That's interesting, but that is a question that your doctor will be able to help you with much better.
I have a sneaking suspicion the two are connected in some way, though. I guess the question is, how long after the end of chronic exposure to inhaled air pollution do your lungs take to a) start rejecting the foreign matter and b) completely finish cleaning it out.
If your doc has any insights, I'd be very interested to see this thread bumped again.