Making the tickbox checked would require more from hosts than just clicking a box. They would be taken to an extra page that has a community-approved checklist that includes:
- no scented candles
- no plug-in or spray air fresheners, no "scent sticks"
- a positive list of cleaning, disinfection and laundry products, only products from that list can be used
- no dish soaps , except products from a positive list
- no toilet bowl freshener
- no soaps, shampoos etc. in the bathroom except possibly products from a positive list (almost all fragrance-sensitive people travel with their own body care products)
The page would ask hosts to commit to playing by these rules, and further educate them that the issue is of a medical nature, and that violating these rules can cause disabling symptoms to guests.
No, it wouldn't be perfect. Properties could still have smells from previous guests. But let's not make the perfect the enemy of the good. The checkbox, with detailed information and education attached as I describe here, would be a HUGE improvement for fragrance free travelers.
Hosts who find all this too burdensome, to unworkable, would be free to simply not activate the box.