For our ZOOM meeting of July 29, Dr. Bert Blocken of Eindhoven University of Technology of the Netherlands and KU Leuven in Belgium presented a summary of his research in both wind-tunnel measurements and computational fluid dynamics calculations with respect to the transfer of droplets and aerosol mist downwind of running and biking participants.  He also told us about the criticism and social outfall from his presentation of the results of that research.  The full meeting and presentation can be viewed at
He started with a discussion of the science behind aerosols and wind engineering.  Aerosols are suspensions of particles or droplets less than 5 microns in diameter in air.  A normal low-energy exhalation has some 50 droplets in it whereas a single cough might have 1000 and a single sneeze some 10,000.  The transmission of the Covid-19 virus may occur through large droplets (greater than 5 microns diameter) directly in the face, by contact with infected surfaces (with the hand transferring the viral load to the face), or by aerosols where the scale of the impact is dependent on both the closeness of the infected individual, the dosage and the length of time of exposure. 
Wind engineering is the study of the interaction between wind and man and his works on the surface of the earth.  Data may be acquired by field measurements, wind tunnel experiments, or computational modeling.  In order to demonstrate that there may be a difference between common sense and real data, he presented several simple examples.  A bicyclist drafting immediately behind a motorcycle sees 75% less air resistance compared to cycling by himself; but drafting in front of a motorcycle also shows a 5% decrease.  With six closely-spaced cyclists traveling in line and at speed, the 5th cyclist will experience the greatest decrease in resistance or effort required. 
Applying the rigors of these two disciplines, he asked what is the appropriate spacing between two individuals either running or cycling to be equivalent to the six feet (two meters) social distancing recommended for minimizing transmittal of the Covid-19 virus?  Basically, what is the impact of the slipstream behind the lead individual on increasing the necessary social distancing?  He addressed this question for a number of arrangements of participants with both wind-tunnel experiment and computational modeling.  He found, as common sense would suggest, that the best arrangement is side-by-side so that neither participant is in the slipstream of the other.  Almost as satisfactory is staggered, one participant behind the other but offset from the line of progress.  If the second participant is immediately behind the first, then the second should be some 5 meters (16 feet) behind the first at a fast walk, 10 m (32 feet) for running fast, and 20 m (65 feet) for fast cycling. 
He and his co-workers presented these results (which would appear to be both common sense and uncontroversial) as non-peer-reviewed results on social media.  They immediately received a flood of criticism and even vindictive comments and stalking as a result of individuals reading incomplete and uncritical summaries of the work or comments from individuals with little or no expertise in the area of the study.  Only later did quality media make an effort to track down actual experts in the area or even the original authors to get a balanced and unbiased review of the results.  His conclusion is that no one should make unqualified comments outside his area of expertise and that communication without evidence at best causes confusion. 
Is indoor stationary cycling (or, for that matter, any indoor stationary exercise) safe?  The more strongly you exercise, the more aerosols you generate, so the facility must have good ventilation in order to reduce the exposure to the virus. 
In presentations about scientific studies and results, who can you trust?  It is incumbent on both the scientists and the media to exercise caution in presenting results or summaries of results.  On the part of the reader, it is necessary to be skeptical not only of scientists who are on the payroll of the organization that is being touted but also of scientists who are commenting for attention and self-profiling. 
How do face shields compare with face masks with respect to reducing risk?  The shields can be effective against large droplets but not against aerosols.  Face masks are effective against droplets but only partly effective against aerosols, depending on quality of design and manufacture. 
What about two walkers passing each other in opposite directions?  Given that neither will be in the aerosol cloud from the other for very long, there is little danger unless one of the individuals turns his head and coughs directly in the face of the other. 
Given the high probability of infection in closely-spaced pelotons in bike races, what is the immediate future of those races?  There is a recently developed test for the Corona virus that provides results in 30 seconds, so it may be possible to test all participants first thing in the morning and immediately after the race in the afternoon and get any infected participants out of the competition.
A link to Dr. Blocken's original paper is below, along with his more recent response to public criticism, All is fair in love and war...and COVID-19.