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Which app is that?
A combination of lack of wind for a few days, extreme pollution output(from cars, factories etc) and the fact that the central heating system is on(the central heating system is government controlled, and they usually use coal in the power-plants, so yea...)
When it comes to Beijing air, safe is a relative concept. As long as it's between 250 and 350 it's normal. Anything below that is amazing, at least in the winter. During the summer it's usually between 150 and 300.
Gotcha. Amazing to think that in 2013 some cities still have to deal with that crap. I'd imagine that's a lot like what Pittsburgh, PA might have looked like in the late 1800's and early 1900's.