WASHINGTON — A growing call for health officials to relax rules about outdoor mask wearing could soon lead to one of the most significant changes in virus guidelines since the U.S. first told Americans to don face coverings to curb the spread of COVID-19.

Outdoor mask wearing has been the subject of controversy in recent weeks. There’s evidence that vaccines not only prevent illness, but also viral transmission. And with the U.S. averaging 2.74 million vaccine doses daily — with a total of 231 million given out overall — even some public health experts are calling for less strict guidelines as warmer weather arrives.

A response could come as early as Tuesday, when President Joe Biden is expected to announce that the U.S. Centers for Disease Control and Prevention will issue new guidance on whether vaccinated people still need to wear a mask outside, according to a CNN report citing unidentified sources.

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It’s long been understood that indoor environments present the highest risk of transmission. SARS-CoV-2, the virus that causes COVID-19, spreads largely via aerosolized viral particles. The size of a space, how many people are in it, length of exposure and air circulation all factor into how easily viral particles might spread. Outside, those particles disperse far more quickly, lowering risk significantly.

But even then, the question of risk is not so black-and-white. The risk of inhaling aerosolized virus from a fellow hiker just passing by on a trail is minimal. If you’re in a situation where it’s harder to maintain distance from others, like, say, an outdoor concert or crowded farmer’s market, that risk goes up. Even outdoors, risk of transmission is a sliding scale.


Your risk also depends on factors such as how socially distanced you are from others, how long you’re around them, your vaccination status and the vaccination status of everyone around you.

Earlier this month, the New England Journal of Medicine published a blog questioning whether it’s time to do away with such mandates written by Paul Sax, clinical director of the division of infectious diseases at Brigham and Women’s Hospital.

“I think it’s best when public health mandates come out that they really follow the science,” he said. “Transmission doesn’t happen when people are outdoors by themselves walking around.” He still recommends wearing masks outdoors if you’re not vaccinated, having a face-to-face conversation with someone you don’t know, or in a crowded place.

“For awhile, the mask outdoors has been more a marker of supporting public health,” he said. “COVID isn’t going away any time soon, so we have to find ways we can live with it more easily. Not wearing masks when you are outdoors by yourself could be one thing that could help with that.”

Marc Lipsitch, a Harvard University epidemiologist, tweeted support for a Slate article making a similar argument. “I am generally a hawk about maintaining rules with a clear benefit,” he wrote. “Outdoor masking has notable costs and really no evidence of benefits.”

Others have argued that masking up in public helps set a good example. Many Americans are fatigued by more than a year of pandemic life and eager to revert to normal even while cases of the virus are surging in some parts of the U.S.

Mask wearing is a reminder that life isn’t quite back to what it was — and that if we fail to maintain vigilance, it could prolong the pandemic.

“There’s no clear rule that can be applied,” said Krystal Pollitt, an assistant professor of epidemiology at the Yale School of Public Health. “There are so many things to take into consideration to best evaluate risk. We still need to be vigilant.