All is fair in love and war…and COVID-19. Part I.


May 29, 2020

All is fair in love and war… and COVID-19. Part I.

Author: Bert Blocken, Professor Civil Engineering at Eindhoven University of Technology, the Netherlands and KU Leuven, Belgium


As any other crisis, also the COVID-19 pandemic brings about the best and the worst in people. In the first part of this “corona story” a moderate example in the category “worst”. In which a well-intended research project on COVID-19 to support running and cycling was misunderstood and misrepresented by a few, sometimes unintentionally, sometimes intentionally, to discredit runners and cyclists. In the second part a more radical and painful example, in which two US citizens went through an excruciating nightmare after a social media actor branded one of them as the source of COVID-19 in the US. But in this second part also an example in the category “best”, a beautiful poem with movie by an Irish family dedicated to the frontline workers in Italy’s Bergamo hospitals.


“Can I safely cross a person when I am doing my running exercise?”
“Is it dangerous to overtake another person when I am cycling?”
“Should I hold my breath when I overtake or cross another person?”
“Should I stay at home when I cannot exercise completely alone?”

These are the types of questions we got in March 2020, by the dozens. We tried to answer them with a new and unfunded engineering research project, in which we set out to investigate how respiratory droplets move around walkers, runners and cyclists. For multiple reasons. First, to provide a contribution – no matter how small – to the fight against COVID-19. Second, because we, as building physics and aerodynamics engineers, have the expertise to do this, having studied droplets in complex airflow patterns for over 20 years and airflow around athletes for over 15 years. Third, we have the facilities: a large wind tunnel and computational hardware and software to study droplet movement around walkers, runners and cyclists.


When a person sneezes, coughs or even exhales, he or she is emitting small droplets – many of which too small to see by the naked eye – that can carry the virus. The receiving persons can be infected by inhaling these droplets, or by getting these droplets on their hands and then touching their face. That is why during the COVID-19 pandemic, many countries world-wide have declared – sometimes by law – a so-called “social distance” of about 1.5 m, 2 m or 6 ft (actual value dependent on the country) to be kept between individuals. While this is not really a social distance but rather a physical distance, this distance is considered important and effective because it is expected that generally, most of the droplets fall down and/or evaporate before having traveled a distance of 1.5 m.


The 1.5 m social distance originates from a study in 1934. It was determined based on two people standing still. It does not take into account the potential aerodynamic effects introduced by strong wind or by rapid movement such as walking fast, running and cycling. Anyone with a minimum level of intuition or knowledge in fluid mechanics will know that when you exhale, cough or sneeze small droplets, and you keep moving, you will leave the droplets behind in your trail. Because they are light and there is no force acting upon them that would allow them to stay in front of you and travel along with you. That is Newton’s second law, fairly undisputed since 1687. If then another person is walking, running or cycling behind you, and this person does not wait until the droplets behind you have had the chance to settle to the ground or evaporate, this other person will walk through this droplet cloud and can inhale these droplets.

That is what we call common sense, and generally, research should be not conducted to confirm common sense. However, what was not known, was the actual distance that should be kept between two people walking, running or cycling behind each other, in order to avoid that the trailing person keeps moving in the constantly exhaled droplet cloud by the leading person. Therefore, we decided to investigate these distances.


In short the results are:

(1) droplets exhaled by breathing, coughing or sneezing by a moving person, will indeed rapidly move behind this person and will stay quite well confined in the narrow area behind this person, called the slipstream, until they evaporate or reach the ground. The slipstream is the area where the flow is disturbed due to the movement, also called the wake, and where a lower pressure is present. This lower pressure is the main driving force keeping the droplets quite well confined inside this slipstream. Which is good news: it means that when you are walking, running or cycling nearby another person, you just have to avoid being in the slipstream behind the leading person for too long.

(2) If you want to be in the slipstream or cannot avoid to be in the slipstream, and you want to avoid droplet exposure, you have to keep a longer distance than 1.5 m between yourself and the person in front of you. Roughly: for walking fast (4 km/h): 5 m, for running fast (14 km/h): 10 m, for cycling fast (30 km/h): 20 m. These are the distances that can be considered to be “aerodynamically equivalent” to the 1.5 m standing still, because they yield a similar level of “non-droplet exposure”. And yes, these are large distances. We cannot make it any nicer or more comfortable than it is. Note that 1.5 m distance for people standing still is also a large distance and in different, non-pandemic times, this would also be considered unfeasible and even unacceptable.


