I think it depends on several factors.
How much of a problem is it for the afflicted person - convenience, comfort, or safety? How much of an imposition is it to other people? How controllable is it?
Take scents for an example.
Having a workplace policy of no perfume/cologne for general comfort is reasonable, I think - wearing perfume is a minor, optional thing, and many people dislike scents, even when it isn't a health issue. Plus, people are often really bad at judging how strong their own scent use is.
Asking people not to wear scent as a special case because someone in the office has allergy issues is also reasonable.
Asking people in an office to go completely scent free is much more unreasonable, because of the sheer prevalence of scented products and the fact that this also extends to people who share accommodation with the coworkers - we're talking changing your soap, shampoo, conditioner, deodorant, moisturizer, sunscreen, laundry detergent, fabric softener, hand sanitizer, acne cream, etc. That is no longer a minor change to accommodate someone, *and* it extends into their home life as well as work behaviour.
Expecting people on the bus to not wear scents is unrealistic because you don't have any control over their behaviour, or way to enforce it.