According to a new study from a team of physicists and civil engineers based in the Netherlands and Belgium, runners and fast walkers appear to create a “slipstream” of air droplets that can be transmitted anywhere from 15 to 30 feet behind them. The research suggests that that commonly-repeated mantra of the last several weeks — “You can run, just stay six feet away from the next pedestrian” — may require reconsideration.
Led by scientist Dr. Bert Blocken, an expert in fluid dynamics, the team created an interactive model to exhibit how quick movement can affect air droplets, in terms of both particle diameter and potential distance. The six-foot rule, a tentpole in social distancing vocabulary, is predicated on air droplets from still people, who are breathing normally. But during heaving sessions out on the sidewalk (Blocken’s group specifically modeled for a six-foot runner cranking out a 6:40 mile), an aerodynamically “safe” distance would need to be nearly 30 feet behind (and 15 for a fast walker).
Watch the video here, which depicts the full sequence. Fascinatingly, two people running side by side is far safer than when one person runs directly behind another, even at a sizable distance.
Blocken’s study isn’t peer-reviewed or published (the scientific community has its hands full at the moment), but according to a variety of infectious disease specialists, the results look “reasonable,” as The New York Times reported.
The implications shouldn’t be a cause for immediate alarm; Blocken’s model analyzes air droplets, period, not the potential viruses contained in them. It’s unlikely that respiratory droplets capable of containing coronavirus can travel quite so far as 15 or 30 feet to infect passersby. But in an age of quarantine, common sense is our collective best friend. Next time you’re out for a jog, try not to run up directly on fellow runners or brisk walkers. Give yourself a wide berth, and don’t return to the middle of the sidewalk until you’re well in front, which will protect others from your breath.
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