Methylchloroisothiazolinone
What It Is
Methylchloroisothiazolinone (MCI, also abbreviated CMIT) is a synthetic preservative in the isothiazolinone family (CAS 26172-55-4). It is a chlorinated derivative of methylisothiazolinone (MIT), with the addition of a chlorine atom on the isothiazolinone ring. MCI is almost always used in a 3:1 ratio with MIT, marketed under the trade name Kathon CG. This combination has been one of the most widely used industrial and cosmetic preservative systems since the 1980s. MCI functions as a broad-spectrum antimicrobial preservative.
Common Uses
The MCI/MIT combination is used in shampoos, conditioners, body washes, liquid hand soaps, household cleaners, laundry detergents, paints, adhesives, and industrial cooling water systems. MCI is rarely used as a standalone preservative -- the 3:1 MCI/MIT blend is the standard commercial form. Its use in cosmetics has been increasingly restricted in the EU and voluntarily reduced by many global manufacturers due to allergenicity concerns.
How It Works
MCI works through the same thiol-reactive mechanism as MIT -- it inactivates essential microbial enzymes by forming covalent bonds with cysteine residues. The chlorine atom on MCI's ring makes it substantially more electrophilic (reactive) than MIT, which increases both its antimicrobial potency and its chemical reactivity with biological tissues. The 3:1 MCI/MIT blend is synergistic: the combination is more effective than either component alone at equivalent total concentrations.
Safety and Regulation
MCI is a strong contact allergen -- stronger than MIT alone. The MCI/MIT combination has been a top-10 allergen in dermatological patch-testing panels for decades (de Groot and Frosch, 1990). The EU restricts the MCI/MIT combination to 15 ppm (0.0015%) in rinse-off cosmetic products and prohibits it entirely in leave-on cosmetic products (Commission Regulation (EC) No 1223/2009, Annex V). MCI is not permitted as a standalone cosmetic ingredient in the EU.
In the United States, the FDA does not restrict MCI/MIT in cosmetics, though the CIR Expert Panel concluded the combination is safe at up to 15 ppm in rinse-off products and recommended against use in leave-on products (CIR, 1992). In household cleaning products, higher concentrations are permitted by the EPA.
The allergenicity of MCI is well established. Clinical patch-testing data consistently show that the MCI/MIT combination produces positive reactions in 2%-4% of consecutively tested patients in dermatology clinics (Geier et al., 2012).
Why Natural Flower Power Does Not Use It
Natural Flower Power does not use methylchloroisothiazolinone or the MCI/MIT combination in any product.
MCI is the most allergenic preservative in the isothiazolinone family. While it is effective at very low concentrations, its sensitization rate makes it a poor fit for products designed for repeated skin contact like hand soaps and dish soaps. We use benzisothiazolinone (BIT) instead, which shares the isothiazolinone core chemistry but has a substantially lower sensitization profile. The choice between MCI/MIT and BIT was one of the earliest preservation decisions we made, and the wave of MCI/MIT allergy that continues to show up in dermatology clinics has confirmed it was the right call.
Related Ingredients
Methylisothiazolinone (MIT) is the non-chlorinated isothiazolinone that MCI is always combined with in commercial use. Benzisothiazolinone (BIT) is the isothiazolinone preservative Natural Flower Power uses, with a different ring structure and lower allergenicity. Phenoxyethanol is a non-isothiazolinone preservative alternative. Methylparaben is a paraben-family preservative that provides broad-spectrum preservation through a different mechanism.
Sources
- de Groot, A.C., and Frosch, P.J. "Adverse Reactions to Fragrances: A Clinical Review." Contact Dermatitis, vol. 36, no. 2, 1990, pp. 57-86.
- Geier, J., et al. "Contact Sensitization to Methylisothiazolinone and Methylchloroisothiazolinone/Methylisothiazolinone." Contact Dermatitis, vol. 67, no. 6, 2012, pp. 334-341.
- Cosmetic Ingredient Review (CIR). "Final Report on the Safety Assessment of Methylisothiazolinone/Methylchloroisothiazolinone." Journal of the American College of Toxicology, vol. 11, no. 1, 1992.
