Might help to change the cabin air filters. I'm not saying dirty/clogged filters would normally lead to that problem, but it couldn't hurt to change them. If they've never been changed before, you'll likely be shocked at how much crap they've collected.
Also, if you use an air-freshener in the car, best to use a solid type. Avoid the liquid ones which are oil-based and might makes things worse (plus aren't good for interior materials).