The air is very bad in most of China's major cities. When I've visited, I usually last about 2 weeks and then fly home.
However, if you want to skip the cities, I suggest visiting the Li River near the city of Guilin, but don't stay in Guilin.
There are river tours that start at Point A and end at Point B and Point B is a small town where the air is clean---the only industry is tourism. They ban all cars on that town's central streets. On the outskirts of the town there are streets that allow traffic but it was never as bad as it gets in China's major cities---at least when I was there in 2008.
To give you an idea of the river Cruise:
http://ilookchina.net/2013/06/27/li-river-cruise-2008/
And here area a few photographs of the town I'm talking about.
WARNING. If you visit my post by clicking the next link, do NOT click any of the links embedded in the actual post. I decided to check them to see if any of them were inactive/broken, and one of them, that was okay when I wrote the post, has become corrupted. My computer through my Google Chrome link is going crazy. I'm sending this updated comment through my Firefox link.
http://ilookchina.net/2013/03/05/impressions-of-liu-sanjie-2/