As communities began considering whether to reopen schools this fall, the American Academy of Pediatrics faced a challenge. The professional organization supports in-school teaching as the best way for kids to develop and learn. But one of the best ways to prevent the spread of Covid-19 is to stay away from infected people. That left members of the organization with conflicting imperatives as they mulled what sort of advice they ought to give school administrators — and with no clear science on what “a safe distance” really means.

Since early January, the World Health Organization has been calling for separation of at least one meter (a little more than 3 feet). The Centers for Disease Control and Prevention, meanwhile, has been recommending nearly twice that distance — 6 feet or more.

When the AAP issued its guidance in late June, followed by an update last week, it recommended that desks be placed at least 3 feet apart, but noted that 6 feet would be ideal. (Face coverings are called for either way.) According to the AAP, “schools should weigh the benefits of strict adherence to a 6-feet spacing rule between students with the potential downside if remote learning is the only alternative.” The guidance recommends a distance of 6 feet for teachers and staff — the CDC’s wider berth.

Some studies have shown that younger children don't transmit the virus as readily, and space is at a premium in many of the nation's crowded schools. And with the AAP’s stated goal of having students physically present in class, enforcing 6 feet of space would be a real challenge for many school systems, especially given the camaraderie among K-12 students. Indeed, some reopened high schools where students have been photographed crowding together have already closed again due to outbreaks.

Mountains of existing research on viruses similar to SARS-CoV-2 were available to the AAP, as well as fresh studies on the new virus, some of which haven’t yet been published in peer-reviewed journals. The group had to weigh staffing and space practicalities and what parents and communities would accept. It’s the same information that’s available to policymakers, business owners, family reunion planners, and pretty much everyone else trying to negotiate the new normal. None of it is definitive.

And it turns out that questions about distancing don’t end at 6 feet. In the past few months, environmental scientists, physicists, engineers, epidemiologists, and others have become increasingly vocal with concerns that the virus might be transmitted through smaller droplets that can reach as far as 26 feet on violent exhalations like coughs and sneezes. The droplets can slow down, dry out, and hang in the air for hours. (No matter what distance they support, pretty much everyone weighing in on distance recommends that people wear face masks when out in public and close to others who might be potentially infected with SARS-CoV-2.)

Scientists describe the risk as a continuum from high to low; few believe the risk is high at 26 feet. But the conundrum facing health agencies and lawmakers in the absence of a consensus on just how far away is far enough is both difficult and consequential. Pediatrician and historian Howard Markel of the University of Michigan, one of the people who started the discussion of physical distancing 15 years ago, said different messages from policymakers can make it tough for the public. He said lack of precision is bound to make people worried about why there’s no one single answer.

T he idea of keeping infected people away from healthy ones shows up in the Bible and in the writings of Muhammad, and has played out on a grand scale through centuries of quarantines. When the 19th century brought the discovery of microbes as agents of disease, scientists learned that infection could spread through the air, propelled by coughing and sneezing, or even through just speaking or breathing.

In the 1930s, American scientist William F. Wells wrote that pathogens could ride out on people’s breath in two forms. The larger form, today often called “droplets,” are subject to gravity. They fall to the ground within a few feet. The tinier aerosols can float in the air, riding air currents. (There’s some debate within the scientific community over the vocabulary of particles in the air, but we’ll use droplets and aerosols here.)

So if large droplets are a distinct category from small aerosols, and if a bacterium or a virus spreads mostly on the large droplets, and if the large droplets fall from the air within just a few feet, then it makes sense to keep just a few feet apart from other people, with about 3 feet versus 6 feet being a key question.

“Air that is hot and moist comes out of the lungs carrying a continuum of droplet sizes, spanning [from] large invisible drops to invisible mist,” said Lydia Bourouiba, who studies fluid dynamics and infectious diseases at the Massachusetts Institute of Technology. “When you are exhaling, sneezing, coughing, singing, or talking, the air that you are exhaling is carrying all of these drops.”

