Methylchloroisothiazolinone

Also referred to as MCI, or Kathon CG (when in an ingredient blend)

Used as a preservative when combined with methylisothiazolinone

Is a well-known skin sensitizer

Can only be used in leave-on products at 7.5 ppm (parts per million) and rinse-off products at 15 ppm

Methylchloroisothiazolinone, AKA MCI, is an organic compound used in skin care and cosmetics as a preservative when combined with methylisothiazolinone (MI). It’s a well-known skin sensitizer.

The combination of these two ingredients often goes by the trade name Kathon CG (among others). When combined with methylisothiazolinone, methylchloroisothiazolinone helps keep cosmetics free of bacteria, fungi and yeast. This blend is used in many products instead of parabens, despite the fact that parabens have a better safety track record and lower incidence of causing a sensitizing reaction.

Although preservatives are integral to skin care and cosmetic formulations (who wants to apply a serum chock full of bacteria to their face?!), there are many other less sensitizing preservatives than methylisothiazolinone and methylchloroisothiazolinone that can help keep your cosmetics free of microorganisms.

Introduced into cosmetics in the mid-1970s, MCI elicited a great number of sensitizations in consumers in leave-on products during the 1980s and 1990s. This led to the Cosmetic Ingredient Review Expert Panel providing guidelines for its usage when combined with MI in leave-on and rinse-off products. They state, “concentrations [of MCI/mi] shouldn’t exceed 7.5 ppm [parts per million] in leave-on products or 15 ppm in rinse-off products.”

Overall, MCI isn’t worth it! Look for less sensitizing preservatives in your cosmetics, particularly any such products meant for use around the eyes or on lips.

See: Methylisothiazolinone