Methylchloroisothiazolinone (MCI) is a preservative that is active against bacteria, yeast, and fungi. It’s used in the manufacture of water-based cosmetics and personal care products. It’s also used in industrial processes, including the manufacture of:

  • paper coatings
  • detergents
  • paints
  • glue
  • cutting oils

According to the U.S. Food and Drug Administration (FDA), methylchloroisothiazolinone is a Standardized Chemical Allergen.

In high concentrations, MCI can cause chemical burns, and it’s a skin and membrane irritant.

As an ingredient in cosmetics, MCI has been associated with allergic reactions. Those reactions were mostly related to leave-in products in the 1980s and 1990s. It has since been largely removed from the majority of cosmetic products and is now used primarily in rinse-off products in much lower concentrations. Since these changes, the rates of allergic and irritant reactions are lower. The rate of contact allergy is about 8 percent.

MCI is often combined with methylisothiazolinone (MI) under the brand name Kathon CG.

In the United States, it’s currently used in concentrations of up to 15 parts per million (ppm) in rinse-off products and 8 ppm in other cosmetics. It’s considered acceptable for use in cosmetics by Cosmetic Ingredient Review (CIR).

In 2014, the European Commission Scientific Committee on Consumer Safety issued a voluntary ban on “the mixture of Methylchloroisothiazolinone (and) Methylisothiazolinone (MCI/MI) from leave-on products such as body creams. The measure is aimed at reducing the risk from and the incidence of skin allergies. The preservative can still be used in rinse-off products such as shampoos and shower gels at a maximum concentration of 0.0015 percent of a mixture in the ratio 3:1 of MCI/MI.”

According to the Canadian government Cosmetic Ingredient Hotlist, MCI is only permitted in combination with MI. If the MCI/MI combination is used in a formulation with MI alone, the total cumulative concentration of MCI/MI is not allowed to exceed 0.0015 percent. In Canada, MCI/MI is permitted for rinse-off products and not permitted for leave-in products.

Methylchloroisothiazolinone is not listed by The International Agency for Research on Cancer (IARC) as a known, probable, or possible human carcinogen.

Although it can be used alone, methylchloroisothiazolinone is often used with methylisothiazolinone (MI). Read the ingredients list on the product label and look for any of the following:

  • 5-chloro-2-methyl-4-isothiazolin-3-one
  • 5-chloro-2-methyl-4-isothiazolin-3-one hydrochloride
  • 5-chloro-2-methylisothiazolin-3-one
  • 5-chloro-N-methylisothiazolone
  • Kathon CG 5243
  • methylchloro-isothiazolinone
  • methylchloroisothiazolinone

Methylchloroisothiazolinone (MCI), especially when paired with methylisothiazolinone (MI), is an effective preservative. In high concentrations it can be a skin irritant and even cause chemical burns. Because of this, many countries — including the United States — have restricted the concentration levels of MCI/MI in products.