Not necessarily. For several reasons. First, this is an aerodynamics study. We did not want to link to virology, because on the virology side, given the complexity involved, many unknowns remain at present. It is not yet clear which level of virus in how many and which size’ of droplets for which duration of exposure can give an infection risk for which person with which level of immune system. We also do not yet know the risk associated with the 1.5 m or 2 m of 6 ft physical distance. But that does not mean we should stop building knowledge using those fields (fluid dynamics, civil engineering) that can also contribute. So we only focused on these “aerodynamically equivalent” social distances. Second, every day, we see people that do not keep the 1.5 m social distance all the time. It is likely that it is not a problem to be closer than 1.5 m briefly, but that it is best avoid to break this 1.5 m rule for too long duration. The same will hold for the 5, 10 or 20 m in walking, running and cycling. It is unwise to be walking, running, cycling, for a long duration in the slipstream of other persons. This is exactly what people do in long-distance walking, running and cycling in groups. On 15 April 2020, top Dutch cyclist Tom Dumoulin, former winner of the Giro d’Italia, stated: “Saliva droplets fly around in the cycling peloton. If in a multi-stage race like Paris-Nice, the first day one rider has a cold, a few days later, the whole peloton has it.” A nd not all cyclists sleep together in the same room or interact with each other outside the race. So you better know how to behave when training amidst a pandemic.


Within the research team, we discussed how we should announce these results. Should we first submit this work to a scientific journal for peer review, or should we communicate them to the public as soon as possible? We unanimously chose the latter option. Because we found it unethical to keep these results a secret, wait until our paper would have been peer reviewed, which, in our field and depending on the journal, could take several months, and only after that communicate to the public, stating: “sorry, we knew this all along, but we did not tell you because we found our publication more important”. Also because this study just confirms common sense, and its only value and the only new addition is specifying the distances to be kept when moving in each other’s slipstream. We honestly thought there was no risk in communicating these results. It was not like we were advising people to taking specific drugs against COVID-19 that could also have negative health effects. We just advised caution in keeping larger distances. Well… we thought wrong.


First, we communicated this common sense study by means of some animations and a short text on a website of one of our partners. Next, the study was picked up by two Belgian newspapers who contacted me for information and wrote a short and very accurate report. One of these reports was read by a Belgian national, working in Asia, who was charmed by the study and who, with the best of intentions, wrote a longer read about it, however without the required scientific “ifs” and “buts” and posted it on Medium, where… it was read by 2.2 million people in record time. That is where the situation exploded and diverged into three groups: the good, the bad and the ugly.


One group understood the study and provided feedback ranging from neutral over very positive to enthusiastic. If they had questions, they did the effort to contact us. The neutral feedback can be summarized as: “nice but this is common sense and we knew this already”. The positive feedback came from many colleagues in fluid mechanics and aerodynamics but also from top professionals in cycling, including sports medical doctors, team coaches and professional riders.

... THE BAD…

However, some, albeit very few, media plunged into the study like a pack of wild wolves, dropping ethical standards in journalism, such as reading the actual study and/or contacting the primary source, the authors, and allowing the original authors to reply to the criticism. Also some scientists found it necessary to comment negatively on the study, also dropping the most basic ethical standards in science and academics: to never comment on items outside your field of expertise, to never comment on a study when you do not know what it is about and to never provide strong black and white statements, let alone emotional statements, when you are not 100% sure.


It can be worse. One medium has fabricated contents and conclusions and attributed these to our study, while these bear no resemblance at all to the actual contents and conclusions of the study. This article has led others to abuse our study to spread the news, in other articles and on social media, that “runners and cyclists are spreading COVID-19”, after which in several cities in a very few specific countries, runners and cyclists started to be accused and insulted by pedestrians and car drivers while performing their exercise. Many runners and cyclists became very upset, and many have also sent very angry emails to us. So while this research project started because we wanted to provide science-based answers to support and ease the mind of runners and cyclists, in some countries, it has been abused by some unethical journalists and scientists to do exactly the opposite.


Being submerged in media requests, I got the request for a phone call by a German medical doctor. He was so kind to be more worried about my reputation than I was. I called him, and he explained me how much difficulties he experienced in explaining our common sense study to his patients and his group of 700 runners. Finally, he resorted to this simple – but striking – analogy:

“If you stand with your car in front of a traffic light, waiting, and you spray your windscreen in order to wipe it clean, you only get droplets on your windscreen. The car that is about 10 or 20 m behind you does not get your droplets on its windscreen. But if you start riding and then spray, the car 10 or 20 m behind you will also receive droplets”.

Indeed our work says exactly the same but then for walking/running. The “only” value of our work is that we determined the actual distances. But apparently in COVID-19 time, even common sense is not straightforward anymore…


Some negative media, in search for arguments to break down the study, criticized that the study had not yet undergone peer review. While colleagues in fluid mechanics and civil engineering world-wide kept emailing us they were surprised that so many people did not understand common sense, others, including some (a very tiny minority of) virologists, epidemiologists and microbiologists, indeed insisted that the study had no value because it was not peer reviewed. This while they themselves, on tv, radio and in newspapers, every day in the COVID-19 crisis, are providing information that is not peer reviewed either, and present them as facts that governments should adopt and that the public should adhere to.
Conversely, our release of results without peer review was branded as “reckless”, “violating all academic principles”, and further statements that I do not wish to repeat here. That is not only inconsistent but also far from reality. Scientists are releasing results without peer review all the time. This is even done at mass meetings, with hundreds, sometimes thousands of participants. Thousands of these events occur every year in countries world-wide. These events are called: scientific conferences. Where sometimes even no papers at all are provided (so no peer review either), only a presentation with figures, animations and results. And where often press is present.

