Is Six Feet of Social Distancing Always Necessary?
By: Markham Heid
Tags: Health, Pandemic, Covid 19, The Nuance, Coronavirus

6 Feet Apart Is the Gold Standard, but Should It Be?

Exploring the origins, and difficulties, of the 6-foot rule

It may have been the most bizarre card game in history.

Groups of men — some sick with the common cold, some healthy — sat around card tables for 12 hours, playing poker. The healthy men wore specially designed arm braces or plastic “collars” that allowed them to handle the cards and chips but made it impossible for them to touch their faces. The sick men were unencumbered and could freely touch the cards, the chips, or their own runny noses. The men were seated about 4.5 feet from one another.

The gonzo poker game was organized by researchers at the University of Wisconsin Medical School for a 1987 study that sought to measure how viral pathogens pass among people via different routes of transmission. Since the healthy men couldn’t touch their faces, the only way they could get sick was by breathing in airborne virus particles expelled by their unwell poker buddies.

Once this first part of experiment was over, the presumably cold virus–infested playing cards and chips that the sick men had handled were immediately transferred to a new lab room, where a fresh batch of healthy volunteers was waiting. These men played poker with the cards and chips for 12 hours and were directed to touch their faces every 15 minutes.

So who got sick? Among the healthy men in the first part of the experiment — the ones who couldn’t touch their faces but were sitting close to ill people — more than half ended up coming down with the common cold. Among the men who had to play with the germ-ridden cards, none got sick. “These results point to aerosol transmission as the most important mechanism of natural spread,” the study authors wrote.

That study is one of several older research efforts that — coupled with more recent work — have helped the Centers for Disease Control and Prevention (CDC) create guidelines designed to stop the spread of viruses and other pathogens. Those guidelines form the foundation of the government’s current SARS-CoV-2 recommendations, including its advice to stay at least six feet away from other people.

Three feet is the “area of defined risk” for health care workers exposed to patients who may carry an infectious disease.

“These studies looked at how likely it was that someone infected would communicate [that infection] to others in a shared environment, and then how far apart people were who became infected,” says Julie Fischer, PhD, an adjunct professor of microbiology and immunology at the Georgetown University Center for Global Health Science and Security. The results of these sorts of experiments are not always perfectly consistent, and most of the data is not specific to SARS-CoV-2. But Fischer says that the CDC’s guidelines are based on the best evidence to date and are designed to afford the public the greatest level of protection.

But guidelines are not laws. And some organizations that are planning to reopen this fall — in particular, some schools — are discussing whether a full six feet of physical distance is necessary to keep people safe from Covid-19. Some experts also say that the question of “what is a safe distance?” and “what distance is safest?” may have two different answers.

Balancing risks and benefits

Back in June, the University of North Carolina at Chapel Hill announced that when classes resume this fall, student desks and seating would be spaced a minimum of three feet apart — not the six feet recommended by the CDC.

The university said that its decision was based on input from infectious disease and public health experts. But the move triggered an immediate backlash among concerned students, their parents, and some faculty members. The school soon revised its policy to conform with the government’s six-foot guideline.

While many observers may have wondered just what the university’s administrators were thinking, some who were close to the controversy say that the debate isn’t as clear-cut as one might assume. “The three-foot rule — that’s classically what’s considered the safe distance,” says Efraín Rivera-Serrano, PhD, a molecular virologist at UNC-Chapel Hill. He’s referring to long-standing infectious disease guidelines, produced by the CDC, that say three feet is the “area of defined risk” for health care workers exposed to patients who may carry an infectious disease.

Rivera-Serrano says there’s no question that maintaining at least six feet of physical distance is optimal. But he points out that the World Health Organization, along with countries such as France and Denmark, have adhered to a one-meter (3.2 feet) physical distancing guideline throughout the pandemic. “Three feet should be enough, especially if [everyone is] wearing a mask,” he says.

The question of “how much distance is enough?” is a challenging one for schools and other institutions or businesses that are attempting to balance public safety with reopening imperatives. Everyone agrees that more distance is better when it comes to reducing exposure risks. But classrooms do not have unlimited space, and as the distance between two people increases, the amount of added risk reduction falls.

