise, it might seem like you’re withholding information.”

That makes one element of the CDC’s communication strategy particularly confounding: While experts interviewed by Fortune and other outlets have pointed to the 2003 SARS transmission study as a likely source, the CDC itself has so far declined to explicitly outline the thinking behind its six-foot guideline. Reporters at the news website Quartz say they sought clarification for nearly two weeks without receiving a response. The CDC also did not reply to Fortune’s request for further insight.

The CDC may be reluctant to show its work out of a desire to keep things simple. “I think people want more certainty,” Johnson admits, instead of endless reminders of how little we know about a deadly, invisible disease.

But certainty isn’t even the true goal of the six-foot rule. “[The CDC] is not saying, ‘If you’re six feet apart then you won’t get sick,’” says Baseman. “Because there’s not actually evidence that that’s true.”

Instead, this and other guidelines are all about improving the odds.

“Scientists in general are going to emphasize reducing risk,” says Johnson. “Because there are no absolutes in science.” Even for those who follow coronavirus guidelines rigorously, she says, “there’s still going to be some amount of risk. You can wear a mask, and it will help protect you and those around you, but you can still become infected.” She says a good analogy is the use of speed limits: They don’t eliminate the risk of injury on the road, but they meaningfully reduce it.

And, like speed limits and seatbelts, the various coronavirus guidelines work best together. “None of them by itself is intended to provide complete protection,” says Baseman. “Each one of them confers one level of protection, and, in combination, they will provide more.”

That appears to be particularly important given our evolving understanding of the novel coronavirus. Even if studies showing longer transmission distances are accurate, the six-foot rule and masks still reduce risk, leading to fewer infections and deaths overall.

Johnson acknowledges changing messages can be confusing for the public, but argues changes should be welcomed, not taken as grounds for mistrust.

“Anytime you see a change in the science, it’s a change for the better,” she says. “Because it means we have more information. It means we can better protect people.”