Our study actually confirms common sense, and just puts numbers to it. Who would have guessed that in times of COVID-19, some scientists in other fields than ours and some journalists would insist on having our common science first peer reviewed, before we can apply it…


Some (again a very tiny minority) virologists, epidemiologists and microbiologists and some journalists have criticized us for getting involved in a field that is not ours. And that civil engineers have no role in this crisis. These critics do not seem to know their literature and their history. Over decades, most recently after the 2002 SARS outbreak, civil and mechanical engineers have been the ones studying how droplets and aerosols are distributed in and around buildings and influenced by ventilation and air-conditioning systems in buildings, ships, etc. Most studies in the past 20 years on the 1.5 m social distance have been done by civil and mechanical engineers, and their findings have been published in thousands of scientific publications… If anything should be clear from previous pandemics, it is that dealing with them requires a multi-disciplinary approach.


While some media did not understand the study and some others did not want to understand it, in order to profile themselves at the expense of the reputation of the authors, about one week after the well-intended Medium article inadvertently unleashed the media storm, the quality media came into action. These media also consulted a more decent sample of virologists, epidemiologists and microbiologists, and they did the effort to also consult the primary source (= us) for a detailed round of questioning and also other experts in aerodynamics. The other experts in aerodynamics generally confirmed that our results are common sense and that the longer distances make sense. Even some virologists and epidemiologists did so. Some examples of articles in high-quality media (non-exhaustive list):

- New York Times (Gretchen Reynolds)
- India Today (Rahul Kanwal):
- iRun (Ben Kaplan):
- Soigneur (Keir Place):
- (Joe Lindsey):
- L’Avenir (Alan Marchal):


Admittedly, we made a mistake. Not in our research. But in our communication. We did not anticipate that in the midst of a world-wide crisis, Murphy’s law should be applied to science communication: “everything that can be misunderstood, will be misunderstood”. Either unintentionally, or intentionally.


In the meanwhile, several national authorities have adopted our findings and – although sometimes more strictly than required - have converted them into guidelines for the public, such as France, Spain and Japan. Other authorities, such as the Dutch RIVM, have stated that movement of people should be considered and that therefore more research is needed for better guidelines.

Some scientists have indicated that running and cycling outside cannot be a risk because time and dose are essential for infection. As mentioned before, it is quite problematic for scientists to make strong statements when they are not 100% sure. And such a statement is indeed incorrect in several ways. In a group of runners of cyclists, large saliva droplets and snot fly around all the time. These are not aerosols that are of most concern here, but the large droplets than have been shown to be very effective in transferring the virus… which is why the 1.5 m distance was imposed in the first place… But even if such a small fraction of droplets would not be enough to cause infection, one should know that recreational and professional runners run for tens of minutes, sometimes more than an hour in each other’s wake. Recreational and professional cyclists ride for hours, 5, 6, sometimes more, always in each other’s wake. That is “time”. And in such a group, due to the physical exercise and effort, a lot of large droplets and snot are ejected and these fly around in the group. That is “dose”. Again referring to Tom Dumoulin’s statement: “Saliva droplets fly around in the cycling peloton. If on the first day of a multi-stage race one cyclist has a cold, a few days later, the whole peloton has it.” We rest our case. The above-mentioned journalists and scientists that do not understand flying saliva and snot in a cycling group, would be wise to maybe once in their life, train together with distance runners and cyclists, before communicating strong statements about infection risks in these sports.


We will publish these research results in several scientific journals with peer review. In the meanwhile, the draft paper and a list of Questions and Answers to avoid the many misunderstandings have been published online: Paper: ( Q&A: (


In crisis times, we should anticipate that everything that can be misunderstood, will be misunderstood, either intentionally or unintentionally. Common sense should not be taken for granted. A tiny minority of journalists and scientists have dropped the basic ethical standards in their field in order to profile themselves at the expense of the reputation of others. Apparently, all is fair in love and war… and COVID-19. I need to stress this is only a tiny minority of scientists and journalists (because we highly appreciate the vast majority of them and their work).

Professional running and cycling races that will resume later this year will certainly have a wide range of appropriate precautionary measures in place. But we pray that no mass infection will occur in those recreational cycling and running events where some of these measures will not be in place, and where people are running or cycling together in closely packed groups for hours. We do advise the tiny minority of questionable scientists and journalists that labelled our study as nonsense to pray extra hard.