Possible exhaust leak? I know when my header was cracked I had to keep the vents closed otherwise I could smell the exhaust and I'd get to where ever I was going with a nasty headache and feeling like I was gonna vomit. **** cleared right up when I got the header fixed and now I can have all my vents open no problem.
First thing i always tell people that see steam/smoke or smell something coming from ac is feel front passenger floor and see if its wet and if it is see if it smells sweet. If it isnt then take it to the shop for a look or lift the car and check your exhaust for cracks.