Air oxidation increases skin irritation from fragrance terpenes

Background: Linalool and limonene are common fragrance terpenes that autoxidize on air exposure. The pure compounds are not allergenic but their oxidation products can cause contact allergy. Little has been investigated regarding the irritancy of oxidized terpenes. Aim: The aim of this study was to investigate the irritating effect of pure and oxidized R-limonene and linalool in concentration series and to study the MNIC (Maximum Non Irritant Concentration) of autoxidized linalool and limonene. Patients/methods: Patch testing was performed in dermatitis patients and controls with sequentially diluted concentrations of oxidized and non-oxidized linalool, and oxidized and non-oxidized R-limonene. Readings were made with visual assessment and using laser Doppler imaging. Results: The non-oxidized terpenes were non-irritating in all tested concentrations. Both linalool and especially R-limonene were more irritating after oxidation compared with the pure compounds. No difference in response was seen between dermatitis patients and controls. Conclusion: Autoxidation of the fragrance terpenes linalool and R-limonene increases irritation. Oxidized linalool is less irritating than oxidized R-limonene. In this study, we found no advantages in using laser Doppler technique compared with visual assessment.