A few articles popped up on the topic so I have a few more for you to look at: I noticed this article in Forbes that blames the problem on diesel engines. No doubt they're a contributing factor, but there are many more, such as heavy use of coal in China, desertification and sand storms, etc.
Then, on a related front, a few interesting papers have come out recently about the effects of air pollution, including this study that finds that burning natural gas saves children's lives by reducing air pollution, and another (by some friends of mine from school) that shows how damages caused by air pollution vary by context. (If those links don't open, just enter your towson.edu email address and they'll send you a link to the paper.)
