In order to identify a compound as a causative agent of contact dermatitis, the following requirements should be met: 1) The patient shows positive reaction to the suspected product. 2) The product contains the candidate compound. And, 3) the patient also shows positive reaction to the candidate compound. However, it is not easy to meet these requirements without co-operations between dermatologists and chemists. Recently, we have experienced four patients of pigmented contact dermatitis from cotton flannel nightdresses, and attempted to make clear the causative agent of these dermatitis by the combination of patch testing to the patients and chemical analysis of the flannels. The results obtained could be summarized as follows: i) In patch testing, all of four patients showed positive reactions to the cotton flannel nightdresses they had worn. ii) By the chemical analyses of these flannels, it was found that these flannels contained large amounts of naphthol AS (2-hydroxy-3-naphthanilide), an azoic coupling component (contents: 2900–11700 µg/g). And, iii) the patients also showed positive reactions to naphthol AS in patch testing. Since the above results satisfied the requirements 1)–3), we could conclude that naphthol AS might be the causative agent of these dermatitis.
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