Resorcinol: a strong sensitizer but a rare contact allergen in the clinic.

Resorcinol is a simple aromatic chemical (1,3benzenediol) that has widespread use as a coupler in hair dyes and food, for example as an anti-browning agent in crustaceans (1), and, because of its antipruriginous, keratolytic and antifungal properties, in topical pharmaceutical products, such as Castellani paint. Moreover, it is present in certain skin markers. According to the local lymph node assay in mice, resorcinol has an EC3 value of 5.5% (2), and was classified as a moderate sensitizer, whereas, according to the Scientific Committee on Consumer Safety, it has an EC3 value of 1.4%, and was classified as a strong sensitizer (1). Here, we report our experience with patch testing (January 1990 to November 2015) with resorcinol 1% in petrolatum (pet.) in 1187 selected subjects: (i) as a part of the hairdressers’ series, (ii) when it had been used topically, or (iii) to check for cross-reactivity. Only 5 positive patch test reactions were observed, and these cases will be discussed here.