Other names: Chloromethylisothiazolinone; Chloromethylisothiazolone; Methylchloroisothiazolinone; Methylchloroisothiazolone;CMI; CMIT; MCI
Methylchloroisothiazolinone (5-chloro-2-methyl-4-isothiazolin-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
The International Agency for Research on Cancer (IARC), does not list methylchloroisothiazolinone as a known, probable or possible human carcinogen, nor have in vivo tests found evidence of carcinogenic activity. Methylchloroisothiazolinone is an allergen.
Methylisothiazolinone or MIT, sometimes erroneously called methylisothiazoline, is a powerful biocide and preservative within the group of isothiazolinones, used in shampoos and body care products. Though long considered safe for use in cosmetics, two recent in vitro studies have shown that MIT is neurotoxic, causing damage to rat brain cells in tissue culture. Long-term health and safety studies have been conducted on animals, and thus far there is no published evidence of nerve damage or neurological effects associated with MIT for consumers or workers. The two in vitro studies were published in peer-reviewed journals  , but the animal safety studies were not. Regulatory authorities in the USA, Japan and Europe and more than 25 other countries have all independently concluded the product is safe. Despite these claims, the studies published in scientific journals suggest that additional testing may be needed. Initially, a similar conclusion was reached by the European Scientific Committee on Cosmetic Products and Non-Food Products Intended for Consumers (SCCNFP) in 2003 (link below). However, in 2004, after receiving additional studies, committee said "The SCCNFP is of the opinion that the proposed use of Methylisothiazolinone as a preservative at a maximum concentration of 0.01% (100 ppm) in the finished cosmetic product does not pose a risk to the health of the consumer." The specific conclusions of this article are that: 1) requested data (by the Committee) provided on physico-chemical properties on methylisothiazolinone are complete, 2) the percutaneous absorption study (requested by the Committee) is inadequate. A 100% absorption (via the skin) is assumed, and 3) the in vivo UDS (unscheduled DNA synthesis) assay is adequate. Methylisothiazolinone is considered non genotoxic/mutagenic.
As such, the overall conclusion listed earlier only addresses the genotoxic and mutagenic potential of this compound under the specific circumstance at which it was tested. There is no other mention of any other form of toxicity. Finally, whether Methylisothiazolinone poses a risk via other forms of exposure or in occupational settings during the manufacture of products containing the biocide has yet to be determined.