Ever hear of the Asian brown cloud? Yeah, it's a huge cloud of nasty stuff hovering over Asia, specifically the China area.
In fact, I recall hearing that breathing the air in China is like smoking two packs of cigarettes a day.
But that's what you get when you rush towards industrialization without caring about the environment. That and all the new cars the Chinese are driving because their economy is booming.
They're also building a huge dam and flooding a lot of land...in order to do that they have to forcibly remove people(and they don't get any money for it, they just have to move). It's called the Three Gorges Dam.
In fact, when I lived in Denver, they put out pollution warnings for people with asthma, or breathing problems, and tell them they should stay inside that day and not go out.
Sounds like the same problem as Mexico City. Mexico City, like Denver is pretty high up on the ol' sea level and in a rather mountainous region. The air is thin, so the pollution spreads quickly(also, more carbon monoxide is produced because of the lack of oxygen) and because there are mountains it can't exactly blow away very effectively.
Kudos to my geography professor from last semester for teaching us all this stuff.