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
    probably an effective way to lower the risk of transmission. But six feet could be much too
    close, said Lydia Bourouiba, an associate professor at the Massachusetts Institute of
    Technology who studies the fluid dynamics of disease transmission.

    “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.

    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.

    The six-feet-away guideline is based on an assumption from 1930s research: that viruses are
    transmitted only 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 and can then be carried farther away.

    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, our exhalations — even if we’re not coughing or sneezing — can still contain a gas
    cloud of viral particles. And a particularly powerful sneeze can send these 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 to definitively say what distance is safe. But in the
    meantime, people can be cautious in public.

    “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.

    Gal Tziperman Lotan can be reached at [email protected] or at 617-929-2043.

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