To know for sure how far an infectious virus like SARS-CoV-2 can travel, you’d have to expose volunteers to infected people at various distances under various conditions, and see what happens. That’s been done with some of the viruses that cause the common cold. But it can’t be done for SARS-CoV-2, given its potential lethality.

Still, public health agencies have to issue advice for Covid-19, even in the absence of complete data or consensus. The CDC issued general guidelines in 2007 for pandemic flu, calling for social distancing for everyone, without specific distances. It wasn’t a popular suggestion. “When we were recommending for the first time social distancing measures as a last ditch effort, as a nuclear option, there were people saying we would ruin the world,” said Markel.

In 2017, the CDC committed to large droplets for pandemic influenza, with a new set of guidelines specifying at least 3 feet for community settings such as schools and workplaces. For its Covid-19 guidelines, the CDC increased its recommendation to 6 feet or more.

An earlier study of the first SARS coronavirus infection on an airplane had found that people three rows away from the index patient (7.5 feet) were at risk. And the CDC referenced a paper by Wells that specified that droplets traveled about 6.6 feet down (though it was not clear about how far out). Several studies in the early 2000s suggested the large droplets landed about 3 to 5 feet away.

The World Health Organization, meanwhile, settled on close-in droplet transmission in a Jan. 5 communication with member states, even before Wuhan, China was locked down.

In the pandemic’s early stages, the WHO asked an international group of researchers led by Derek Chu of McMaster University in Canada to consider the questions of distancing, masks, and eye protection. The researchers reviewed 172 studies of SARS-CoV-2 and other coronaviruses in health care and public settings. In a June 1 article in The Lancet, they concluded that “a physical distance of more than 1 meter (just over 3 feet) probably results in a large reduction in virus infections.” For every 3 feet more, the relative effect “might” increase as much as two times.

The Chu study didn’t look closely at the question of whether aerosols could carry the virus as well or whether droplets could travel further, other than to note that some studies of air samples from patients’ rooms had found viral material, but others had not, and that there was no evidence yet showing that the viral material found was viable enough to cause an infection.

F or now, both the CDC and WHO guidelines presume transmission is primarily through large droplets. That assessment is opposed by a number of researchers who believe that finer particles that can dry up and stay in the air for hours must be considered.

Part of their reasoning comes from epidemiological studies of Covid-19 outbreaks. One non-peer reviewed study posted in late April described an incident in Guangzhou, China, where the virus spread to three families at separate tables in a poorly ventilated restaurant; only those in the airstream of the index patient were affected, suggesting the airborne virus could travel some distance. (A note of caution: Studies that have not been peer-reviewed should not be considered to be established information.)

Another study posted in mid-July (and also not yet peer-reviewed), reported on what happened on the Diamond Princess cruise ship early in the Covid-19 outbreak. Nineteen percent of the 3,711 passengers and crew became infected, apparently from a single passenger from Hong Kong. It’s unlikely that all the victims passed within a few feet of each other. Using computer models, the researchers determined that the primary mode of transmission was aerosols.

And several studies have suggested that viruses similar to SARS-CoV-2 might spread beyond 6 feet.

Studies like these led the Center for Evidence-Based Medicine (CEBM) at the University of Oxford to conclude in an analysis that fine aerosols can transmit infection across distances longer than just a few feet. The CEBM scientists also considered physical experiments in which the virus remained stable in air for hours, and even migrated across rooms, though the evidence that the virus is capable of infection is indirect. The analysis noted evidence from Bourouiba that small droplets in a sneeze or cough can travel in fine mists up to 6 to 8 meters (20 to 26 feet). While the researchers admit the evidence is limited, they noted that, “there is no direct evidence that it is not spread this way.”

In fact, many scientists share a growing concern about aerosols — and by extension, about the adequacy of current precautionary warnings. In early July, 239 scientists published an appeal to the medical community, directed primarily at the WHO, warning that there is a real possibility that tiny droplets could carry the virus beyond a couple of meters, to “room scale.”

