Limonene is not usually an ingredient someone chose. It is a major component of orange, lemon and bergamot oils, and of a great many other botanicals, so a formula containing a citrus oil contains limonene whether or not anyone intended it. That is why it appears on so many ingredient lists, and why a reader can encounter it in a product that describes itself as naturally fragranced.

The entry adds a condition that no other allergen in the set carries: the peroxide value of the substance must be less than 20 mmoles/L. Limonene oxidises on contact with air, and it is the oxidation products — hydroperoxides formed over time — that sensitise, more than limonene itself. The regulation therefore constrains the material's state rather than its quantity.

The entry is explicit that this limit applies to the substance and not to the finished cosmetic product, which makes it a raw-material specification question. It is answered by the supplier's certificate of analysis and by how the material is stored and handled, not by anything measurable in the tube at the end.