title: α-Terpinene, an antioxidant in tea tree oil, autoxidizes rapidly to skin allergens on air exposure.
abstract: The monoterpene α-terpinene is used as a fragrance compound and is present in different essential oils. It is one of the components responsible for the antioxidant activity of tea tree oil. α-Terpinene is structurally similar to other monoterpenes, e.g., limonene, known to autoxidize on air exposure and form allergenic compounds. The aim of the present study was to investigate the possible autoxidation of α-terpinene at room temperature. To investigate the sensitization potency of air-exposed α-terpinene and the oxidation products formed, the murine local lymph node assay was used. Chemical analysis showed that α-terpinene degrades rapidly, forming allylic epoxides and p-cymene as the major oxidation products and also hydrogen peroxide. Thus, the oxidation pathway differs compared to that of, e.g., limonene, which forms highly allergenic hydroperoxides as the primary oxidation products on autoxidation. The sensitization potency of α-terpinene was increased after air-exposure. The allylic epoxides and a fraction, in which only an α,β-unsaturated aldehyde could be identified, were shown to be strong sensitizers in the local lymph node assay. Thus, we consider them to be the major contributors to the increased sensitization potency of the autoxidized mixture. We also investigated the presence of α-terpinene and its oxidation products in four different tea tree oil samples of various ages. α-Terpinene and its oxidation products were identified in all of the tea tree oil samples. Thus, from a technical perspective, α-terpinene is a true antioxidant since it autoxidizes rapidly compared with many other compounds, preventing these from degradation. However, as it easily autoxidizes to form allergens, its suitability can be questioned when used in products for topical applications, e.g., in tea tree oil but also in cosmetics and skin care products.
authors.affiliation: Department of Chemistry and Molecular Biology, Dermatochemistry and Skin Allergy, University of Gothenburg, SE-412 96 Gothenburg, Sweden.
authors.forename: Johanna
authors.initials: J
authors.lastname: Rudbäck
authors.forename: Moa Andresen
authors.initials: MA
authors.lastname: Bergström
authors.forename: Anna
authors.initials: A
authors.lastname: Börje
authors.forename: Ulrika
authors.initials: U
authors.lastname: Nilsson
authors.forename: Ann-Therese
authors.initials: AT
authors.lastname: Karlberg
mesh_terms: D000388:Air; D000485:Allergens / Q000737:chemistry* / Q000633:toxicity; D000818:Animals; D000975:Antioxidants / Q000737:chemistry* / Q000633:toxicity; D000081005:Cyclohexane Monoterpenes; D005260:Female; D006241:Haptens / Q000737:chemistry* / Q000633:toxicity; D008198:Lymph Nodes / Q000187:drug effects / Q000276:immunology; D051379:Mice; D008808:Mice, Inbred CBA; D039821:Monoterpenes / Q000737:chemistry* / Q000633:toxicity; D010084:Oxidation-Reduction; D012867:Skin / Q000187:drug effects / Q000276:immunology; D020947:Tea Tree Oil / Q000737:chemistry
chemical_list: D000485:Allergens; D000975:Antioxidants; D000081005:Cyclohexane Monoterpenes; D006241:Haptens; D039821:Monoterpenes; C018669:gamma-terpinene; D020947:Tea Tree Oil