How, in times of COVID-19, you can safely ride your bike: a conversation with Professor Bert Blocken

Chances are you have already seen it: Belgian-Dutch Study: Why in times of COVID-19 you cannot walk/run/bike close to each other. That Medium article, written by Jurgen Thoelen, a self-styled ‘entrepreneur, building clouds in all forms and shapes’, has gone viral in recent days. Accelerated by algorithms and the likes and shares of millions of readers, it has filled Facebook, Twitter, and Instagram feeds all over the internet, while inspiring stories in outlets such as The Sun and Bicycling, as well as a virulent take-down in Vice.

The problem, says Professor Bert Blocken, author of the original study, is that Thoelen’s Medium article, although well intended, did not accurately represent the actual practical consequences of his research.

Professor Blocken is a member of the Department of the Built Environment at Eindhoven University of Technology in the Netherlands and the Department of Civil Engineering at Leuven University in Belgium, where he researches urban physics, wind engineering, and sports aerodynamics. He has published 188 peer-reviewed papers and is the scientific director of the Eindhoven Atmospheric Boundary Layer Wind Tunnel. A keen cyclist himself, he provides aerodynamic advice to World Tour teams such as Jumbo-Visma and Groupama-FDJ. He has also advised athletics projects such as Eliud Kipchoge’s sub-two-hour marathon.

When he first shared his findings with the Belgian newspaper Het Laatste Nieuws, Professor Blocken never expected that his work would attract so much attention or generate such heated discussion. He is especially troubled by the way that it has been used to berate people who exercise outdoors.

“Some media have really cracked down on people for walking, running, and cycling, saying that they should stop doing that and that they are spreading the coronavirus, which is, of course, complete nonsense,” he says. “That is just a very wild extrapolation of our study.”

In fact, Professor Blocken and his team meant to reassure cyclists, runners, and walkers with their research.

“The initial goal,” he says, “was actually to ease people’s minds, because I had gotten a lot of questions from people, asking me, What should I do when I overtake a person with my bicycle? What should I do when I walk past a person? Should I hold my breath? they asked me. And then they said, Well, you know about aerodynamics and you studied droplets; can you say something about that? That’s why we did this research—to indicate to people where the droplets are and where they are not, so that they can make informed decisions. They needn’t freak out about droplets being everywhere, because that’s not the case. The droplets are just in the slipstream, and if you stay out of this slipstream, there are no droplets.”

To carry out their study, Professor Blocken and his team employed methods developed for sports science and civil engineering.

“In droplet studies, we always combine experiments and simulation,” he explains. “Simulation is computer simulation, where you are solving the fluid flow equations. And the experiments are usually wind-tunnel experiments, but, because the wind tunnel is now closed and the university is also closed, we used wind-tunnel experiments from the past where we investigated the flow around runners and wind-tunnel experiments done by other people about how droplets move and evaporate in a complex flow. We used those to calibrate the computer model, and then the computer model was used to calculate where the droplets go.”

Research done for World Tour teams, as well as Kipchoge’s sub-two-hour marathon project, was vital for this work.

Still, Professor Blocken insists that their findings are not rocket science. In fact they should be quite intuitive to most active people.

In short: the droplets that you exhale when you breathe, and could possibly be infected with the virus, hang in your slipstream as you move through the air. When you are travelling at speed, your slipstream extends further than the 1.5 metres that governments recommend for social distancing. 

The relative risk of infection that such droplets pose was not something into which Professor Blocken and his team delved, apart from assuming, in accordance with common sense, that avoiding all contact with such droplets must be safer than getting them on your face or inhaling them.

“The droplets stay pretty finely enclosed in the slipstream,” Professor Blocken explains, “so that means that they are quite confined to a specific area. This can raise concern, because their concentrations there can be quite high, but actually it should ease people’s minds, because it means that in all the other positions around the runner [or rider] there is no problem.”

Nevertheless, Professor Blocken and his team did find that government-mandated social-distancing requirements are insufficient for cyclists, runners, and walkers when they are travelling directly in front of and behind one another.

