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Six-foot rule to protect against coronavirus may not be enough, MIT professor says

Calls research it’s based on outdated, finds that droplets can travel much farther

Customers stood six feet apart outside Trader Joe's supermarket in Boston.
Customers stood six feet apart outside Trader Joe's supermarket in Boston.David L. Ryan/Globe Staff

The oft-repeated guideline to stay six feet away from other people in order to reduce the risk of coronavirus transmission is based on decades-old research that has sorely needed updating for years, according to an MIT professor who studies the issue.

Avoiding people who have the virus — or might have the virus — is indeed an important and effective way to lower the risk of transmission. But six feet could be much too close if the person is sneezing or coughing, said Lydia Bourouiba, an associate professor at the Massachusetts Institute of Technology who studies the fluid dynamics of disease transmission.

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“Although such social distancing strategies are critical in the current time of pandemic, it may seem surprising that the current understanding of the routes of host-to-host transmission in respiratory infectious diseases are predicated on a model of disease transmission developed in the 1930s that, by modern standards, seems overly simplified,” Bourouiba wrote in a piece published in the Journal of the American Medical Association last week.

In certain conditions, a particularly violent sneeze can send particles flying 23 to 27 feet away, Bourouiba said.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, weighed in on Bourouiba’s research, emphasizing that most of the risk of transmission still comes from droplets that fall close to the infected person.

“I was disturbed by that report because that’s misleading,” Fauci said in a White House news conference this week, talking about headlines that touted the new safe distance as 27 feet away. "That means that all of a sudden, the six-foot thing doesn’t work. That is a very, very robust, vigorous, achoo sneeze. That’s what that is. And that’s not what we’re talking about.”

Bourouiba’s lab has produced fascinating slow-motion videos of people sneezing, captured at 2,000 frames per second, that show how far their emissions can spread. Her research over the years has focused in part on the flu, not on the new-in-humans coronavirus that causes COVID-19. But the fluid dynamics of sneezes and coughs still apply, even if the virus’s concentration in emissions and lifespan outside the body could vary between viruses.

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The six-feet-away guideline is based on an assumption from research in the 1930s by Harvard sanitary engineer William Firth Wells: that viruses are transmitted through droplets expelled when a carrier coughs or sneezes; and that those transmissions can be classified in two groups: larger droplets, which fall closer to the infected person and really pose a risk only to people who get very close, and smaller droplets, which evaporate before settling on a surface.

There are a few inaccuracies there, Bourouiba said.

First, the division between large and small droplets is arbitrary, both in classifying their size and in determining how much risk they carry.

Second, a particularly powerful sneeze can send these viral particles, both in droplets and in a puff of gas traveling through the air, flying much farther than six feet. Under the right conditions, they can go 23 to 27 feet, Bourouiba has found.

Much more research is needed to know for certain how to slow the spread of the virus, Bourouiba said. There’s not enough data now to definitively say what distance is safe. But in the meantime, people can be cautious in public.

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“When possible, if it’s a confined space, then maintaining larger distances would be wise,” Bourouiba said. In a hospital setting, health care providers who have access to personal protective equipment may want to put it on as soon as they can when near infected patients.

And as governments and hospitals scramble to find more N95 masks and other protective gear, Bourouiba is raising questions about how effective masks are on people already carrying the virus. She pointed out that N95 masks have not been tested to determine how effective they are against repeated sneezes and coughs. More testing needs to be done to determine whether giving a mask to someone who is already infected can protect people nearby, she said.

Dr. Daniel Kuritzkes, chief of the Division of Infectious Diseases at Brigham and Women’s Hospital, said the hospital likely won’t dramatically change its existing practices.

“Even before COVID-19, as flu season got underway, we were asking patients who came into the emergency department or a doctor’s visit, if they had symptoms of a cold or flu, to put on a mask,” Kuritzkes said.

People who are carrying the virus but not coughing or sneezing likely don’t post a risk to someone 20 or more feet away, he said. But he did encourage anyone who does have respiratory-related symptoms, like coughing or sneezing, to stay home unless they need urgent medical attention.

And if people who have the virus do leave the house — say, to see a doctor — wearing a regular surgical mask (as opposed to an N95 respirator) can help keep others safe.

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“I still think that the 6 feet distance is a reasonable, practical measure," Kuritzkes said. "If someone is actively coughing and sneezing, you probably want to keep your distance a little bit more. And of course, outdoors, there’s much less chance of coming into contact with respiratory secretions.”



Gal Tziperman Lotan can be reached at gal.lotan@globe.com or at 617-929-2043.