I’m not sure how heavily “clean air” weighs in peoples’ minds compared to other factors such as employment opportunities, housing costs, traffic congestion and the like. It’s not as if the air in Fairfax and Arlington, which rate F for ozone, is remotely as noxious as that of Beijing or Kolkota. Once air quality reaches a tolerable level, people may not place the same value on incremental gains. I don’t recall anyone ever citing air quality as a reason to live in one metro over another. Still, clean air is a factor, even if a small one.