Methylisothiazolinone, an emerging allergen in cosmetics?

A few cases on primary sensitization by, and occupational contact dermatitis from, methylisothiazolinone in paints and glues have been published. In cosmetics, methylisothiazoline (MI) is permitted in a concentration of 100 p.p.m., while 15 p.p.m. for the mixture of methylchloroisothiazolinone and methylisothiazoline (MCI/MI). To present cases of sensitization to, and allergic contact dermatitis from, cosmetic products containing methylisothiazolinone only. Seven patients with suspected contact dermatitis - six of them with (peri-)anal lesions and one with facial dermatitis - were patch tested with the baseline series, the own products exposed to, cosmetic ingredients, as well as with methylisothiazolinone 1000 p.p.m. and MCI/MI 200 p.p.m. The patients with anal lesions had become sensitized by wipes for intimate hygiene, and one patient with facial dermatitis by a make-up remover, all containing methylisothiazolinone only. Three out of seven cases would have been missed if only MCI/MI 100 p.p.m., as present in the baseline series, had been tested. The inclusion of methylisothiazolinone as a preservative in cosmetics might not represent the solution to the problem of allergic contact dermatitis from isothiazolinones, since it leads to primary sensitization.