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Are Running or Cycling Actually Risks for Spreading Covid-19?


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Are Running or Cycling Actually Risks for Spreading Covid-19?

Last week, a Belgian-Dutch research team self-published a report advising runners and cyclists that they should take extra care while passing others on the road, warning them that respiratory droplets that could potentially contain the novel coronavirus might spread further than the 6 feet buffer recommended by public health officials. Their paper quickly initiated an…

Are Running or Cycling Actually Risks for Spreading Covid-19?

Last week, a Belgian-Dutch research team self-published a report advising runners and cyclists that they should take extra care while passing others on the road, warning them that respiratory droplets that could potentially contain the novel coronavirus might spread further than the 6 feet buffer recommended by public health officials. Their paper quickly initiated an online debate over both its conclusions and the value of circulating unpublished papers, given the superheated coronavirus research environment and the need to give the public clear information about how to stay healthy.

Normally, an unpublished paper from a research group would not get much attention. This one gained traction after one of the researchers, Bert Blocken, a professor of civil engineering at Eindhoven University of Technology in the Netherlands and also at KU Leuven in Belgium, gave an interview to a Belgian paper and also tweeted about their results. The study has not yet been peer-reviewed or accepted for publication by a scientific journal, the normal route for researchers.

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Later, his group posted an initial three-page summary on Blocken’s university lab webpage, and on Thursday night posted his entire 12-page study on the same site. In the meantime, a Medium user named Jurgen Thoelen amplified the attention the team was getting by posting an overview of their conclusions and urging runners and cyclists to maintain their distance. The post gained social media attention and more news outlets began to cover the story, even as the paper itself drew criticism for its lack of data.

Blocken said he decided to convey his findings first on social media and in interviews because he thought it was important public health information. One of the other researchers on the study, a graduate student at Eindhoven, lost his grandfather to Covid-19 as they were conducting the experiments, Blocken said. In their first self-published report, the team called their work a “modest contribution by engineers/aerodynamicists to help a bit in the worldwide fight against Covid‐19.”

By now, the story and accompanying graphic has spread widely and swiftly (OK, yes, virally), both among publications trumpeting Blocken’s results and those critiquing them. The Daily Mail, for example, ran with the headline “Horrifying simulation reveals the dangers of jogging during the coronavirus pandemic.” But others like Vice News ran much more critical articles, inviting experts to weigh in on whether outdoor exercise can spread the disease, with a Harvard epidemiology professor telling Vice that Blocken’s “modest contribution” argument for circulating his research “made my blood boil.”

On a local level, the story has made many people worry about cycling or running—for instance, it has sparked a huge debate on my town’s listserv, with our mayor now asking cyclists to wear cloth masks to protect pedestrians on sidewalks. That kind of policy prescription doesn’t follow the science of Blocken’s paper.

Contours of air speed in the vertical centerplane when running at 4.5 meter distance at a speed of 4 meters pr second.Courtesy of Bert Blocken

Blocken says that things got a bit out of hand, and that he wasn’t expecting the outbreak of media coverage. “The intention was to encourage people to be more aware that they should be a bit safer in terms of distance,” he added. “I’m not saying you shouldn’t go out and run and cycle. I’m trying to give the opposite message, which is to stay beyond 1.5 meters [6 feet].”

So what did his study actually say? Blocken’s research focuses on air flow around professional cyclists, a sport that attracts a huge fan base and commercial sponsorship in Europe. He’s previously published studies about dispersal of urban pollutants in downtown environments, as well as thermodynamic flow around wind turbines. For this study, he and his team were able to access the wind tunnel at Eindhoven University to take measurements of the air flow around runners and cyclists at three speeds: a normal walking pace, a fast run, and a moderate cycling speed. They also factored in the effects of evaporation of respiratory droplets, although they did not compute what would happen with crosswinds, headwinds, or tailwinds.

Then they combined the data with existing studies about how respiratory droplets spread during exercise. It’s important to note that their study does not attempt to estimate the risk of infection from exposures during exercise; it just describes the aerodynamics of respiratory particles.

His team concluded that cyclists and runners have to stay much farther than 6 feet from a runner or rider in front of them to avoid inhaling droplets or having them land on their bodies. He calculated safe distances for each sport: That 65 feet is needed when riding a bike at 18 miles per hour, 33 feet while running at a 6:44 minutes-per-mile pace, or 16 feet while walking at a normal pace. “By that time, the droplets will have moved down to the ground and you won’t get them in your face,” says Blocken. What about riding or jogging side by side? “It’s no problem unless you turn your head and cough in their direction,” Blocken added.

