Paraphenylenediamine Contact Allergy and Post‐Diagnostic Outcomes: A Single‐Centre Study From the United Kingdom

ABSTRACT Background Paraphenylenediamine (PPD) is a potent allergen, frequently responsible for allergic contact dermatitis (ACD) from hair dyes and temporary ‘black henna’ tattoos. Misleading ‘PPD‐free’ labelling may expose sensitised individuals to structurally related aromatic amines. Objectives To characterise clinical features, cross‐reactivity and outcomes of patients diagnosed with PPD ACD following diagnosis of PPD allergy, and to evaluate the accuracy and safety of ‘PPD‐free’ products marketed in the United Kingdom. Methods Retrospective study of PPD positivity between 2018 and 2025 at a university hospital was conducted. Demographic, clinical and exposure data were extracted from records, and outcomes post‐diagnosis were surveyed by telephone. Ingredient lists of permanent ‘PPD‐free’ hair dyes/tints identified through a structured online search were reviewed for cross‐reactive amines. Results Of 1306 patients tested for PPD over 7 years, 56 (4.3%) had a positive reaction. The mean age of affected patients was 49 years; 82% were female, and hair dye was the most frequently reported source of exposure (77%). Eight (14%) had occupational exposure. Among 37 tested to para‐toluenediamine (PTD), 10 (27%) were positive. Among 31 followed up, 24 (77%) reported remission after avoidance. Only 1 of 10 permanent ‘PPD‐free’ hair dyes/tints examined was found to be free of any potentially cross‐reactive amines. Conclusions PPD remains a prevalent allergen causing chronic and occupational ACD. ‘PPD‐free’ products frequently contain cross‐reactive amines, highlighting the need for improved labelling and careful counselling.