Public health experts are reevaluating guidelines for safe social distancing amid growing evidence that the novel coronavirus can travel farther than six feet under some conditions.

A team of infectious-disease experts argues in a new analysis, published this week in the journal BMJ, that six-feet protocols are too rigid and are based on outmoded science and observations of different viruses. Other researchers say six feet is a start — but only a start — warning that more space is almost always better, especially in poorly ventilated areas indoors.

Factors such air circulation, ventilation, exposure time, crowd density, whether people are wearing face masks, and whether they are silent, speaking, shouting or singing should be part of assessing whether six feet is sufficient, experts say.

"I think six feet is a fine number, but we need to convey that this is a starting point," said Linsey Marr, a Virginia Tech civil and environmental engineering professor who has studied airborne viruses and was not involved with the BMJ report. "Beyond six feet doesn't mean there's zero risk."

The conventional wisdom behind six-foot separations originated from research by a German biologist, Carl Flügge, who in the late 1800s suggested that was as far as microbe-containing droplets could travel. His hypothesis missed farther-flung particles invisible to the naked eye — in particular, the tiny gobs of bodily fluid and virus that float on the air as aerosols.

If the novel coronavirus can float in the air as a vapor, earlier assumptions of its range are inadequate. Airborne transmission is still not conclusively proved, but a growing number of experts see persuasive evidence in super-spreading events that have transmitted the virus to people scores of feet away from the infection source.

"Distance alone will never solve the aerosol problem. If you are in the same room, you can get infected," said University of Colorado aerosol expert Jose-Luis Jimenez. The infection reached a person 45 feet away at a March choir practice in Washington state, where a singer spread the coronavirus to 52 people.

"Outdoors, distanced and with well-fitted masks," Jimenez said, "is the only thing close to a silver bullet."

Yet six feet, and sometimes less, remains the default guide. The Centers for Disease Control and Prevention defines social distancing as "at least six feet (about two arms' length) from other people who are not from your household in both indoor and outdoor spaces." The World Health Organization has recommended at least one meter, or three feet. Some countries in Europe set social distances at 1 1/2 meters, almost five feet; others at two meters, or 6 1/2 feet.

The United Kingdom earlier in the pandemic implemented a two-meter mandate for diners and drinkers. Under pressure from pubs that feared this rule would limit patrons to unprofitably low numbers, Prime Minister Boris Johnson in July compressed that to a "one-meter-plus" separation.

Because U.S. public health officials have so frequently recommended six-foot separations, the measurements have been mistaken for goal lines beyond which exists total safety. But that does not reflect how this virus spreads, numerous researchers said.

Another key factor is the motion of the air. "It becomes very important to not think just about a fixed distance. It's very important to think about the air flow," said Lydia Bourouiba, an author of the BMJ report who studies the fluid dynamics of infectious disease at the Massachusetts Institute of Technology.

One helpful analogy is the cloud from a cigarette smoker. "As you move farther away, you're exposed to less because it becomes more diluted," Marr said. "The smoke doesn't stop, though, at six feet."

Bourouiba and her colleagues created a chart to delineate low-, medium- and high-risk scenarios. Jimenez and other experts unaffiliated with the report welcomed the table as a valuable tool to assess surroundings.

The chart was made to empower people "to evaluate relative risk in a way that is more subtle than just a one-rule-hammers-it-all," Bourouiba said. Six feet is "not enough protection" in some environments, she said. On the other hand, rigid adherence to that in relatively safe situations "creates, in some sense, more strains." For example, she said, "it's not the end of the world" if people are a little closer in the lowest-risk settings.

What might those scenarios look like? "If you're outside, it's very well-vented, it's completely open air, there are no stagnation points in the airflow and people are wearing masks," Bourouiba said. Even in those situations, she suggested keeping encounters brief and avoiding groups of people who are packed tightly together.

Bourouiba cautioned that the chart was designed as a guide for people who are showing no symptoms. It also does not account for factors such as indoor air patterns or a person's susceptibility to the virus.

What counts as a short duration also remains ill-defined. "People talk about five to 15 minutes, but there's still not enough science to say," Bourouiba said. That's because time is linked to dose — the greater duration spent near someone, the greater potential for more exposure to virus. Researchers are trying to determine the dose of coronavirus required for an infection.

Asked whether promoting a more nuanced view of social distancing might lead to noncompliance in a country where some people refuse to wear masks or take other precautions, Bourouiba said her intent was to help hypervigilant people relax, not to give rule-flouters additional opportunities. She said it was aimed especially at the managers of shared spaces, such as school leaders.

"Keeping the highest level of awareness is something that we can do just for so long," she said. "And if we're going to stay with this for a whole year or two . . . what's really important is people understand when there is a very high risk."

The WHO has hesitated to classify the virus as airborne, while acknowledging that airborne transmission of the virus can occur in some circumstances, such as when medical procedures generate small, virus-containing aerosols. More than 200 scientists wrote an open letter to the WHO, as The Washington Post reported in July, urging the organization to acknowledge the role of small aerosols in transmission. The agency has since cited "emerging evidence" of airborne transmission.

One hurdle is the difficulty of detecting living, sufficiently infectious virus in aerosols. Coronavirus pathogens in the air may be underreported because the virus is so fragile, said Julian Tang, a virologist at Great Britain's University of Leicester and the National University of Singapore. Scientists use machines to draw virus out of the air. But some air sampling devices can shred the pathogens they are meant to capture.

Despite the difficulty in capturing and measuring live virus, a few research groups have done so. Tang noted two recent observations in hospitals: An air sampling study, published in late July, found enough virus floating near the rooms of patients with COVID-19, the disease caused by the coronavirus, at the University of Nebraska Medical Center, including in hallways, for scientists to grow the pathogen in cell cultures. Another study, which has not yet gone through peer review, found living virus up to 16 feet away from coronavirus patients at a hospital in Gainesville, Fla.

“Infected people, whether they have symptoms or not,” said Marr, the Virginia Tech professor, “are releasing virus into the air. And you can get it by breathing that in.”