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.
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.
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.”
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