More than 1100 patients were tested with methylchloroisothiazolinone-methylisothiazoli-none, 100 ppm, in aqueous and petrolatum-based patch test materials from 1985 to 1987 by members of the North American Contact Dermatitis Group. Thirteen reactions to the aqueous materials and 10 to the petrolatum-based materials were observed. Irritant reactions were infrequent, and about half the reactions were deemed relevant. From 1984 to 1985, patch tests with this substance at a concentration of 250 ppm in petrolatum were conducted. Thirteen persons were identified as allergic, but three others were sensitized by the patch test procedure. Sensitization as not observed in tests with aqueous or petrolatum-based substance at a concentration of 100 ppm, and this concentration appears to be the best compromise between safety and sensitive detection of allergy. Use tests are helpful but not infallible as a guide in establishing relevance with methylchloroisothiazolinone-methyhsothiazolinone. Wash-off products are frequently well tolerated by patients with positive reactions to this substance.
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