NOEL KING, HOST:
Do you need to wear a mask when you're outside? There's still a federal mask mandate in place. But the CDC is expected to update its guidance on masks later today. NPR health correspondent Maria Godoy is following this story. Good morning, Maria.
MARIA GODOY, BYLINE: Good morning, Noel.
KING: So the CDC is issuing an update on masks. Why?
GODOY: Well, one big reason is the growing number of Americans who have now been vaccinated against COVID-19. More than half of the adult population has gotten at least one dose. And so that's helping pave the way for the beginning of a return to normal. Here's Dr. Anthony Fauci, America's top infectious disease expert, speaking on ABC.
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ANTHONY FAUCI: It's pretty common sense now that outdoor risk is really, really quite low, particularly if you're vaccinated.
GODOY: I spoke with Dr. Monica Gandhi. She's an infectious disease specialist at the University of California, San Francisco. And she notes that other countries with high vaccination rates don't have outdoor mask mandates.
MONICA GANDHI: Israel released their outdoor mask mandates a week or so ago. The U.K. does not mask outdoors. And so we're also probably looking at other countries who have very high vaccination rates, even higher than we, and they're saying, OK, this is the time.
GODOY: And the other thing is studies have been showing that, unlike being indoors, when you're outdoors, the risk of transmission in general is lower.
KING: How much lower?
GODOY: Well, I asked that question to Linsey Marr. She's a researcher at Virginia Tech. She studies how viruses travel in the air. And she's also studied masks. And here's what she said.
LINSEY MARR: Virus just cannot accumulate in the air outdoors. It's like putting a drop of dye into the ocean. You know, if you happen to be right next to it, then maybe you'll get a whiff of it. But it's going to become diluted rapidly into the huge atmosphere.
GODOY: One analysis suggested that the risk of transmitting the coronavirus is 19 times lower outdoors than indoors. But Marr says that's likely a conservative estimate. And the real risk is probably a lot lower. In fact, another study out of Ireland looked at more than 230,000 COVID cases in that country through March of this year. And it found that just one in a thousand cases could be traced to outdoor transmission.
KING: Like putting a drop of dye into the ocean. So interesting. Are there any conditions, though, in which health experts say when you're outside, you still should wear a mask?
GODOY: Well, so first, I should note that, actually, some areas of the U.S. still have outdoor mask mandates. Although, the CDC guidance may change that. And also, I should note that Dr. Fauci has signaled that the guidance will address masking by vaccinated versus unvaccinated people. That said, in general, in terms of the science, all the doctors and researchers that I've spoken with say it depends on the situation. So if you're just passing someone on the street or whizzing by on a bike, there's no need to wear a mask outdoors. But if you're in a crowded situation like, say, an outdoor farmer's market or you're at a concert and it's packed, it still makes sense to wear a mask. You know, here's Linsey Marr again.
MARR: My general rules of thumb would be if I'm having a face-to-face conversation with someone and if I can, you know, put my arms out and they're within arm's reach for more than a minute or two, then I would mask. If I know everyone around me and I am vaccinated, then I wouldn't worry about it.
GODOY: So if you don't know someone's vaccination status, just follow that arm's length rule for when to mask outdoors, especially if you're in an area with high transmission. If you're both fully vaccinated, no need for a mask. And, of course, keep those masks on if you're in a public space indoors, like the grocery store.
KING: NPR health correspondent Maria Godoy. Thanks, Maria.
GODOY: My pleasure. Transcript provided by NPR, Copyright NPR.