Paging Dr. Hamblin: How Close Is Too Close While Biking?

Dear Dr. Hamblin,

My partner and I are both blind. Since March, our primary means for getting anywhere has been walking. We haven’t ridden public transit and very rarely ride as a passenger in a car. However, we do have two tandem bikes. In normal times, our sighted friends would captain them (which is to say, be the front riders). Because this is an outdoor activity, and not face-to-face, we are wondering what the level of risk would be if both riders were masked. Is it safe to go tandem biking?

Lisa Larges
Minneapolis, Minn.

For all the volumes of research that have been published on COVID-19 so far, none has specifically focused on tandem bicycles. The Centers for Disease Control and Prevention has issued no guidelines. But we’re all making decisions based on imperfect information these days, and I think we know enough to confidently apply some other studies to your scenario.

Generally speaking, biking is an ideal pandemic activity. It’s a great transportation alternative to riding in a car, and the presence of the bike itself mostly enforces physical distancing. Cycling has proved so opportune for the moment that in many places, the demand for bikes and the subsequent strain on the supply chain has made a decent bike almost as hard to come by as hand sanitizer that doesn’t smell like urinal cakes and vodka. I’m far from the only person endorsing pandemic cycling; in July, New Yorkers recorded 80 percent more bike rides in the exercise app Strava than they did at the same time last year. Riding a bike feels manifestly good in a world where almost nothing else does.

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The issue of tandem biking—or riding in close proximity to other people, as other cyclists have asked me about—is more interesting. It would, hypothetically, create some potential for transmission. You’re closer than six feet, unless you have the world’s longest tandem bicycle. Your friend in the front would be generating little aerosolized particles while they breathe—as we all do—and the airflow would direct those particles back toward you. The number of particles would increase if the other person is breathing hard (as people on bikes tend to) or talking loudly over the din of a busy street. This could all theoretically raise the chances of you inhaling some virus if your friend is contagious.

That sounds bad. But we also know that several factors would be working in your favor. First, you have safety in the fact that you’re not facing each other. Because the front rider is projecting their breath forward, and you’re sitting directly behind, their head serves as a physical barrier that parts the current of air. You’d get more of their breath in your face if you were at a 45-degree angle behind them (in their “slipstream”). Second, because you’re moving, the air around you is constantly turning over. Your breaths are being washed away into the sea of air around you. The situation is entirely different than if you were on a stationary tandem Peloton in a basement. (I don’t think those exist, and I’m not sure they ever should, but the pandemic would be an especially inopportune moment to launch them.)

Though no one has studied your specific scenario, some Dutch engineers published a controversial model earlier this year that suggested safe distances for outdoor exercise. Based on experiments on airflow patterns in wind tunnels, they concluded that to be entirely safe from ever inhaling anything that came out of another person’s mouth, you would need to be 16 feet behind someone who’s walking, 33 feet behind a runner, and even farther behind someone moving faster, including most bikers. These numbers made news and terrified more than a few people.

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The big problem with the model is that it didn’t measure whether anyone actually got infected or sick. It’s interesting to know that you could theoretically be inhaling a microscopic speck of some stranger’s sputum at such a distance, but it doesn’t mean that anything contagious is traveling that far. Nor does it mean that you could be exposed to enough of the virus to be at risk of getting infected. Researchers have yet to document cases of outdoor transmission at anywhere near 16 feet. So forget those specific figures above. Just remember that no number of feet is magic, not even six, and that any distance guideline depends on the context and length of time you’re exposed. Personally, I don’t worry about passing an oncoming runner or biker at a close distance; but I do try not to trail people closely for long. (It’s generally a weird thing to do anyway.)

Technically this is sort of what you’d be doing, so I can’t tell you that tandem biking is zero risk. I can’t recommend that you go tandem cycling with a new captain every day. But if it’s one friend whom you’d trust to let into your bubble as a monogamous cycling partner, I’m going to suggest that you go ahead and saddle up the tandem bike—if you can enjoy it without worrying too much. With masks on, and assuming your friend isn’t sick or having lots of high-risk contacts, you can worry less about the virus than you would about potholes or oblivious people opening car doors into the bike lane. Whatever risk you might incur would have to be balanced against the risk of not biking. The exercise has value, socializing has value, sunlight has value, and just getting out of your same surroundings as we drag into the ninth month of the pandemic absolutely has value.