Towards aerodynamically equivalent COVID‐19 1.5 m social
distancing for walking and running

Questions and Answers
Authors: Bert Blocken, b.j.e.blocken@tue.nl, Thierry Marchal
Question 1: What was the objective of the study?
The starting point was the 1.5 m (or in some countries 2.0 m or 6 ft) social distance that is recommended
to be kept between two people standing still to avoid respiratory droplets of person A reaching person B.
It is common sense that when person A exhales droplets and person B moves forwards, person B canmove into the droplet cloud exhaled by person A. So people movement can affect droplet exposure. The
objective of this study therefore was to find out to which extent the social distance of 1.5 m is enough or
not for two people walking, running, cycling in each other’s vicinity. And if not, how it should be adjusted
to yield a similar (non‐)droplet exposure risk as for the case of 1.5 m with two people standing still.
Question 2: What it meant by “aerodynamically equivalent social distance” for walking, running, cycling?
The standard social distance is 1.5 m, 2 m or 6 ft (depending on the country). The aerodynamicallyequivalent social distance is the social distance that two persons need to maintain when
walking/running/cycling to have the same level of (non‐)droplet exposure risk as in the case of 1.5 m, 2m or 6 ft for two persons standing still and facing each other.
Question 3: What are the main findings of this study?
First, is was found that the droplets (range 40 m – 200 m) exhaled by a moving person are mainly
entrained in the slipstream (wake) behind this person.
Second, in absence of strong wind, the equivalent social distance for walking/running/cycling can remain
1.5 m (or 2 m or 6 ft) if the two persons are moving side‐by‐side or in a staggered formation. However, if
person B is positioned in the slipstream (or wake) of person A, this person can be exposed to thedroplets emitted by person A. Then the equivalent social distances are: 5 m for walking fast (4 km/h), 10
m for running fast (14.4 km/h), 20 m for cycling fast (30 km/h). The social distance to be kept when in theslipstream increases with increasing speed of person B.
Question 4: What are the virological, medical or epidemiological conclusions from your study?
None. This study does not draw any conclusions on the infection risk associated with particular social
distances or droplet exposure. We are sharing these results with healthcare authorities and remain attheir disposal for further information. It is obvious that no or reduced droplet exposure is better thanlarger droplet exposure. Therefore, the adjusted and equivalent social distances are to be preferred overthe single value of 1.5 m.

Question 5: Why is it important to have these updated and larger social distances in walking, running andcycling?
These distances should not be larger than 1.5 m, except when person B is in the slipstream of person A.
Then, the larger social distances are not only common sense but also a matter of consistency. If one now
argues that this distance when moving fast (walking, running, cycling) should not be larger than 1.5 m,
this logically implies that the 1.5 m when standing still and talking is also too large – which is not thecase, as shown by the vast literature since 2002 on travel distance of drops.
Note that organizations like CDC (Center for Disease, Control and Prevention) are asking for additional
evidence and technology to give better recommendations in the future.
Question 6: What is the value of this study?
The value of this study is not to indicate that person B moving closely behind person A can inhale thedroplets emitted by person A. That is common sense for most people with a minimum fluid dynamics
background and/or intuition. The value of this study is to indicate where exhaled droplets go (i.e. in theslipstream) and which specific social distances should be used in walking/running/cycling to beequivalent to 1.5 m standing still.
Question 7: Was this study only done by engineers and not by medical experts?
Not really. The study was done by four engineers (2 x civil, mechanical, aeronautical) but one of these,Thierry Marchal, is also seen as a top medical expert in simulation, formally a e‐Health expert to EU
authorities, leader of Avicenna Alliance and therefore spokesperson for the discussion with variousauthorities (https://youtu.be/Sig5Q7rMFfY). The medical world is moving towards in silico (computermodel) to complement in vitro and in vivo data. These models have been reviewed by other experts such
as Marc Horner, co‐author of the V&V 40 standard and presented to the US healthcare authorities
before sharing them publicly. The medical world is now moving fast towards computer models.
Therefore our study fits in these new developments.
Question 8: Should you not have involved virologists, epidemiologists and medical experts?
Not for the scope of this particular study. This study is an aerodynamics study, not a virology study. The
only two facts we adopt from virology are two basic and well‐established facts: (1) Respiratory droplets
are an effective way of transferring this type of viruses (well‐established in scientific literature); (2) It isbetter to not be exposed to other people’s saliva droplets than to be exposed to them (common sense).
Question 9: Does it make sense that mostly engineers performed this study?
The role of civil and mechanical engineers in current and previous SARS epidemics should not be
underestimated. Engineers have been heavily involved in SARS since the outbreak in 2002 in Asia
because the transfer of droplets in the indoor air in hospitals and houses is a key civil and mechanicalengineering expertise. Thousands of papers on this topic have been published by civil and mechanicalengineers in the peer reviewed literature. Note that the 1.5 m social distance has been defined in thepast decades based on studies by engineers, not medical professionals. Engineering is now an importantpart of all biomedical and pharmaceutical research as illustrated by the fast and growing success ofbiomedical engineers.

