Limonene

A component of essential oils, often used for fragrance, but can be a skin irritant and allergen. Limonene primarily functions as a fragrance ingredient in cosmetic formulations, imparting a citrus scent. While it may possess mild antioxidant properties by scavenging reactive oxygen species in vitro due to its terpene structure, its most significant interaction with skin relates to its role as a penetration enhancer for other cosmetic ingredients. Furthermore, upon air exposure, limonene readily oxidizes to form highly sensitizing hydroperoxides, which act as haptens to trigger type IV hypersensitivity reactions in susceptible individuals.

The CIR Expert Panel considers Limonene safe for use in cosmetics when formulated to prevent oxidation. However, its oxidized derivatives are potent contact sensitizers, making it a common cause of fragrance allergy, especially in leave-on products.

Look for products with 0.001-1% (typically much lower when added as a pure fragrance ingredient to minimize sensitization risk, often below 0.1%, but can be significantly higher as a natural component of essential oils like orange or lemon peel oil). for clinical efficacy.