Last week, Belgian researcher Bert Blocken released a graphic showing the slipstream of saliva droplets as they linger behind runners and cyclists. He determined cyclists traveling together should remain about 20 metres apart and runners, traveling 14K-per-hour (which would be a 3-hour marathon time; 1:30 half), should remain 10 metres apart when in each other’s slipstreams. The study, released before being peer-reviewed and subject to the best and worst aspects of going viral, created a stir. Ben Kaplan caught up with Blocken from his home.
How are you?
I’m hoping things will slow down a bit. The attention is nice, but what I notice is—and maybe I should’ve known this—it’s extremely important to be super, super, super clear, because all kinds of misinterpretations can happen and people misunderstand. I absolutely do not want to forbid people from running, cycling or walking.
What did people get wrong?
I think some people misunderstood this study to being an infection study when it’s purely an aerodynamic study. Aerodynamics are my field. I only know virology from what I read in the news articles and scientific papers.
How can you contract the novel coronavirus, or give it to someone else, when you’re running, walking or cycling?
Droplets inhaled by somebody else are an effective way to transmit the virus, so we looked at equivalent distances of how far you should run or walk or cycle behind another person if you want to have the same non-exposure as the recommended social distancing.
How far should you be behind someone if you’re just walking down the street?
1.5 meters, that’s if you’re just standing still, and to some extent, my research is common sense. When you exhale a cloud of droplets and walk away, and another person walks toward the cloud before the cloud has time to settle down, you walk into the cloud and inhale the droplets and get them on your face, get them on your body.
So what you did was take the 1.5 metres and extrapolated the exit velocity of droplets for runners, walkers and cyclists?
Precisely, it’s about the slipstream. The matter of time it takes for the droplets in the slipstream of a moving person to settle before the next moving person comes to the spot where the droplets were left behind. 1.5 metres when moving is not as safe as 1.5m when standing still. So what we did was figure out what the proper distance should be when moving behind each other in sports.
It’s crazy to think about how far you need to be away from the virus to avoid it.
It is, but it’s so important. It’s all about when you’re moving to the cloud and when you give the cloud time to settle.
Were you surprised by the response?
In a way. This is not rocket science. Studying droplets and air flow around people is something I’ve been doing for 20 years, but apparently the timing is right for a lot of attention on this.
It’s important.
I was a little bit surprised that no government has mentioned something about this. I really do think for everybody involved in running and cycling, we all know the slipstream. We know how aerodynamics work, but it appears to be the first time the issue was raised somehow in the public and on social media, that’s just weird. But OK. So be it.
What have you learned?
Keep walking. Keep running. Keep exercising. Keep cycling. But make your training a bit tougher than usual—stay out of the slipstream of the people in front of you. I don’t know how it is in Canada, but in Belgium, you’re not allowed to cycle in big groups anymore.
Same here.
That makes a lot of sense and it’s completely in line with our study. The first one coughs, the droplet cloud travels through the entire group and this is a massively effective way of getting people infected. Don’t cycle with more than one friend, and make sure it’s the same one each time. I would add that I think it’s perfectly fine to cycle next to each other. The droplets you exhale will appear behind you.
Running the same thing?
Run next to someone or in a staggered formation—not in a line.
You’re a cyclist. What else have you heard?
Keep a distance. I’ve been talking to people in my village, runners and cyclists, and some have said they hold their breathe when they overtake somebody and that does make sense to some extent. It doesn’t guarantee you don’t get droplets on your face or body, but I think we should choose to be a little bit more careful then our governments tell us when exercising.
You’re still exercising?
I will not drive one kilometres less on my bicycle because of this.
What if you lived in New York, or somewhere crowded, where you couldn’t avoid the slipstream?
A lot of people said, after I posted, I live in New York and it’s impossible to keep a distance. I can’t solve that problem. I’m just telling you the scientific facts that we found in the computer simulations.
It’s funny because we don’t run with tape measures, so who’s to say the exact distances? But to reiterate, what are the safe distances for avoiding slipstreams?
If you’re running very fast—14k-per-hour—it’s about 10-meters distance apart. If you’re walking, let’s say 4K-per-hour—it’s 4 to 5 meters. With higher speeds, a bit more distance; lower speeds, a bit less. Cycling at 30K-per-hour requires 20 metres. But only when you are in the slipstream.
There’s been some criticism of your study, that it hasn’t been peer reviewed.
Yes. I got a lot of criticism. People saying that I spread this on social media and made them more anxious, that maybe I shouldn’t have told them, but on the other hand, if we waited to be peer-reviewed and published and didn’t tell this to people and published in six months I would have felt horrible not giving people practical information for healthy exercising. I had to share what I know.
I agree.
I talked to our research team and the press agencies at my two universities (Eindhoven University of Technology in the Netherlands and Ku Leuven in Belgium) and asked them their point of view, and everyone thought, given the urgency of the situation, and that I’m 100% sure of my results, and that it’s my risk and my reputation, they all said, “We trust you and your results—put it out.” You know, I have been doing this for 20 years.
Do you have any doubts in your research?
If I had any doubt, I wouldn’t have put it out. If the study is wrong, I will resign at both my universities. But it’s not. This is routine for me.
Do you wear a mask?
No. Maybe I should. In fact, I have a colleague in China, and he promised to send me 200 masks, but I’m going to bring them to an elderly home. I think people working in the elderly homes need all the support they can get.
To read the most recent research by Bert Blocken, please click here.
To read the questions and answers addressing the work, PLEASE CLICK HERE.
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You blew it on the run speed calcs. 14kph would be a 3:00 marathon.
Why are doubling down on this debunked paper. It was not peer reviewed and many of those who are qualified to review this type of research say that it flawed research producing flawed results. This type of sensationalist reporting just raises anxiety levels. Please stop.
Why did he not consider how wind affects the spread of the droplets? Surely this is an important aspect when determining where it’s safest for u to be in relation to other people while outside?
Interesting animation. Wind is important as droplets up to 150 um almost become aerosols in an appreciable wind, as they do in spray drift, (coughing is an extreme coarse agricultural spray as the animation shows by the colours) especially an atmospheric boundary layer (i.e. outside in the wind) at the surface where the turbulence intensity is around 0.3 so they will spread out further upwards, downwards (onto the ground) and sideways. This is important in community transmission. This is one pathway of many that the virus can travel exactly how important it is, is difficult to determine. Better be safe than sorry. Then there is the Finnish animation in the supermarket..