Question 10: Why did you choose to not follow the standard academic procedure to first get the study
peer reviewed and only announce results in the media later?
This is an exceptional situation. Even the extremely careful FDA (Food and Drug Administration) has
wisely adjusted some guidance to ensure the rapid approval of any device or treatment / vaccine related
to COVID‐19 (e.g. Guidance document for ventilators and accessories) without compromising patient
safety. Time matters a lot here. So it is important to temporarily adjust processes to make sure new
products are reviewed faster. Similarly here, once the results were properly validated, the priority was toshare the results with the public to help reduce the risk of propagation of the COVID‐19; a few days later
the scientific publication was shared in open access to allow all other scientists to review and challenge
the approach and contribute to its future extension. The peer review publication will follow next. But weare not on the same time line when there is a pandemic storming the world. We thought that the priority
was on people’s health.
If we follow the normal procedure, we would first write a research proposal. If we are lucky we receive
funding to start within a year. Let us say we can produce all results in one day (never happens). Then wewrite the article in one day (nearly impossible) and submit for peer review. In our field, this can takemonths, e.g. 6. So 1.5 years from now we have our peer‐reviewed paper. Should we only thencommunicate to the public, we do not think this would be ethical. We made the choice to do validate ourresults first, then share the output publicly, before submitting the article for peer review. Not the most
comfortable order for us, for multiple reasons, because of no funding, some aggressive and unjustified
criticism, more debate on the format than on the study, etc. It appeared more important to us to share
this precaution advice to the public.
Note that not following the normal procedure is not so exceptional, it happens frequently in scientificresearch. Scientists do this all the time, in the form of mass meetings, where often also press is present.
These are called scientific conferences and thousands of these events are organized every year world‐wide. There, often research is presented without publication at all or where no peer review or no decent
peer review is performed for the conference publications…
Of course, we will have our study peer reviewed, because we wish to publish it in a journal, but that did
not seem most urgent to us now, certainly because the study confirms common sense and adhering to
the guidelines of the study cannot have harmful consequences.
Question 11: Don’t you think your advice is dangerous, especially given the study is not yet peer
reviewed?
This is not a study related to a treatment. There is no risk in following our guidelines. This is not asituation that can be compared to advising people to take certain medicine that has been insufficiently
tested and that can have adverse health effects. There are no adverse effects for you to walk, run or
cycle in staggered arrangement instead of inline, except… increased air resistance. We believe that inthis exceptional situation increased air resistance should be preferred over potential health risks.

