The optimal patch test concentration for ascaridole as a sensitizing component of tea tree oil

BACKGROUND: Tea tree oil is used as a natural remedy, but is also a popular ingredient in household and cosmetic products. Oxidation of tea tree oil results in degradation products, such as ascaridole, which may cause allergic contact dermatitis. OBJECTIVES: To identify the optimal patch test concentration for ascaridole, and to investigate the relationship between a positive reaction to ascaridole and a positive reaction to oxidized tea tree oil. PATIENTS/MATERIALS/METHODS: Three hundred and nineteen patients with eczema were patch tested with ascaridole 1%, 2%, and 5%, and 250 patients were patch tested with oxidized tea tree oil 5%. Readings were performed on D3 and D7 according to a patch test calibration protocol. RESULTS: With an increasing ascaridole test concentration, the frequency of positive reactions increased: ascaridole 1%, 1.4%; ascaridole 2%, 5.5%; and ascaridole 5%, 7.2%. However, the frequencies of irritant and doubtful reactions also increased, especially for ascaridole 5%. A positive reaction to ascaridole was related to a positive reaction to tea tree oil. CONCLUSIONS: This study is in support of ascaridole being a sensitizer. We recommend patch testing with ascaridole at 2%. The finding that every positive reaction to oxidized tea tree oil is accompanied by a positive reaction to ascaridole suggests that ascaridole might be a contact allergen in oxidized tea tree oil.