The wind tunnel tests concluded that the zone of potential danger falls in a narrow slipstream behind runners and cyclists, instead of forming a wide V-shaped cone. In theory, Blocken says, this means that runners or cyclists can limit their exposure even more by staggering their position to avoid that slipstream.

The root of the debate over Blocken’s study isn’t whether his team correctly gauged where the droplets might fall; it’s that it’s led to such intense speculation over how likely these particles are to make anyone sick.

Droplet spreading when running at a speed of 14.4 km/h when (a,b) running behind each other; (c) side-by-side; (d) in staggered arrangement.Courtesy of Bert Blocken

Right now, we know that the coronavirus is passed from person to person when someone coughs or sneezes, or when the virus lands on a surface and is touched by a person who then touches their face. The amount of time the viruses can survive outside the body depends on the surface, according to the National Institutes of Health. It’s also not clear what amount or density of viral particles it takes to infect someone, although that’s a question that scientists really want to answer. They know that the density of particles, or viral load, plays a big role; crowded indoor spaces carry a bigger viral load than open outdoor spaces. Virologists also note that the time of exposure is important—a brief hello to a neighbor on the sidewalk poses less of a risk than sitting next to your friend at an outdoor café for a few beers.

But so far there are no published studies of the spread of the novel coronavirus from one person to another in outdoor settings. One recent study of 318 outbreaks of three or more Covid-19 patients found all but one transmission occurred indoors—but as with many studies being conducted right now, that report was published as a pre-print in MedRxiv by a team of researchers at Hong Kong University and Southeast University in Nanjing, China, which means it has not yet been peer-reviewed.

Linsey Marr, an expert in airborne transmission of viral diseases and a professor of civil engineering at Virginia Tech, says the issue of whether people can become infected from cyclists or runners is still undecided. “We need to keep in mind, though, that we don’t yet know what size particles released by an infected person actually contain virus and whether that virus is ‘alive,’ or can still infect others,” Marr wrote in an email to WIRED.

She agreed with Blocken’s advice to walkers and runners to allow for greater spacing if traveling right in front of or right behind another person. However, she notes that the study assumed no wind. “Basically, if you’re directly upwind or downwind of others,” Marr wrote “allow for more space.”

Marr wrote that she wasn’t upset that the researchers decided to publicize their work through the media instead of through the traditional route of submitting the study to a peer-reviewed journal. “Given the situation we’re in, I think it’s fair that the researchers shared the results because they could be immediately useful,” she wrote.

person lathering hands with soap and water

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A half dozen other virologists contacted by WIRED declined to comment on Bracken’s study, saying they were busy reviewing papers or conducting their own research, or had responded to reporters’ requests last week. But as many researchers in this field had previously told WIRED’s Roxanne Khamsi, there’s still a vigorous debate among researchers over how likely the virus is to spread through air. Some argue that if it’s spread through larger blobs or “droplets” that are coughed, sneezed or exhaled, it will fall quickly to the ground; others argue that if it is spread through finer “aerosols” it can linger aloft for much longer, creating a higher infection risk. And some say there’s no clear division between the two categories, anyway.

In the meantime, experts say it’s still a good idea to exercise, and that it makes sense to distance as much as you can, whether that means avoiding crowded spaces or using technology as a way to exercise in the (remote) company of friends. Anne Hyman, a commercial laboratory scientist and president of a Washington, DC-area cycling club, is concerned about the potential risks of spreading Covid-19 while riding. “People are taking to wider areas to walk and congregate, and it’s creating a dense situation on multiuse paths,” said Hyman, president of the 2,000-member Potomac Pedalers Touring Club. Instead, she rides her stationary bike with friends using the Zwift virtual competition software that allows users to race against each other. The triathlete hasn’t ridden on the roads near her suburban Maryland home since the state was put on a lockdown March 30.

Experts are also reminding the homebound that 30 to 60 minutes of daily exercise—even when done alone or inside—boosts the immune system. “We do know that the number one objective is to reduce exposure,” says David Nieman, an exercise immunologist and director of the human performance lab at Appalachian State University. “But I believe it’s possible while we still get our physical activity.”

Nieman, a marathon runner, has reviewed Blocken’s study and agrees with the concept of keeping a bigger buffer zone between fellow runners or riders. “You can say I’m going to go with one or two buddies who I know well and claim they are not sick and just trust that it’s OK, but it’s not foolproof,” Nieman says. “People you know and trust may have been exposed. Instead, try exercising outside by yourself. The risk is extremely low, and all the benefits are there.”


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