In case you missed it, a New York Times article says U.S. Olympic athletes may be given masks while in Beijing to help them deal with the smog.
Someone please correct me if I'm wrong, but I don't recall any of this kind of talk in 1984 (LA hosted the Games that year), and as an LA native, my lungs remember the soupy, disgusting air in SoCal during the 1980s. In fact, Beijing air has never really bothered me after growing up in LA. I'm not saying LA was worse, but apparently my lungs gave up the fight long ago.
Maybe if those athletes trained in the San Gabriel Valley (East of LA), where I went to college, their lungs wouldn't care less what the air in Beijing was like. On the other hand, there would probably be a 1 in 3 chance of developing lung cancer or emphysema. It's all a matter of risk assessment.