How dirty are China's cities? Here's one way of looking at it:

Even in China’s most polluting city, Daqing, per household [carbon-dioxide] emissions are just one-fifth of those in San Diego, the greenest city in the US.

Of course, there's more to bad air than just carbon-dioxide. China easily beats us on the more immediately noticeable pollutants like sulfur dioxide or particulates, as any smoggy photo of Beijing will show. But I was surprised to hear that San Diego is America's "greenest" city. Presumably its per-capita emissions are so low because the weather's so nice (at least on the coast), and many households don't need much in the way of heating or cooling, something this 2008 study seems to confirm.