Ford Expedition AC Odors

First of all, our model is a 2002 & it is not from mildew or mold, etc...
It is more of a chemical smell that actually burns your nose & eyes.
It doesn't happen all the time. Mostly when the AC is not on automatic & is
on recirculation air. The Service dept. has replaced the evap., evap case,
& seal.
This seemed to help for awhile, but now it is back. I've had it in the
garage 6 times
& they say they can't find anything wrong that would make this smell.
Any ideas on this? It really makes me sick to drive very long in it.
Carol