Question: My Facebook feed is littered with people admonishing others for running outside and I think it’s gone a bit too far. Can you please do a Q&A on whether people need to fear getting the virus from that runner who just ran past them?

Answer: This is a perfect example of why we need better and widespread health communication about the coronavirus. There’s so much information for folks to sift through, including disinformation, and there’s no one CDC resource or the like that describes the risks of transmission under various scenarios. The best thing I’ve read on risk is this blog post from Professor Bromage that he published a few days ago.

Professor Bromage reminds us that “Successful Infection = Exposure to Virus x Time.” He writes (among other valuable takeaways) that “Social distancing rules are really to protect you with brief exposures or outdoor exposures. In these situations there is not enough time to achieve the infectious viral load when you are standing 6 feet apart or where wind and the infinite outdoor space for viral dilution reduces viral load. The effects of sunlight, heat, and humidity on viral survival, all serve to minimize the risk to everyone when outside.” Please take a read through if you haven’t had the chance yet. It will become all the clearer that running outside as an individual is a very low risk activity.

As pathologist, Dr. Kasten, recently described it, “[Consider the difference in the risk between taking a stroll through the park or a climb up a steep cliff face] sure, you could slip, fall, strike your head, and die on that path in the park. Likewise, you could free-solo successfully to the top of El Capitan. But most of us would accept the risk of the stroll and not accept [the risk of] dangling from the cliff. Breathing in someone’s sneeze cloud, close by, without a mask — that’s the cliff face. Jogging several feet away, or getting the mail — that’s the park.”

And now here’s a bit more information about some of the studies showing risk of indoor vs. outdoor activity:

1. A study of transmission dynamics in a restaurant in China showed that “droplet transmission was prompted by air-conditioned ventilation.” Indoor conditions and air ventilation matters a lot when it comes to viral transmission risk.
2. Two pre-print (not yet peer reviewed) studies released last month showed that the preponderance of transmission occurs indoors.

• The first study identified outbreaks of 3 or more people in January/February in municipalities in China to assess environmental exposure types. Researchers found that of the 318 outbreaks identified (7,324 cases), only 1 occurred in an outdoor environment and involved two people. “A 27-year-old man had a conversation outdoors with an individual who had returned from Wuhan on 25 January and had the onset of symptoms on 1 February.”
• The second study examined 110 cases in Japan to identify transmission settings and found that “the odds that a primary case transmitted COVID-19 in a closed environment was 18.7 times greater compared to an open-air environment (95% confidence interval [CI]: 6.0, 57.9).” This study manuscript is quite weak — missing lots of key details — so I wouldn’t put much weight on it until after peer review, but in the meantime, the results do fit within the trend of what we’re seeing.