Methylchloroisothiazolinone

Methylchloroisothiazolinone (5-chloro-2-methyl-4-isothiazolinon-3-one) is a preservative with antibacterial and antifungal effects within the group of isothiazolinones. It is effective against gram-positive and gram-negative bacteria, yeast and fungi.

It is found in many water-based personal care products and cosmetics. It is also used in glue production, detergents, paints, fuels and other industrial processes. Methylchloroisothiazolinone is known by the registered tradename Kathon CG when used in combination with methylisothiazolinone.

It can be used in combination with alcohols and other long-chain organics in compounds such as methylchloroisothiazolinone ethylparaben benzalkonium chloride or methylchloroisothiazolinone 2-bromo-2-nitropropane-1,3-diol benzalkonium chloride.

It was first used in cosmetics in the 1970s. In high concentrations it can cause chemical burns and it is a skin and membrane irritant and so it was largely removed from most cosmetic products except for those with only short duration skin contact such as rinse-offs. Its inclusion in certain forms makes it more acceptable to sensitive users, so it can be found in cosmetic creams and lotions which require skin contact. In the US accepted concentrations are 15 ppm in rinse-offs and 8 ppm in other cosmetics.

Safety concerns
Like a number of other common cosmetics ingredients, methylchloroisothiazolinone has been considered to be a carcinogen. Methylchloroisothiazolinone is an allergen.