INCI name
Limonene
Also listed as: A fragrance allergen, often from citrus peel
Limonene is a terpene found in the peel of oranges, lemons and other citrus fruit. The smell is the familiar citrus-peel note. Like linalool, it is both a natural constituent of essential oils and a material that can be added on its own, and it must be named on EU and UK packs above a set threshold.
Limonene oxidises in air, and the oxidation products are a known cause of fragrance contact allergy. Citrus oils left in a warm bathroom for months are more likely to have oxidised than a fresh bottle.