500
“Last year, the U.S. Embassy installed an air monitor on the roof of one of its buildings, and every hour it posts the results to a Twitter feed, with a score ranging from 1, which is the cleanest air, to 500, the dirtiest.
American cities consider anything above 100 to be unhealthy. The rare times in which an American city has scored above 300 have been in the midst of forest fires. In these cases, the government puts out public-health notices warning that the air is “hazardous” and that “everyone should avoid all physical activity outdoors.”
As I type this in Beijing, the Embassy’s air monitor says that today’s score is 500.”
