Measuring the harm

shanghai air 4
The Shanghai skyline shrouded in smog.

At its worst, the city's air quality index topped 500. The U.S. categorizes any reading over 300 as "hazardous."

It is notoriously difficult to get accurate pollution readings in China. When severe smog hits Beijing, residents rely on the U.S. Embassy's own readings published via social media.

First published December 10, 2013: 3:59 AM ET

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