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Methylchloroisothiazolinone (and) methylisothiazolinone

(MCl/MI ; Kathon CG, Euxyl K100)

The isothiazolinones are, perhaps the most potent allergens on the consumer market. They are very effective preservatives. However, under suitable dilution and exposure conditions, even the most potent allergen can be used safely.

The commercial product (Kathon CG) contains only 1.5% of the active ingredients (the ratio of which is 3:1) with the remainder being water and magnesium salts that stabilise the system. Methylchloroisothiazolinone is the more potent allergen of the active components.

The patch test concentration of the actives is 100ppm in water.

The preservative was responsible for an epidemic of contact sensitivity, in some geographical areas, in the 1980s and early 1990s. Problems occurred mostly when the preservative was used in leave-on products at concentrations higher than 15ppm. It is now mainly used in rinse-off products at lower levels (up to 15ppm but usually <10ppm). At these lower levels and use conditions, the prevalence of hypersensitivity is acceptably low and individuals allergic to the preservative may tolerate rinse off products (eg shampoos) preserved with it.

The patent on the original system is expiring. New systems may be introduced that are stabilised with other salts (copper) or contain the less allergenic fraction methylisothiazolinone.


Methylchloroisothiazolinone (and) methylisothiazolinone is the INCI name for the preservative and this is the name that must appear on cosmetic ingredient listing in the European and USA markets if a product contains it. Therefore, it is easy to determine consumer exposure (qualitative) to the preservative from use of cosmetic products. The absence of any meaningful ingredient listing on household products means that qualitative assessment of exposure from such sources cannot be made.


As the preservative is now essentially used only in rinse off products, any dermatitis with relevance to contact allergy to it will affect those areas mostly exposed to rinse off products: head, and hands. Labelling permits qualitative assessment of exposure to some product types that may contain it. There is no easy mechanism to assess quantitative exposure; it is exposure to an allergen in dose per unit area that is of pivotal importance for induction and elicitation. Because of this, for an individual allergic to the preservative, it is most practical to simply avoid using products identified as containing the preservative from the label. Levels of the allergen in a product may be too low to be able to elicit a reaction but the level will not be known.