If people are wearing masks, it’s unclear whether there’s a large amount of additional risk reduction when people move from three feet to six feet.

To illustrate this point, imagine that someone has tossed a water balloon up in the air. If the balloon lands and explodes within three feet of your legs, you’ll probably get hit with some droplets. The farther away you move, the lower your risk falls of being splashed. But at a certain point, the odds of any water hitting you become so small that putting more distance between yourself and the balloon doesn’t do much to lower your risk.

The same basic rules apply to virus transmission. Rivera-Serrano says that when an infected person talks, sneezes, coughs, or even just breathes, that person expels droplets of saliva. The closer someone is to that person, the more likely they are to inhale one or more of those droplets. But if people are wearing masks, it’s unclear whether there’s a large amount of additional risk reduction when people move from three feet to six feet.

To his point, a July study published in the Lancet looked at data related to SARS, MERS, and Covid-19. It found that people’s risk of infection dropped from 13% to 3% when they maintained at least three feet of physical distance. “[P]rotection was increased as distance was lengthened,” the authors of that study concluded. But graphs included in the study suggest that the magnitude of the risk reduction beyond three feet may be quite small.

Rivera-Serrano says that whenever six feet or more of physical distancing is possible, people should follow that guideline. But he also says that if people maintain at least three feet of distance and are also wearing masks, it’s not yet clear whether the additional three feet of distance provides a significant added layer of protection — or, at least, one that is significant enough to keep a large percentage of U.S. students out of the classroom this fall.

The origins of the six-foot rule

As recently as the 1950s, health officials didn’t have a solid understanding of the ways in which common respiratory illnesses spread from person to person. That changed during the 1960s and 1970s when some pioneering research at the Common Cold Unit — a former initiative of the British Medical Research Council — revealed that close proximity to infected individuals, more so than touching infected surfaces, seemed to carry the greatest transmission risk.

A later study of English school children, published in 1982, suggested that virus transmission was elevated when students sat three feet or less from one another. “These studies formed the foundation of the standards developed by CDC and [the National Institutes of Health] and used in health care facilities, which are that anything closer than three feet carries the most risk,” Georgetown’s Fischer explains.

Before Covid-19, the three-foot guideline was still widely used in health care settings. So how did U.S. health authorities come up with the current six-foot recommendation?

“During the SARS epidemic, epidemiologists realized that three feet might not be enough to prevent droplet transmission,” Fisher says. “There was evidence that health workers who had moved through spaces within about two meters, or six feet, of SARS patients had become infected.”

SARS and Covid-19 are transmitted by related coronaviruses. Assuming that they are passed from person to person in similar ways, it follows that three feet might not be enough space to effectively lower the spread of SARS-CoV-2. But if everyone’s wearing masks, that could change the math. It’s uncertain how much added Covid-19 protection a person enjoys if they’re wearing a mask and they maintain six feet of distance from others, as opposed to three feet.

“People love to think about things in absolutes, but in biology there are always exceptions. With distance and risk, it’s a continuum.”

Experts say the answer likely depends on dozens of different variables. “We understand that unique air patterns, like the way air-conditioning flows, can make a big difference in how far droplets can move,” Fischer says. (At a restaurant in China, a person infected with Covid-19 sat close to an air-conditioning unit, which was believed to have carried the virus to diners sitting “downstream” in the path of the unit’s air flow.)

Also, a person who is sneezing, as opposed to talking or breathing, may expel droplets much farther. When people are outdoors, Fischer says that several different factors — such as UV light or humidity — may cause virus droplets to degrade or fall to the ground more quickly than they would indoors, and so transmission may be less likely. But the risks associated with all these scenarios are “hard to quantify” and highly situation-dependent, she says.

To sum all this up, public health authorities are doing their best to provide people easy-to-follow, evidence-supported guidelines that minimize the risk of virus spread. The best evidence to date suggests that maintaining six feet of physical distance is likely to be a highly effective way to reduce the odds of SARS-CoV-2 transmission. But some open questions remain, and debates about what distance is appropriate — especially when coupled with masks and instituted among low-risk groups — are sure to continue.

“People love to think about things in absolutes, but in biology there are always exceptions,” Rivera-Serrano says. “With distance and risk, it’s a continuum.”