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.