Warmer weather means more outdoor exercising. Is 6 feet of distancing enough?

Beautiful spring weather—partly sunny with highs in the 70s—has arrived in the Sacramento region, which means that many of us will be breaking free of our shelter-in-place swaddle and heading outdoors. Even during the stay-at-home mandate, the official state and county guidelines consider outdoor activities like walking, hiking, biking and running “essential,” provided social distancing is involved. But is the 6-foot rule enough when it comes to potentially virus-carrying respiratory particles that we naturally emit when we breathe or talk, much less cough or sneeze? (It’s the height of pollen season, after all.)

Recently, a group of 37 global scientists petitioned the World Health Organization to raise the distancing regulations to more than 6 feet. One scientist in that group, a University of Colorado chemistry professor, says he tries to keep at least 25 feet from anyone while outdoors.

We sat down, via Zoom of course, with UC Davis professor of chemical engineering William Ristenpart, who studies the movement of respiratory particles that may be carrying viruses like SARS-CoV-2, to ask about the safest ways to take in that gloriously sun-soaked weekend bike ride or hike.

We’ve been schooled to avoid handshakes, hugs, handrails and the like to prevent the spread of COVID-19. There’s less focus on airborne transmission of SARS-CoV-2 (the virus that causes the disease), but it’s a silent and invisible danger.

I think everybody's seen the spray when you cough or sneeze. You've probably heard the phrase, “Say it don’t spray it.” Well, the fact that you can even see a spray means that those particles are huge compared to the things we're talking about—invisible respiratory particles called aerosols that come from normal breathing and speech and are large enough to carry a virus. They’re too tiny to see, but they persist in the air for a long time—more so indoors, but outdoors, too. And these invisible particles are still much, much larger than viruses like SARS-CoV-2, so they're plenty large enough to carry a virus. And those things are just floating around on air currents.

Let’s say you run into a friend at the park, and you start up a conversation with them. Are you in danger of being infected if they’re asymptomatic, or vice versa?

If there's a slight breeze, the particles will probably just get carried away and dispersed. Just keep in mind that if you're stopping to have a face-to-face conversation, there's still going to be this invisible cloud of respiratory particles between you. So, even if there's a breeze, you might still breathe something in. Estimating the actual probability of transmission from a situation like that depends on a whole lot of things that are not well known.

What if you’re just passing an asymptomatic stranger that’s on the same running path or bike route that you’re on?

The overall probability of transmission is also going to be proportional to the exposure time. If you just happen to pass somebody on a path, yes they could be infected, and yes, they’re probably breathing out respiratory particles, but there’s a much lower probability that you’re going to inhale one of those virus-carrying particles. The risk of transmission is in proportion to the duration of exposure.

What’s your rule of thumb for social distancing outdoors? The 6-foot rule has been ingrained into our behavior, but not unlike the group of scientists petitioning the WHO, there’s an MIT professor who wants to expand that measure to 27 feet—the distance that particles can travel after a particularly violent sneeze. Allergies are afoot!

Six feet is a good guideline, but it's not a magical distance. Aerosol particles can travel much, much further than 6 feet, getting pulled around by air currents. Trailing 20 feet behind somebody for a few minutes or an hour is likely more dangerous in terms of potentially inhaling a virus-laden respiratory particle than passing within 6 feet of them for just a few seconds. Passing people is probably the lowest-risk encounter in an open-air situation. If you're at the point where you're worrying about 6 feet versus 27 feet, it would be better to stay home. Don't feel like you're safer at 28 feet than you would be at 27 feet. There isn’t a hard cutoff like that.

Would wearing a mask while running or hiking help?

Certainly the CDC and the Surgeon General have both recommended masks. How you wear the mask is very important. I've seen a lot of people put on their masks, but then their glasses or sunglasses fog up. Right off the bat that indicates that there isn’t a good seal, which means they're not wearing the mask properly. Air follows the path of least resistance, so if there are gaps between your mask and your face, air will just enter and exit through those gaps. In general, if you wear a mask, it is going to block larger droplets that would come from coughing or sneezing. But you're still emitting those tiny, invisible aerosols from breathing or talking. I think it’s kind of an open question as to how a lot of these homemade masks work in terms of preventing transmission. But I’ll just say that wearing a mask is better than nothing.

There’s also been some talk about closing public parks and outdoor spaces to minimize exposure to SARS-CoV-2. What do you think about park closures in general?

Closing the type of park where people congregate closely [such as playgrounds] is warranted. Large open areas with minimal people, however, should be fine. A mostly empty hiking trail should be fine. I take my family for long bike rides along mostly empty paths and roads like the Greenbelt in Davis, where we live. And we don’t wear masks out there on those mostly empty paths, in the open air.

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