Sensitization, and why it does not reverse

The regulation describes it precisely

The Federal Hazardous Substances Act defines a strong sensitizer as a substance which will cause, on normal living tissue through an allergic or photodynamic process, a hypersensitivity which becomes evident on reapplication of the same substance. Read it twice: the harm is not the first exposure, it is what the first exposure sets up for the next one.

What that means in a workshop

Every contact with uncured resin or hardener adds to a total. For a long time nothing happens. Then at some point the immune system responds, and from then on it responds every time, at doses that previously did nothing. The threshold is individual and unknowable in advance.

It does not go away

There is no desensitizing, no building tolerance, no working through it. Once sensitized, a person reacts to exposures far below what they handled for years. For somebody whose living is this craft that is the end of it, and it is the normal reason experienced makers stop.

Which is why protection is not proportional to risk felt

Nothing hurts at the time. There is no sting, no smell that warns you, no feedback of any kind. The craft gives you no signal that you are accumulating exposure, so the discipline has to come from understanding rather than from sensation.

The hardener usually deserves more respect than the resin

Amine hardeners are frequently the more aggressive half, both as sensitizers and as skin irritants, and they are the part people measure casually because it is the smaller volume. Check section 2 of the data sheet for each component rather than treating the kit as one material.

Sanding uncured or partly cured work is an exposure too

Dust from a piece that has not fully cured carries reactive material into the air and onto skin. Full cure before sanding, and dust extraction when you do, is part of the same control.

References