The days are getting longer, temperatures are climbing, and daffodils are in full bloom. After a long, dark winter shuttered indoors with the threat of COVID-19 all around us, springtime promises a chance to finally break free from the confines of our homes. But do we have to take those deep breaths of fresh air from behind a mask?
A year after cities closed playgrounds and public parks, fearing the spread of the virus in shared outdoor spaces, plenty of evidence has accumulated showing that outdoor transmission is rare. That means recommendations around wearing a mask outside don’t need to be as strict either.
“One of the strongest findings from the literature is that transmission is reduced outdoors relative to indoors,” says Jonathan Proctor, a postdoctoral fellow at the Harvard Data Science Initiative and the Center for the Environment. The reason is fairly intuitive: The virus has plenty of places to go besides up your nose.
“There’s a lot of air in which the droplets and the viral particles can disperse,” says Lisa Lee, a public health expert at Virginia Tech and former official at the Centers for Disease Control and Prevention.
A systematic review published in February found that fewer than 10 percent of reported SARS-CoV-2 infections occurred outdoors. Indoor transmission, by comparison, was more than 18 times more likely. Infections that did occur outside usually involved other risks, such as people mixing indoor and outdoor activities.
Distance, duration, and intensity matter most
Still, the risk isn’t zero, says Saskia Popescu, an epidemiologist at the University of Arizona in Phoenix.
“Outside is protective, but it’s not a total risk eliminator,” Popescu says. “When we’re seeing transmission outdoors, it’s people who are close to each other, talking face to face.”
The three key factors to consider are distance, duration, and intensity, she says. The closer people are, the more droplets an activity is generating, and the longer people are close to one another, the more the risk increases and the more important a mask becomes. As with so much else in the pandemic, infection risk—and the need to wear a mask—hinges on the context.
“It depends a lot on a number of things, including how crowded an outdoor activity is, how much movement there is, if everybody’s facing the same direction versus everybody facing each other, how vigorously people are exhaling,” Lee says. “If everyone is breathing really heavily, their droplets will travel further, so you’ll want a wider berth than six feet.”
Popescu gave a few examples of outdoor situations in which she always wears a mask versus those where she simply keeps one on hand if needed.
“If I’m in a farmers market, I’ll wear a mask because I’m around other people. Even if I can kind of distance periodically, I’ll wear a mask the whole time,” Popescu says. “If I’m walking with my husband on the beach or walking down the street with my dog, I will have my mask with me and I’ll put it on as I see people approaching.”
Basically, if Popescu will be in close proximity to people outside her household, she puts it on. If she can remain at least six feet from others—at a minimum, she emphasizes—the mask isn’t necessary.
Weather seems to have little impact
As the Northern Hemisphere heads into summer, the risks of outdoor transmission may drop even lower, suggests some preliminary research on the weather’s impact on SARS-CoV-2 transmission. The strongest factor, albeit still with weak evidence overall, appears to be ultraviolet light.
“Our findings suggest that UV might be deactivating the virus and therefore making it more difficult for the virus to spread,” says Proctor, who coauthored a study that showed the association of higher levels of UV light with lower levels of COVID-19 transmission. The reasons for that association are difficult to pin down, he says.
“In terms of empirical studies on climatological factors, sunlight, temperature, humidity, windspeed, etc., it appears that days when you have higher sunlight, you see reduced transmission of COVID over the following two weeks,” says Proctor. “That two-week lag is consistent with the time it takes someone to transmit the virus and then the symptoms to show and to get tested.”
“In general, respiratory viruses like cold, dry temperatures, especially SARS-CoV-2,” says Luca Cegolon, a medical epidemiologist at the public health department in Treviso, Italy, and senior author of a commentary on the rationale for wearing masks outdoors. “The mask is not only providing a physical barrier, but it also helps maintain the relative humidity and temperature of the mouth and especially the nose,” Cegolon says. “That interferes with the settlement [the virus’s ability to get a foothold] and replication of the virus and keeps the immune system of the upper airways stronger.”
But those findings are more relevant in the winter and, again, when people gather close together for a long time. Overall, evidence about the virus’s seasonality remains muddled, says Gaige Kerr, an environmental and occupational health scientist at George Washington University, who led a recent research review in this area.
“At this point, we don’t really have a good grasp on the exact impact that various meteorological variables have on the disease,” Kerr says. Influenza and other coronaviruses have seasonality, Kerr says, and lab experiments have shown the SARS-CoV-2 virus lasts longer in cold, dry conditions with low UV radiation. “But those results have not really been reflected in real world data,” he says. “Despite dozens or hundreds of studies, the results are just all over the place, and there’s not a consensus at all. Using meteorology as a basis for changing or relaxing government interventions is really not supported by the science.”
Spending time outdoors, even maskless, offers health benefits
Instead, the weather’s biggest impact is likely on human behavior. While Proctor’s findings, for example, suggest UV light could be deactivating the virus, other reasons could explain the association they found.
“It could be that when it’s sunny, people go outside,” Proctor says.
People gather inside when the temperature climbs too high or drops too low, Lee says, and that leads to more transmission. In fact, that’s the very reason it’s important for policymakers not to require masks outdoors, says Muge Cevik, an infectious disease physician and clinical lecturer at the University of St. Andrews in Scotland.
“For over a year, people have been under some sort of restriction, our lives have changed significantly, and everyone is just so tired,” Cevik says. “If we ask people to regularly use masks outdoors in every circumstance, it will exacerbate that mental fatigue. Then there’s no place where they can enjoy themselves without any restrictions.”
Outdoor mask mandates also push people indoors, where they won’t be seen gathering but where the risk is much higher, Cevik says. And meeting outside has other public health benefits.
“We can’t really think about public health as only infection control,” Cevik says. “Allowing outdoors to be a place where people recharge their energy through fresh air, joy, physical activity, and social connection is also important from a public health perspective.”
Mask up if the situation calls for it
So, whether people choose to don a mask outside should, again, depend on the activity. Popescu doesn’t see much reason for solo cyclists to wear a mask, though cyclists traveling in tighter groups or around many pedestrians should wear one. Similarly, joggers really only need to put on a mask or pull up a neck gaiter when passing someone.
“If you are running up behind someone jogging, you’ll be running into what they exhale in their slipstream,” Lee says, so wear a mask until you’re past them.
If swimming, masks are obviously impractical, but they’re also unnecessary if people from different households are staying at least six feet apart and no one is shouting directly at someone else’s face, Lee says.
Hikers can leave masks off unless they’ll be passing within six feet of one another. Even then, masks aren’t that necessary for brief encounters or passing unless hikers are breathing heavily. That said, donning your mask has symbolic value as well.
“I do think it’s an important expression of solidarity,” Lee says. “It is an important message that it doesn’t take much to pull your mask on for a few seconds while you’re passing someone.”
Popescu agrees. “It’s a mixture of respect and acknowledging that, even if it’s brief and the risk is very low, it’s the right thing to do,” she says.