The WHO has yet to offer a full explanation regarding its decision to opt for a guideline of just over 3 feet, and the agency didn’t respond to a request from Undark seeking further explanation. But following the scientists’ appeal, WHO officials said they’d been considering the issue and released a detailed brief noting that while there’s evidence that droplets of other respiratory viruses can turn into aerosolized particles and travel farther, aerosol transmission hasn’t been shown yet for SARS-CoV-2. And it suggested that studies showing that SARS-CoV-2 samples spewed out of high-powered jet nebulizers and lasting three to 16 hours in the air might not reflect what happens in the real world.

There are others who are waiting for more evidence as well. John Conly, a member of the WHO’s advisory group on guideline development for Covid-19 and a professor at the University of Calgary, hasn’t seen convincing evidence of cultivatable virus in aerosol samples yet in quantities high enough to cause infection. “If we don’t have cultivatable, infection-competent virus in the air samples, how can we decisively conclude that it is in the small aerosols?” he asks. “The weight of the scientific evidence at this point would not support airborne [transmission] as a predominant route.” He is, though, open to the possibility: “Not to say that it could not have occurred.”

Meanwhile, evidence for aerosolized spread has continued to accumulate. Joseph Allen, an associate professor of exposure assessment science at the Harvard T.H. Chan School of Public Health, said scientists have been warning about it for months. “It’s not like at 6 feet everything drops or 3 feet everything drops off,” he said.

In early August, a University of Florida team said they had measured viable virus — that is, virus capable of reproducing — in air collected 6.5 to 16 feet away from two hospitalized patients with Covid-19. Their paper has yet to be peer reviewed, and no one knows if the concentration of the virus in the air was high enough to infect people, but the study suggests the possibility that far-flung aerosols might be a problem.

Distance should be looked at in combination with other factors, not in isolation, said Bourouiba. Air changes, occupancy, length of exposure, indoor vs. outdoor, whether it’s a health care setting with infected patients or not, all are important. “An indoor space usage for an extended amount of time with poor ventilation, even sitting more than 6 feet apart would not be sufficient,” she said. “If you are in a big park with air flow and winds and everybody wearing a mask, then 6 feet could be OK.”

Linsey Marr, a civil and environmental engineer at Virginia Tech, wrote an opinion piece recently in The New York Times supporting the idea that the aerosolized virus is infectious. But, she wrote, “accepting these conclusions wouldn’t much change what is currently being recommended as best behavior.” What makes the concept of aerosolization important is that it highlights the importance of opening windows, improving indoor airflow, and making sure masks fit properly, she noted.

S cience is usually a piecemeal process until the mosaic reveals the full picture. No one can say for sure how much riskier 3 feet is than 6 feet. And while evidence is accumulating that the virus travels farther, no one can say for sure how quickly the risk drops. Even once the risks are better understood — they no doubt will be, with time — acceptable risk is a value judgement.

Markel, the pediatrician and historian, prescribes erring on the side of caution, and patience with evolving rules. “We don’t have a lot of experience with this. We’re learning as we go along.”

The AAP had to figure out where caution begins, in a situation where the evidence is low and the consequences are high. Schools that can’t achieve the recommended distance may decide to remain closed — or reopen, then end up sending students home again — leaving parents with difficult decisions about childcare and home schooling. Already, many school districts are opting to begin the school year remotely or offer a combination of virtual and in-person learning.

“This is not a risk-free world,” said Mobeen Rathore, a pediatrician and AAP spokesperson. “This is tough for parents, this is tough for teachers, and this is tough for all of us who worry and care about kids.”

But one thing is certain: Keeping some sort of distance is crucial. “I keep telling anyone who will listen to me that we are not going to have safe, but safer, schools,” Rathore said.

Joanne Silberner is a Seattle-based reporter who covers global health, mental health, medical research, and climate change. Her work has appeared on NPR and in STAT, Discover, Global Health Now, and the BMJ, among other publications.

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