Patterns of Sensitization and Cross-Reactivity in Hair Dye Allergy: An 18-Year Tertiary-Center Experience from Greece

<u><b><i>Abstracts:</i></b></u> <u><b><i>Background:</i></b></u> Hair dye-related allergic contact dermatitis (ACD) is a common problem among both consumers and professionals. The main sensitizers are aromatic amines such as para-phenylenediamine (PPD) and toluene-2,5-diamine (TDA), which can also cross-react with chemically related compounds.<u><b><i>Objective:</i></b></u> To analyze patch-test results from patients with suspected hair dye-related ACD, focusing on the prevalence of sensitization to hair dye allergens, cross-reactivity patterns, and occupational differences.<u><b><i>Methods:</i></b></u> We retrospectively reviewed 666 patients patch-tested between 2005 and 2023 at the Patch Test Clinic of Andreas Sygros Hospital, Athens, Greece. All patients were tested with the European baseline and extended hairdresser series. Results were evaluated according to the International Contact Dermatitis Research Group and European Society of Contact Dermatitis criteria. Logistic regression was used to calculate odds ratios for co-sensitization, adjusted for sex and age.<u><b><i>Results:</i></b></u> Overall, 82.4% of patients reacted to at least 1 allergen, with a median of 3 positive reactions. PPD was the most frequent allergen (47.9%), followed by TDA (35.9%) and nickel (41.1%). Significant co-sensitization was observed between PPD and several chemically related compounds, including 4-aminophenol, 3-aminophenol, benzocaine, textile dyes, hydroquinone, and pyrogallol (<i>P</i> < 0.001). Similar associations were observed for TDA. Hairdressers were younger (mean 32.2 vs 40.7 years, <i>P</i> < 0.001) and had predominantly hand dermatitis (62.9%). Patients reacting to multiple hair dye components were significantly more likely to be sensitized to PPD. Co-sensitization between PPD and cocamidopropyl betaine likely reflected cosmetic co-exposure rather than structural cross-reactivity.<u><b><i>Conclusions:</i></b></u> PPD and TDA remain the dominant sensitizers in hair dye-related ACD, with strong cross-reactivity to aminophenols, benzocaine, and textile dyes. Extended patch testing is essential for accurate diagnosis, as 7% of para-amino-compound-sensitized patients would be missed by PPD testing alone. Occupational exposure and multi-allergen sensitization highlight the need for safer formulations and preventive strategies in hair dye use.