Question 12: One could argue that you are not medical experts and that your guidelines cannot be
considered as medical guidance.
The first part is not really correct. Thierry Marchal is not only an engineer but also a top medical expert insimulation, formally a eHealth expert to EU authorities, leader of Avicenna Alliance and therefore
spokesperson for the discussion with various authorities (https://youtu.be/Sig5Q7rMFfY).
We do not give medical guidance. We report aerodynamic results and suggest guidance in terms ofexposure to droplets. We do not draw conclusions in terms of infection risk. This study did trigger aninterest from quite some virologists and epidemiologists, which can lead to future collaboration whereexperts of different disciplines are joining forces.
Question 13: Did you have sufficient past performance to execute this type of study with high quality?
The main investigator, Bert Blocken, has been studying droplets (of micrometer to millimeter sizes) inairflow for more than 20 years, and airflow around cyclists and runners for more than 15 years.
The other lead investigator, Thierry Marchal, has been leading the healthcare activity of a leading
simulation company for 14 years and working closely with US and EU authorities for over 5 years, relying
on his vast ecosystem to continuously comment on new results and challenge the hypotheses.
Question 14: Should all people that want to exercise outdoors start wearing masks?
That is not unwise if people want to move very closely behind each other. However, there are serious
concerns about the shortage of masks and professional masks should primarily be reserved for thehealthcare workers. Assuming that masks are not strictly needed when talking at 1.5 m distance, ifpeople follow our aerodynamic recommendations and the updated equivalent social distances when
walking/running/cycling in the slipstream, then also in those cases masks would not be strictly needed.
Question 15: Are there other simple rules that people should consider?
When there is substantial cross‐wind, the suggestion to stay out of the slipstream remains valid. Thismeans that it is advised to not walk/run/cycle directly behind others but offset to the upwind side.
When overtaking a person, be kind to this person and only when you have reached a certain distancefrom this person (5 m when this person is walking, 10 m for running, up to 20 m for cycling), move backon the same straight line as this person.
Question 16: How you do react to some media that have given negative feedback on this study?
It is always advisable to challenge any results, question the hypotheses and consult other experts in thefield to collect different opinions. We were however surprised by some personal attacks or judgementson the format of the study rather than discussing the content itself or its conclusions. Not contacting thekey researchers for further information and constructively challenging them or when seeking a second
opinion on this aerodynamics modeling, asking persons that do not have proper aerodynamics expertise,often leads to embarrassing situations and useless polemics.

About the study not being complete: no scientific study is ever complete. There is an infinite number ofcombinations of runner body geometry, wind speed, wind direction, turbulence, droplet size, relativehumidity, etc. We chose to start from the most basic choices: two identical runners, no external wind,typical droplet size from literature, etc. as this situation is not only the most common but could also be aworst case scenario. Evidently further work is needed considering different values for these parameters.
We expect that various authorities will be monitoring these future studies once this wave of COVID‐19 isover to better understand these phenomena so that we would be ready for a likely future pandemic.
Question 17: Should people not decide to stop exercising outside?
No. The crisis is very large and mental and physical health are important and walking, running and cycling
do contribute to both mental and physical health. Our study is intended to indicate how social distancing
should be done in those situations. From the beginning, the World Health Organization WHO and theCDC have recommended to keep a distance of at least 1 m or 2 m, acknowledging that this is not always
possible especially in public transportation. We are obviously not these prestigious organizations but weare providing recommendations using reliable results to minimize the risk of contamination
acknowledging that they cannot always been followed.
Question 18: Should walkers turn away until a jogger is 5‐10 m away or until a cyclist is 20 m away?
If a runner or cyclist passes a walker, by the time this runner or cyclist is again on the straight line of thewalker (i.e. the walker is in the slipstream), the runner is often 10 m away from the walker and the cyclist
20 m (unless the runner or cyclist discourteously moves back into the walker track), which are precisely
the distances that are equivalent to the social distance of 1.5 m for two people standing still and talking
to each other. Therefore the risk would be minimum. So, no; there is no need or walkers to be afraid ofrunners or cyclists; more than ever this is a period where solidarity between people and respect of eachother are important.
Question 19: Should walkers avoid being in the vicinity of runners and cyclists?
Same answer as for Question 18.
Question 20: Should we wear a mask while exercising or walking in a place where there are runners andcyclists?
This decision belongs to the local authorities wisely advised by scientific experts. We encourage you toconsult their advice. From all publications that we have seen, it is never prohibited to wear a mask;
however, wearing a mask does not guarantee that you would have a total protection.

‐‐‐ END OF DOCUMENT ‐‐‐