Ten Years of Patch Testing to Hydroperoxides of Limonene and Linalool: Prevalence, Patterns and Clinical Insights

Background: Limonene and linalool are widely used fragrances in cosmetic and household products. Upon air exposure, they autoxidise into potent sensitisers – hydroperoxides of limonene (Lim-OOH) and linalool (Lin-OOH). Objectives: The objective of this study is to investigate patch test reactions to Lim-OOH and Lin-OOH, and associated characteristics, reaction patterns, including doubtful reactions and co-reactivity with other fragrance allergens. Method: A retrospective analysis was conducted using registry data from 6719 consecutively patch-tested patients at Gentofte Hospital, Denmark (2014–2025). Doubtful reactions were clinically subclassified as erythematous, only (E) or infiltrated (I). Results: The prevalence of positive patch test reactions was 1.6% for Lim-OOH and 3.1% for Lin-OOH. Positive Lim-OOH reactions were less likely in men and in patients with atopic, occupational or hand dermatitis, whilst positive Lin-OOH reactions were more likely in those > 40 years and less in atopic dermatitis. Most reactions peaked on day 3/4 and were weakly positive. Doubtful I reactions more frequently progressed to positive than doubtful E. Co-reactivity was observed between Lim-OOH, Lin-OOH and other fragrance allergens. Lower test concentrations reduced diagnostic sensitivity. Conclusion: Co-reactivity with other fragrance allergens supports true allergenicity. The currently recommended patch test concentrations (0.3% Lim-OOH and 1.0% Lin-OOH) seem optimal and qualify for inclusion in the European baseline series.