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caption: Screengrab of simulation of a runner's exhalation splattering a runner behind him. Red represents the largest droplets, while blue represents the smallest. 
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Screengrab of simulation of a runner's exhalation splattering a runner behind him. Red represents the largest droplets, while blue represents the smallest.

6 feet away might be too close, especially if you’re on the move

Local and national health authorities recommend staying at least 6 feet away from other people to avoid spreading coronavirus. Some new studies suggest much greater distancing is needed.

“The 6 foot rule is frustrating,” Portland State University engineering professor and dean Richard Corsi said in an email. “Medical doctors and those in the health sciences repeat it over and over again as if it is gospel, but I am convinced they have no real knowledge of aerosol dynamics."

“A magic shield does not appear at 6 feet,” Corsi said. “If the conditions are right, one can inhale viruses emitted by an infector at distances far greater than 6 feet.”

He said the recommendation to stay six feet apart doesn't account for very fine exhaled particles, known as bioaerosols, that can contain viruses and stay suspended for long periods.

Researchers with the National Institute of Allergy and Infectious Diseases found, in a study published March 17 in the New England Journal of Medicine, that the novel coronavirus can survive in airborne particles for three hours.

Fluid dynamics researcher and Massachusetts Institute of Technology professor Lydia Bourouiba found that a sneeze can deliver a pathogenic payload much farther than six feet.

“The gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet,” she writes in the Mar. 26 Journal of the American Medical Association.

While their work has not been peer-reviewed or formally published, aerodynamics researchers at Eindhoven University of Technology in the Netherlands and Catholic University Leuven in Belgium have produced animated simulations of runners and walkers exhaling as they go.

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Simulations show how exhaled droplets large (in red) and small (in blue) can travel from one person's mouth toward their exercise buddy.
Credit: Ansys / Bert Blocken

The simulations depict invisibly small droplets from the frontrunner’s breath splattering the person right behind him.

Based on those simulations, the European researchers recommend staying at least 15 feet behind another walker, more than 33 feet if you’re running, and more than 66 feet behind another bicyclist.

If there’s room, the researchers say spreading out sideways — either side by side or staggered diagonally — would be safer.

Lead author Bert Blocken, a civil engineering professor at Eindhoven, and colleagues have not published a scientific paper to present their findings and methods.

Instead, they released what they called a “white paper” summarizing their work. Most of it is a translation of a Belgian news article about their research.

Ansys, the Pennsylvania company that produced the animation, also published a press release on the work.

“We are facing a worldwide crisis with many deaths,” Blocken said in an email to KUOW. “The coronavirus will not wait for 6 months or more until our publication has been peer reviewed and published.”

The study does not attempt to address what the risk of catching Covid-19 from a nearby exerciser’s hard breathing might be.

Whether someone would be infected by such exposure would depend on the viral load of the frontrunner, and how much virus reached the follower’s eyes, nose and mouth, among other factors. Virologists are still attempting to determine the “infectious dose” necessary to catch this coronavirus.

“This is a purely aerodynamics study, not virology,” Blocken emailed.

The study defines the distances that exercisers would need to keep to avoid exposure as well as two stationary people following the Dutch government’s recommendation to stay 1.5 meters (4.9 feet) away from others.

Blocken, a cyclist himself, said people on social media have misinterpreted his conclusions as being anti-outdoor-exercise.

“This study is intended to support people going outside to walk, run, cycle, but asking them to be more safe,” he said in an email.

Blocken said he has been studying the aerodynamics of droplets for 20 years, with 189 peer-reviewed papers.

"Sorry to be less modest but I am not just anyone posting anything on social media," he emailed.

Blocken’s main advice is to keep exercising but stay out of the slipstream — the atmospheric wake — of people in front or upwind of you.

Corsi described his trick for maintaining proper distance to the Oregonian: “If you’re going outside, pretend everybody’s a smoker and you don’t like cigarette smoke.”

UPDATE 4/10/2020, 9:15 a.m.: Blocken and colleagues have released a pre-peer-review draft of their full study.