What it is
Limonene is a colorless liquid hydrocarbon classified as a cyclic monoterpene, and it is the main fragrant component of citrus peel. In cosmetics it is used chiefly as a fragrance ingredient, contributing a fresh citrus note, and it also has solvent properties. It exists as two mirror-image forms: d-limonene (the R-enantiomer), the predominant natural form with a sweet citrus odor, and l-limonene (the S-enantiomer), which has a more piney, turpentine-like smell. Limonene oxidizes readily when exposed to air, and its oxidation products are what make it relevant from a skin-sensitization standpoint.
Safety & warnings
Limonene is one of the fragrance allergens that must be declared on cosmetic labels in the EU when present above set thresholds. It is considered a prehapten: in its fresh state it is a low sensitizer, but on exposure to air it autoxidizes and forms hydroperoxides that are the major allergens. Patch-test data identify oxidized limonene as a common contact allergen of high concern, so people with fragrance-sensitive or eczema-prone skin, and those reacting to citrus-scented products, may wish to avoid it; older or oxidized products carry more risk.
Origin & sourcing
Limonene occurs naturally in the peels of citrus fruits, where it contributes to the characteristic odor of orange peel and orange juice. Commercially it is obtained from citrus peel, largely as a byproduct of juice production, using methods such as centrifugal separation and steam distillation. The name derives from the Italian word limone, meaning lemon.
Questions everyone asks
Why is limonene in sunscreen?
It is used mainly as a fragrance ingredient to give a fresh citrus scent, and it can also act as a solvent. It is not a UV filter.
Is limonene an allergen?
It can be. Fresh limonene is a weak sensitizer, but on air exposure it oxidizes into hydroperoxides that are potent contact allergens, which is why it must be label-declared in the EU above set thresholds.
Is limonene natural?
Yes, it occurs naturally in citrus peel and is commercially obtained from citrus, though synthetic and purified grades are also used.