“If we agree that this 1.5 metres is a good distance for two people talking to each other, what then should be the equivalent distance to have the same, ‘non-droplet exposure’, for two runners behind each other or two walkers behind each other or two cyclists behind each other? It appears that this 1.5 metres is fine if you are running next to each other or staggered, so not in a straight line behind each other, but if you want to be in a straight line behind each other, the social distance needs to be farther,” he says. “With fast walkers, it should be five metres. With fast runners, it should be about ten metres. And with fast cycling—at 30 km/hr—it should be 20 metres. That should not freak people out, because if you cannot keep the 20 metres, you can just move to the side, and it’s also fine.”

It should be noted that Professor Blocken’s research has not yet accounted for crosswinds.

Still, as sensible as his conclusions might be, Professor Blocken has come in for some heated criticism.

The most persistent charge is that his study has not yet been peer reviewed. This was the main point of the Vice article.

To this, he offers several rebuttals.

“The first response is that this happens all the time.” he says. “Researchers release work that has not yet been peer-reviewed all the time when there is a need for it. I hope that we are not suddenly going to forbid scientific conferences, where results are often shared without peer review. My second response is that some reporters are very negative about our work, but they haven’t had their own interpretations of our study peer-reviewed either. So, that is inconsistent. The third reaction is that the study is common sense. The outcome is common sense. We discussed it in the research team. We asked the two press officers of my two universities, and they said, The risk you are taking here is zero. You’re just giving advice, so people can be safer.”

Professor Blocken also questions the costs of not releasing his research.

“That’s the main reason,” he says. “What would people think of us as researchers if we would have kept it a secret? We would first have applied for funding, which usually you would get then, if you were lucky, within a year, and then we would have done the study, submitted the article for peer review, and it would have been published a year and a half later. Then, we would have to say, Well, sorry guys and ladies; we knew it all along, but we kept it a secret, because we found funding and avoiding uncomfortable reactions in the media more important than public safety. I think that would have been very unethical.”

The fact that one member of his research team has already lost his grandfather to the coronavirus and another has four of his family members in intensive care in Italy also motivated Professor Blocken to go public.

The other main line of criticism of Professor Blocken’s work is that, as a civil engineer, he is not qualified to comment on epidemiological matters. This was the other main charge levelled against him in the article in Vice.

This, he says, ignores all of the work that engineers have done during previous epidemics.

“People tell me that you should not do this work; it is epidemiological work, but this is very wrong,” he says, “because much of this work in the past, after the 2002 epidemic of SARS in Asia, was done by building engineers, who study droplets in buildings… All these people who criticise that don’t know their literature and don’t know their history. As I said, during the SARS outbreak in 2002, it was the civil engineers, my colleagues actually—most of whom are still active—who determined the social distance of 1.5 metres and two metres. They did the measurements.”

Asked why he thinks his research has caused such a storm, Professor Blocken says, “I think it’s a controversial topic, which makes many people react emotionally and not rationally anymore. Even professionals do this. And that goes for professional journalists, luckily not the majority.”

That said, Professor Blocken does have some regrets about how the research was communicated. When the Het Laatste Nieuws story first appeared and the Medium article began to spread around the internet, he and his team had merely posted a white-paper on his website. “If I would have known how this would explode, then of course the article would have been ready before we spoke to the media. And we would have set up a long Q&A. That could have avoided a lot of problems.” 

Professor Blocken has since posted a pre-print version of the full scientific article as well as answers to common questions about his study online. He has received apologies from the author of the Medium article as well as from an academic who was cited in the Vice article and now says he criticised Blocken too hastily.

As for his own cycling, Blocken says, “I don’t take the routes along the canal anymore, because it is very crowded there now. I now take the roads that are usually for heavy traffic. There is almost no traffic because of the lock down. And it’s actually very easy to keep these long distances. I meet almost nobody when I go cycling, but if I do overtake somebody, I follow my own advice.”

That advice, summed up for everyday riders, is this:

“Don’t cycle in big group; that is indeed not very responsible. If someone in front sneezes, well,  the droplet cloud will move throughout the entire group. But, by all means, go out and walk and run and cycle, but keep your social distance, which is the social distance recommended by the government, 1.5 metres, supplemented by the additional social distances that we have determined, which are just like what the government is saying, but then adjusted for movement.”