First, the bad news.
"I check the BeijingAir Twitter every time I'm headed there for work. I thought I'd report an anecdote from a friend who has worked in China since the 1970s and lived there for many years (though moved back partly to raise children in a more healthy environment!). She had MRIs performed on her lungs some time ago and they indicated significant scarring and other damage, despite the fact that she has never been a smoker. She has never complained of any symptoms or health problems but clearly some damage was done."
FWIW, I heard similar stories from a variety of people who had been in and out of China since the 1980s, but I don't know of any systematic data. Maybe I'll have another data point two weeks from now, when my appointment with my own doctor for a welcome-home physical exam finally rolls around. Only has taken three months to get on his schedule! Good thing we don't have Canadian-style socialized medicine in this country, what with its long waiting lists and rationing-by-delay etc.
Now, the better news:
"We were back in China for a couple of weeks this past summer to visit my former students in Beijing and then to travel in Hunan for a week or so. I think the air has improved. It was mostly blue skies, even in Beijing, which I rarely saw when we lived there for 10 months in 2003-04. I think you are right to conclude that expats do get over the problems once they leave. At least we haven't had lasting health problems -- at least not yet."
As a side note, based on my experience anyone who wants to visit Beijing in particular should go in October. Even though the current BeijingAir Twitter reading is deep into the "unhealthy" zone, this seems reliably the nicest time of the year.