Everything else on the declaration list is a defined substance with a single CAS number. Oakmoss is not: it is an extract of the lichen Evernia prunastri, a mixture whose composition varies with where the lichen grew and how it was processed. The declaration attaches to the extract as a whole.

The sensitisation concern is narrower than the material. Atranol and chloroatranol, two constituents present at low levels, account for most of the allergenic response, and the industry response has been to supply oakmoss absolutes purified to remove them rather than to abandon the material. A low-atranol oakmoss is still oakmoss for declaration purposes.

Treemoss, Evernia furfuracea, is handled separately at entry 92 with the same thresholds. The two are often used together and are frequently confused, but they are different lichens and different entries.

Both are foundational to chypre and fougère perfumery, which is why the restrictions on them are among the most discussed in the industry. A classic fragrance reformulated over the past two decades has very often been reformulated around these two entries.