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Masks Remain Extremely Effective Indoors, But Are They Necessary Outside?

Dr. Ashish Jha says most evidence points to the risks of coronavirus transmission outdoors being very low.
Karen Ducey
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Dr. Ashish Jha says most evidence points to the risks of coronavirus transmission outdoors being very low.

Scientists and public health experts agree that masks are effective at lowering the spread of the coronavirus indoors, where the vast majority of transmission is likely to occur.

But what about outside?

About two dozen states have statewide mask mandates that generally require people to wear masks outside when they're not able to stay at least 6 feet apart. Many cities have their own rules.

But with vaccinations accumulating, some health experts and journalists are arguing that now is a good time for authorities to ease up on outdoor mask requirements. Studies have linked transmission to indoor settings far more than outdoor ones, though data is limited and there are plenty of caveats.

Dr. Ashish Jha, dean of Brown University's School of Public Health, notes that case numbers aren't going down and they've been largely stagnant for the past month or two. But he says the situations driving it are groups of people gathering indoors without masks.

"Once you get outside, it starts becoming really, really uncommon for the virus to spread," he tells NPR's All Things Considered.

Some exceptions could involve people at packed rallies, standing or sitting close together for long periods of time. But beyond those scenarios, "there really just is not much spread happening outdoors."

This interview has been edited for length and clarity.

Interview Highlights

Is there a number we can put on this? How uncommon it would be to pick up a case of COVID-19 if you were outside?

There are estimates that suggest maybe 1 in 1,000 infections happen outside. There are reasons to believe that if you just think about your risk, if you're just out and about walking around, it's probably even much less than that. So those rare instances 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. I think you really have to have a lot more exposure than that.

I am a jogger. ... Do we know how much the risk increases when you're breathing really hard, when you're huffing and puffing, trying to get up that hill?

I think if somebody were right next to you and spending, let's say, 10, 15 minutes running in that little stream of breath that you're exhaling, there might be a risk. But somebody you're running by who is there for just a second, the risk is — it's extremely rare.

There will be people listening to us who are screaming at their radios right now saying it's still spreading and the variants are out there and so far, most people aren't fully vaccinated. You shouldn't be having this conversation yet. What do you say to them?

I understand that first of all. But it's really important to be able to have a nuanced discussion of what is safe and what is not. Because one of the problems is if we can't have that discussion, then some people will adhere to all the rules, even ones that are not necessarily very useful, and other people will just ignore all of them.

And right now, while cases are spreading, while the variants are out there and very contagious ones like B.1.1.7 [are] really dominant, I do want people to do things that are safe. And part of that is telling people, you know, what restrictions they can let go of. And so I think it's critically important that we keep indoor mask mandates in for a while. We can't give up on those, not while infection numbers are high. But it also means telling people what they can relax on. And wearing masks outside, again, unless you're in a very, very crowded space for extended periods of time, probably doesn't do much to protect you or protect others.

Ayen Bior, Courtney Dorning and Elena Burnett produced and edited the audio interview.

Copyright 2021 NPR. To see more, visit

Mary Louise Kelly is a co-host of All Things Considered, NPR's award-winning afternoon newsmagazine.
James Doubek is an associate editor and reporter for NPR. He frequently covers breaking news for and NPR's hourly newscast. In 2018, he reported feature stories for NPR's business desk on topics including electric scooters, cryptocurrency, and small business owners who lost out when Amazon made a deal with Apple.