In February, a man in Chicago brought food to and hugged two friends who had recently lost a family member. The next day, the man went to the funeral, where he comforted other mourners and shared a potluck meal. A few days later, he attended a family birthday party.
The man had symptoms of a mild respiratory illness. Later he’d learn he had COVID-19.
His acts of condolence and celebration set off a chain reaction that sickened at least 16 people, three of whom died. At the time, social distancing measures weren’t yet in place in Chicago. COVID-19 had yet to circulate widely in the area.
The case now serves as a cautionary tale, underscoring recommendations for people to keep their distance from anyone outside their immediate household, researchers report April 8 in Morbidity and Mortality Weekly Report. But how much distance is needed to avoid spreading the coronavirus?
Six feet (or two meters) has become the mantra. The World Health Organization and other experts have said SARS-CoV-2, the virus that causes COVID-19, is spread mainly by large droplets sprayed when people cough or sneeze, contaminating surfaces. So that degree of separation, combined with frequent hand-washing, was thought to be enough to halt or at least slow the spread of the virus.
But new evidence suggests six feet of distance may not be enough. If SARS-CoV-2 is airborne, as scientists think it may be, people could be infected simply by inhaling the virus in tiny aerosol droplets exhaled by someone talking or breathing.
What’s actually safe is unknown. It may depend on many factors, including whether people are inside or outdoors, how loudly people are speaking, whether they are wearing masks, how well-ventilated a room is, and how far the virus can really fly.
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Say it, spray it
When people exhale, talk, sing, cough or sneeze, a cloud of droplets of various sizes leaves the mouth or nose, says Lydia Bourouiba, a fluid dynamicist at MIT. Most simulations of droplet behavior have considered big and small droplets separately. Researchers have worried mainly about bigger droplets — 5 to 10 micrometers in diameter or larger — as vehicles for transmitting viruses, bacteria or other contagious organisms.
Bigger droplets can pack in more infectious organisms, giving a greater chance of infection if someone comes into contact with them. But the bigger the droplets are, the heavier they become, dropping fairly quickly to the ground. Such droplets are thought rarely to travel more than a meter or two before hitting the ground or another surface.
Those droplets might infect people by direct contact, such as when someone coughs or sneezes right in your face. But researchers think indirect contact is the main way people catch viruses, says Qingyan Chen, a mechanical engineer studying how infectious diseases spread at Purdue University in West Lafayette, Ind. Indirect contact might involve an infected person using their hand to cover a cough or a sneeze, then touching a cup or another object. If an uninfected person handles the object, the virus could transfer to that person’s hands. An unwitting nose scratch, eye rub or finger food snack could then infect that person. That’s why handwashing is so important.
Breathing in smaller droplets, known as aerosols, exhaled or coughed up by an infected person may also cause infection. Tiny droplets have a hard time overcoming drag from air and are thought to hang around a person, within a meter (a few feet).
Hence the six-foot rule: It was thought to be far enough to be safe from both occasional long-range spit bullets and invisible clouds of smaller particles.
But droplets spewed from people’s lungs come in a continuum of sizes, from those big enough to see with the naked eye to microscopic droplets churning through the air as a turbulent cloud, Bourouiba says.
“This cloud, in fact, changes everything about the dispersal of the drops that you don’t really see,” she says. The warm, moist exhaled air within the turbulent cloud has forward momentum from breathing, coughing or sneezing, carrying droplets of all sizes much farther than previously thought. In the case of a sneeze, droplets can travel up to eight meters (23 to 27 feet), Bourouiba reports March 26 in JAMA. That means even small droplets may spread throughout a room.
And if droplets fly that far, the virus may, too. “There’s no reason to believe that the virus only stays in those [droplets] that fall close by,” she says.
Coughs also can propel aerosol droplets beyond six feet, evidence suggests. Over three flu seasons, fluid mechanics engineer Eric Savory at the University of Western Ontario in London, Canada, and colleagues persuaded sick people to cough into a large box that allowed the researchers to measure how fast and far respiratory viruses travel. The volunteers coughed while they were sick with influenza, RSV or cold-causing coronaviruses. Some came back after they were feeling well to cough for science again.
Even a meter away from the mouth, coughs are still traveling at about a meter per second, the researchers discovered. “It’s not a speed you can avoid by turning your head away,” Savory says. Volunteers who were either ill, convalescent or healthy all coughed at about the same velocity. Results of the study will appear in an upcoming issue of Indoor Air.
The small droplets do slow down as they get farther from the mouth, Savory says. But his data don’t suggest what’s a safe distance. “A good guidance is you’re lessening your risk [of infection] the farther you are away from someone.”
Singing to the choir
The smallest airborne droplets may be more of a worry than scientists previously recognized.
Aerosol droplets containing infectious SARS-CoV-2 particles can hang around in the air for hours, a March 17 study in the New England Journal of Medicine found. The experiment, conducted under lab conditions, measured air samples for only three hours, but found still-infectious viruses. Some researchers have criticized that study because the virus-laden droplets were made using a medical machine, not by methods that more closely mimic breathing.
But people experience the spread of aerosol particles every day, says William Ristenpart, a chemical engineer at the University of California, Davis. If someone on one side of a large room lights a cigarette, puts on perfume or opens a box of chocolate chip cookies, the smell eventually reaches the other side of the room. “It’s not because [the smoker/perfume wearer/cookie eater is] coughing,” he says. Turbulence created by air mixing carries aerosol droplets around the room.
Ristenpart investigates whether influenza and other respiratory diseases can spread by airborne particles. The coronavirus’s contagiousness is a clue that it might, he says. Researchers at the U.S. National Academies of Science, Engineering and Medicine also concluded in an April 1 report that the virus might spread through aerosol particles (SN: 4/2/20).
