Even as many states continue to mandate mask-wearing both indoors and outdoors, some experts are calling for more nuanced guidance, citing a growing body of research that suggests outdoor coronavirus transmission is extremely rare.

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What research shows about the risks of outdoor coronavirus transmission

Several lines of research support the notion that the coronavirus is rarely transmitted in outdoor environments, the New York Times reports.

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One published in Indoor Air found that, among 1,245 Covid-19 cases in China, just one was a result of outdoor transmission—and that transmission resulted from two people conversing face to face. And another paper, from the Health Protection Surveillance Centre, found just 262 Covid-19 cases associated with "locations which are primarily associated with outdoor activities" out of a total of 232,164 cases—accounting for about 0.1% of cases overall.

Further, a meta-analysis published in the Journal of Infectious Diseases suggested the percentage of cases occurring as a result of outdoor transmission is at most 10%. But Nooshin Razani, an author on the study, said that number was "probably lower" than 10%, as many of the cases occurred in circumstances where there was likely time spent inside as well as outside, such as construction sites and sleep-away summer camps.

According to Ashish Jha, dean of Brown University's School of Public Health, the rare instances of outdoor transmission appear to "occur in those contexts of sort of the large, packed rallies. I don't know that we've seen really any cases of somebody who was just, let's say, out for a walk or out for a run and picked up the infection that way."

Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, similarly said the chances of catching the coronavirus from an unmasked person during a brief encounter—such as passing on the sidewalk—are very low.

According to Marr, viral particles disperse quickly in outdoor air, significantly reducing the risk of inhaling aerosolized virus from a passerby. Even if that person sneezes or coughs as they pass by, Marr said, bystanders are highly unlikely to inhale enough virus to fall ill.

"I think the guidelines should be based on science and practicality," Marr said. "People only have so much bandwidth to think about precautions. I think we should focus on the areas that have highest risk of transmission, and give people a break when the risk is extremely low."

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What might a more nuanced rule for outdoor masking look like?

While experts broadly advised people to make mask-wearing decisions based on local public health guidance, vaccination status, their own risk factors, and the local rate of infection, some suggested approaches that could incorporate nuance—and perhaps a little more leeway—into risk calculations.

For instance, Marr shared her "two-out-of-three rule," which involves meeting at least two out of three conditions in every situation: being outdoors, masked, and socially distanced. "If you're outdoors, you either need to be distanced or masked," she said. "If you're not outdoors, you need to be distanced and masked. This is how I've been living for the past year."

"I think it's a bit too much to ask people to put the mask on when they go out for a walk or jogging or cycling," Muge Cevik, a clinical lecturer of infectious disease and medical virology at the University of St. Andrews School of Medicine, said. "We're in a different stage of the pandemic. … It's not where the infection and transmission occurs."

Similarly, Nahid Bhadelia, an infectious diseases physician and medical director of the special pathogens unit at Boston Medical Center, tweeted, "Let me go for my run, maskless. … Keep the masks on you for when you are stationary in a crowd and headed indoors."

Meanwhile, CDC Director Rochelle Walensky on Thursday said the agency is considering revising its outdoor masking guidance. "We'll be looking at the outdoor masking question, but also in the context of the fact that we still have people who are dying of Covid-19," she said (Parker-Pope, "Well," New York Times, 4/22; Palus, Slate, 4/17; Kelly/Doubek, NPR, 4/21; Syal, NBC News, 4/22).