Identification of contact allergens in C.I. Solvent Red 23 (commercial Sudan III) by chemical analysis and animal testing

C.I. Solvent Red 23, commercial Sudan III. is widely used m cosmetic products. Chemical analyses and guinea pig sensitization tests were carried out to identify its contact allergens. In the Magnusson & Kligman guinea pig maximization test. C.I. Solvent Red 23 showed 20% positive reactions. By conducting chemical analyses with HPIJC and GL.C. 2‐naphlhol (82 ppm). azobenzene (48 ppm). Sudan I(570 ppm) and many unknown impurities as well as the main constituent pigment Sudan ill (87%). were found. The chemical structure of one unknown impurity was identified as an isomer of sudan III. We found that purified Sudan III showed no positive reaction, while the isomer elicited 30% positive reactions, in the same guinea pig test. Furthermore cross‐sensitization with P‐phenylenediamine was investigated using the guinea pig test. Animals sensitized with P‐phenylenediamine also showed positive dictation reactions with purified Sudan III. From these results. the contact allergenicity of C.I. Solvent Red 23 is considered to be due to impurities, including the isomer of Sudan III, 1‐(o‐phenylazophenylazo)‐2‐naphlhol. Positive reactions to Sudan III previously demonstrated in hairdressers are due lo cross‐sensitivity with P‐phenylenediamine