Isothiazolinone preservative as important contact allergen in cosmetics

Kathon CG (K-CG) containing as active ingredients 2-methyl-4-isothiazolin-3-one and its 5-chloro analogue, is a very effective and widely used preservative system for cosmetics and toiletries. Of 243 patients routinely patch tested because of suspected contact dermatitis 8 (3.3%) reacted to K-CG 100 ppm in water. All but 1 patient allergic to the preservative used cosmetics of 2 brands very widely distributed in the Netherlands, both containing K-CG. The observed positive patch test reactions were shown to be relevant in all these cases. Repeated open application tests were positive in 3/5 of the patients tested. The pertinent literature is reviewed. It is concluded that contact allergy to K-CG is common. Sensitization usually occurs from creams and lotions applied to damaged skin (irritant dermatitis, atopic dermatitis), but some patients become sensitized by cosmetic products used on healthy skin, especially on the face and around the eyes. The risk of induction and/or elicitation of contact dermatitis from the use of rinse-off products such as soap, shampoo and shower foam is small.