Add to that the fact that people can spread the virus before they develop symptoms, or without ever developing symptoms (SN: 3/13/20). In fact, COVID-19 may be most contagious one to two days before symptoms appear, when people don’t even know they are infectious (SN: 4/15/20). Almost half of people in Iceland who tested positive for COVID-19 didn’t have symptoms when diagnosed, researchers report April 14 in the New England Journal of Medicine. Many would probably develop symptoms later, says Kári Stefánsson, founder of deCODE Genetics, a Reykjavik-based company screening anyone who wants a test.
By definition, people without symptoms aren’t coughing and sneezing. But they are talking and breathing.
Aerosol particles released when people sing may have led the coronavirus to spread to 45 members of the Skagit Valley Chorale in Washington. Two died.
Some choir members met for practice on March 3 and 10 before Washington state issued a stay-home order and before Skagit County had any known COVID-19 cases. Choir members reportedly kept six-foot distance from each other. But in belting out tunes, whistling and talking to one another, infected choir members may have propelled the virus into each others’ safety zones.
“A good singer knows how to use all the air in their lungs,” says Donald Milton, an infectious disease specialist at the University of Maryland School of Public Health in College Park. That may lead to exhaling lots of coronavirus, or breathing it deep into the lungs during breaths between refrains.
Even just talking face-to-face with an infected but asymptomatic person may be enough to spread the virus, Ristenpart and colleagues propose April 3 in Aerosol Science and Technology. Standing six feet apart might cause people to raise their voices to be heard, and people produce more aerosols and larger droplets the louder they speak, Ristenpart and colleagues reported in February 2019 in Scientific Reports. “There’s a compelling case that one should be suspicious of conversation as a possible vector for transmission,” he says.
Speech generates hundreds of big, wet drops, researchers report April 15 in the New England Journal of Medicine. “Stay healthy” is, ironically, a phrase that sprays a lot of saliva droplets. But that study was not capable of measuring droplets smaller than 20 micrometers across, says Matthew Meselson, a biologist at Harvard University who wrote a comment on the study, also appearing April 15 in NEJM. Other studies have determined that talking produces thousands of aerosols from the lungs for every saliva droplet from the mouth, he says.
It’s too early to tell whether aerosols and big droplets produce different severities of infection, he says. The pandemic may change doctors’ and researchers’ view of how respiratory viruses in general spread, Meselson says. “I think we’ll find we really were behind the curve when it comes to thinking about how disease is transmitted through the air.”
Masks to the rescue?
Findings about aerosol spread and symptom-free spread led the U.S. Centers for Disease Control and Prevention to recommend that everyone wear masks in public. With medical-grade masks in short supply and needed for health care workers, that recommendation has led to a rush to create homemade cloth or 3-D printed masks (SN: 3/27/20).
Studies of medical grade masks have demonstrated that they can block the forward flow of large particles, but aerosols still shoot out of the sides and top of surgical masks. Few studies have addressed the effectiveness of homemade masks (SN: 4/9/20). And no one knows exactly how many coronavirus particles are necessary to start an infection. For some viruses, as few as one virus may be enough. But even if a cloth mask filters out only 10 percent of virus particles, “in a pandemic, maybe it’s worth doing,” says Milton, the University of Maryland infections disease specialist.
A mask of any sort may help the mask wearer release fewer droplets into the air, which helps to protect people around the wearer. Masks reduce the momentum of the exhaled cloud of droplets, diverting flow and reducing the range the particles can travel. But a mask “does not replace social distancing,” Bourouiba says. “It is not high-grade protection that people should feel overconfident about.”
Going with the flow
Even with a mask, six feet may not be a safe distance. Airflow is a big factor in determining how far is safe enough, says Chen, the Purdue mechanical engineer.
Indoors, heating and air conditioning units may draw virus-laden air toward certain parts of a room. But the systems may also bring in fresh air and cycle out stale air containing viruses and other particles. Ceiling fans and other fans may also blow the virus around a room. In such settings, it may be hard to figure out where best to evade the virus. Just as with indoor smoking, Milton says, “you’ll be trapped in that atmosphere.”
It’s a good idea to keep six feet between grocery store patrons, but whether it’s really safe is another matter, Meselson says. “There’s no magic about six feet,” he says. “It’s better than two feet, and 10 feet is better than six, but for aerosol, I don’t know what to say.” Aerosol particles can linger long after a shopper has gone home, potentially infecting workers or people who visit the store later. His advice: “Try not to go into any enclosed space if you have any reason to believe there was an infected person inside.”
Outside, six feet is probably a safe distance.
Six feet may be OK when asymptomatic individuals are talking outdoors, Bourouiba says, particularly if both are wearing masks.
Savory, the Western University engineer studying coughs, agrees. “The advice that is being given is really sound. It’s a good compromise to be two meters away from someone,” he says. “This distance dramatically reduces the transmission capability of this virus and indeed any virus. If you want to be more safe, you should stay away as far as you can.”
Six feet may also be OK when it comes to the huffs and puffs of exercise. People exercising outdoors run the risk of stepping into another person’s exhalation cloud, but then outdoor airflow may quickly dissipate the cloud.
“When you’re walking, you’re walking through your exhaled breath with every step, but it’s quite well diluted,” says Julian Tang, a virologist and fluid dynamicist at the University of Leicester in England. And there is little side-to-side air movement, so stepping off the path to walk by someone else carries little risk, he says. “It’s quite transient. They’re coming towards me, and then they’re gone.”
But the idea of a “safe” distance outdoors depends on if, and which way, the wind is blowing. A breeze may carry virus-laden breath farther than six feet, Milton says. “If there’s a strong breeze blowing, probably nobody should be downwind from each other.”