I always explain my situation before making a reservation. Something like I am chemically sensitive and would want to ensure linens are washed in a "free and clear" detergent and that no fragrances are used in cleaning products.
Unfortunately, even after taking such precautions I often have hosts who say "of course," we never use fragrances. Then when we arrive we find dryer sheets in the closets, chemically saturated potpourri sachets in every drawer.
We end up bagging the stuff up and putting it in a faraway closet, going out and buying sheets from a 2nd hand store. (we bring our own towels) Then hosts downgrade us as uncooperative guests.
Yes, I wish AirBnB would create a checkbox category. "Pure Rooms," which are far from perfect at several hotel chains is at least a good start.