With more cities and states enacting stay-at-home orders to prevent the spread of the coronavirus, one of the few sanctioned outings is exercise. As a result, outdoor spaces have been swamped. In Seattle, where I live, city officials have closed parking lots, ball courts, and playgrounds at popular local parks to dissuade people from swarming, but people are still showing up in droves. Over the weekend, I took advantage of a rare sunny spring day and ran at a nearby park, where I found hundreds of other people doing the same. The density of people was astounding—there were people everywhere, walking four across on the paved path, with cyclists, rollerbladers, and skateboarders weaving around pedestrians. There was no way everyone was staying the CDC-recommended 6 feet away from others at all times, and I wondered how likely it was that parkgoers might infect one another.
In wide-open spaces, where people can maintain adequate space, the risk of infection is vanishingly low. But it may be slightly higher in crowded spaces where people are walking close to one another or zigzagging through one another’s paths. “I’m not going to be a false optimist and say the risk is zero,” says Dylan Morris, a Ph.D. candidate at Princeton University who co-authored a recent study about the novel coronavirus’s ability to live in the air and on different surfaces.
One major consideration is whether you can encounter virus particles in the air. The short answer is yes, in theory; Morris and his colleagues’ paper found that potentially infectious virus particles we breathe, cough, or sneeze out can remain in the air for up to three hours. That scary-sounding fact made headlines, leading many to believe infection was inevitable.
Three hours might sound like a long time, but there’s an important caveat: The density of virus particles that remain in the air isn’t necessarily enough to infect you. There’s a steep drop-off in the number of particles that remain after just an hour, says Morris, and how much virus is left after that time—or three hours, for that matter—depends on how much virus there was to begin with. The amount an infected person breathes out on, say, a casual walk around the park might not be enough to infect a passerby, but more studies are needed to understand this. Scientists are actively working to understand the minimum virus load to infect a human, says Morris.
So, it’s certainly possible, though not terribly likely, that you could encounter virus particles during exercise. Using an analogy borrowed from Virginia Tech professor Linsey Marr, Morris says that exposure to virus particles is similar to exposure to cigarette smoke. “You’re going to inhale a lot if you’re standing right next to someone and they puff in your face,” he says. “You’re not going to get a lot of secondhand smoke 6 feet away from them, and you’re really not going to get a lot if you’re 12 feet away.”
The farther away you can stay from others, the better. While 6 feet is the CDC-recommended distance, MIT professor Lydia Bourouiba warns that virus particles can travel beyond than that. In a recently published JAMA paper, she found that we produce a cloud of droplets, and that cloud is propelled out of our bodies with varying force. Droplets from a simple exhale appear to travel about 6 feet, but sneezes and coughs expel particles from our body with more force. Clouds produced by our sneezes and coughs have much more forward momentum, potentially carrying particles up to 27 feet. “Six feet is a baseline, and something we’ve started to cling to as ‘safe,’ but it’s important we don’t get lulled into a false sense of security,” says Bourouiba. “We can’t pretend there’s this wall at 6 feet.”
The same caveat to Morris and colleagues’ work applies here: While Bourouiba’s research shows virus particles could travel quite far, it’s unclear whether the concentration of those particles would be enough to be infectious. (Researchers are now stressing the importance of viral load; exposure to small amounts of virus particles may not trigger infection.) That depends on the amount of virus being expelled. It’s also unclear whether those particles would ever even make it to you while you’re walking, running, or biking outside. “The moment you have a draft or wind, that [droplet] cloud is being mixed and diluted,” says Bourouiba. How open the space you’re in can make a big difference. “If you’re hiking and it’s a trail full of people walking in the same direction, full of trees with little airflow, that’s a higher potential load than an open area. If there’s no airflow, clouds can remain in concentrated pockets.” In an enclosed space, like a hospital room, the long distances droplet clouds can traverse is much more worrying, and underscores the importance of proper protective gear for health care workers. Bourouiba’s findings could have implications for other enclosed spaces, too, like grocery stores or public transit.
Your mode of exercise might also change the risks you encounter as well as the risk you pose to others. If you’re running, biking, or otherwise exerting yourself, that changes your breathing. “Breathing heavily will increase the particles you’ll expel,” says Morris. Bourouiba agrees, pointing out that exhaling more forcefully might also increase the momentum of your droplet cloud. Plus, she says, your breathing cycle speeds up: “You’d inhale the same amount much faster because you’re going through multiple cycles at a higher frequency.” Again, that doesn’t necessarily mean that you’ll hoover up enough virus particles to infect you, but it’s not impossible. And if you’re breathing heavily and traveling past others at close distances, it’s unlikely but still possible that you could be shedding virus particles. (Scientists have posited for weeks that people with no symptoms are contributing to the spread of the virus.)
As a result, Morris suggests exercising with a mask, or staying at home. The scientific community is in the midst of a heated debate about masks right now, but both Morris and Bourouiba agree that they can’t hurt, as long as we’re reserving N95 masks for health care workers on the front lines of the pandemic. Surgical masks, homemade cloth masks, or even bandannas can be somewhat useful. “Any kind of breathing, talking, coughing, sneezing, or breathing heavily when you’re doing a hard workout—all those things put respiratory secretion out into the environment, and a simple cloth mask could block some,” he says. “It’s not going to go down to zero, but this is so much a numbers game of concentrations and dispersion that the more you block, the more you protect others.” Exercising with a bandanna tied to your face may be slightly unpleasant, but it’s a pretty minor inconvenience to endure if it protects others from your flying spit.
The important thing, though, is to be just as cautious in a mask as you would be without one. Bourouiba worries mask wearing might lure us into a false sense of security. “Masks are not protecting you from inhalation of residual particles in the air, and they’re not sealed, so droplets still escape through the sides,” she says, so wearing a mask does not give us license to get closer to people than we otherwise would. But, she says, “it can’t hurt.”
The upshot? Get outside, if you can, but think carefully about where you’re going, what you’re doing, and how you’re interacting with others. As for me, I’ll be avoiding that crowded park until our social distancing guidelines are relaxed.
Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.
