About That Germy Slipstream…

Anyone who’s ever done a bike race, or a running race, a triathlon or a swim competition knows this: drafting is awesome. Tucked in the slipstream, you get sucked along behind the person in front of you. They break the wind, you get the benefit.

There’s also something else you get: their germs.

New research that went public this week is showing that even if you are walking behind someone at a pace of 4 kilometers an hour, you can be catching water droplets and be exposed to germs from the person who might be less than 10 feet ahead of you.

Researchers at KU Leuven and at the Eindhoven University of Technology created simulations showing how those working out outdoors could be exposing themselves to the new coronavirus (Covid-19) even when staying 1.5 meters apart.

The simulations show that the respiratory droplets of someone potentially infected with the virus could come into contact with anyone located behind them by traveling through a slipstream.

“When people speak, exhale, cough or sneeze they generate droplets, and while the largest droplets tend to fall to the ground first, the smaller ones can remain in the air a bit longer, so it is important that a person who is behind another does not walk into this cloud of droplets,” study coordinator Bert Blocken wrote in the published white paper.

The white paper released on this was created with the help of experts in aerodynamics and sports who simulated the release of saliva particles from persons in motion (walking and running) and in different configurations (two people side by side, diagonally behind and right behind each other). As the paper stated:

A simulation of the germs carried in the slipstream. Courtesy Bert Blocken

“Normally these simulation models are used by the research team to improve the performance level of elite athletes, both runners and cyclists because it is very effective for athletes to stay in the wake of leading runners and cyclists.

With regard to COVID‐19 infection risk, it is actually recommended to do the opposite, as it now turns out: to stay out of that slipstream. The researchers displayed the results of their research in a series of animations and figures.

The cloud of drops left behind by a moving person is clearly visible. “People who sneeze or cough spread the droplets with greater thrust, but also those who only exhale emit droplets.” The red dots on the images represent the largest particles. These are generally considered to be the most contagious – although virology research should confirm this for COVID‐19 – and fall down faster. “But when someone walks through that droplet cloud, they can still end up on that person’s body”, explain the scientists.”

The simulations show that social distancing plays less of a role for two people who walk or run side by side in calm weather. The drops then end up behind the duo. Those who move in a staggered arrangement are also less likely to catch saliva droplets from the predecessor, at least when there is no substantial cross‐wind.

The risk of contamination is greatest when people walk or run closely behind each other and therefore in each other’s slipstream.

Based on the results, Blocken advises keeping a distance of at least four to five meters behind the leading person while walking in the slipstream, ten meters when running or cycling slowly and at least twenty meters when cycling fast. “If you want to overtake someone, it is also recommended to start “pre-sorting” into a staggered arrangement from a fairly long distance – twenty meters with bicycles, for example, so that you can overtake carefully and at a proper distance by moving in a straight line.”

Compare it to driving: if you want to overtake, you should also not wait until the very last moment.”

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