The CDC says that “less than 10%” of Covid transmission occurs outdoors.
Which sounds like a lot of outdoors transmission. If anything close to 10% was correct, it would mean thousands of deaths were from outdoors transmission.
But the number doesn’t stand up to scrutiny. 
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If you read the academic research that the CDC has cited in defense of the 10% benchmark, you will notice something strange. A very large share of supposed cases of outdoor transmission have occurred in a single setting: construction sites in Singapore. How could that be?
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In one study, 95 of 10,926 worldwide instances of transmission are classified as outdoors; *all* 95 are from Singapore construction sites. In another study, four of 103 instances are classified as outdoors; again, all four are from Singapore construction sites.
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It turns out that academic researchers defined places that were a mix of indoors and outdoors as outdoors. One study defined all of these settings as outdoors: “workplace, health care, education, social events, travel, catering, leisure and shopping.”
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In the case of Singapore, some of the supposedly outdoor construction sites had many enclosed spaces. So there is a very good chance that many of these transmissions classified as outdoors were actually indoors.
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And yet even counting all the Singapore cases as outdoors still suggests only about 1% of transmission was outdoors. Other studies — from Ireland and China — put the share at 0.1% or less.
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Saying that less than 10 percent of Covid transmission occurs outdoors is akin to saying that sharks attack fewer than 20,000 swimmers a year. (The actual worldwide number is around 150.) It’s both true and deceiving.
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The CDC’s exaggeration of outdoor transmission isn’t just a gotcha math issue. It is an example of how the agency is struggling to communicate effectively, and leaving many people confused about what’s truly risky.https://www.nytimes.com/2021/05/11/briefing/outdoor-covid-transmission-cdc-number.html?referringSource=articleShare …
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The government list of recommendations is so long and complex that it’s useless to many people, as
@zeynep has noted. All the while, the scientific evidence points to a much simpler conclusion: Masks make a huge difference indoors and rarely matter outdoors.51 replies 114 retweets 873 likesShow this thread -
Replying to @DLeonhardt
Also on the Singapore construction sites. (I also chased down the same number for my recent NYT piece). Besides the shared dormitories, they do work at loud environment all day: yelling at very close distance. That's probably how you can get those rare outdoor transmission cases.
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At very close distance, (0.2 to 0.5 meter) you *are* spraying people a bit more with larger particles, and the smaller aerosols are more concentrated there, too (will dilute quickly outdoors but at first that' where they are.) See correct graphic:pic.twitter.com/z13WXEH33m
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Replying to @zeynep @DLeonhardt
So construction or similar people singing/yelling at very close distance may allow transmission even outdoors, especially if prolonged. (Which fits what we know from the epidemiology). So that's the outdoor rule for me: avoid close, prolonged contact between the unvaccinated.
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Replying to @zeynep @DLeonhardt
But also that agree with Muge Cevik that the Singapore sites also had shared dormitories (which is what my own chasing down of that number revealed) so that was likely indoors/outdoors in many cases. Thanks for getting the more correct number out.
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