Reaction to any chemical runs from zero to severe. The point at which one can notice a reaction is the point at which one can say they have an allergy! In my experience, using photographic chemicals daily for over 30 years, and working with thousands of people who did the same, we saw a very very tiny fraction of people who had any noticeable reaction at all. Very tiny.
In fact, most allergies that we heard of came from the home. Hair dyes were at the top IIRC, and Formalin next from new shirts and furniture. A person would be fine at work with formalin stabilizer but would become sensitized after contact with a new shirt, or after sleeping in a new bed. The formalin there did the trick after years of contact with the same chemical at work. But again, I stress that this was a very tiny minority.