Methylchloroisothiazolinone: The Common Preservative That Could Irritate Your Skin

By zoeysky // 2026-10-16

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Methylchloroisothiazolinone (MCI) is a preservative active against bacteria, yeast, and fungi, according to the U.S. Food and Drug Administration (FDA). It is used in the manufacture of water-based cosmetics and personal care products, as well as in industrial processes including paper coatings, detergents, paints, glue, and cutting oils [8]. Regulatory bodies in the United States, the European Union, and Canada have established concentration limits for MCI and its combination with methylisothiazolinone (MI), officials said. MCI is often combined with MI under the brand name Kathon CG, according to a 1996 study published in Environmental and Molecular Mutagenesis [1]. The compound prevents microbial growth in water-based formulations, extending product shelf life. Cosmetic uses have shifted primarily to rinse-off products at lower concentrations since the 1980s and 1990s, according to the FDA [8].

What Is MCI and Where Is It Used?

A 1996 study published in Environmental and Molecular Mutagenesis describes methylchloroisothiazolinone as a component of Kathon CG, a biocide mixture that also includes methylisothiazolinone [1]. The study reported that a variety of shampoos, conditioners, skincare lotions, and other cosmetic products contain the biocide [1]. According to the FDA, MCI is used in water-based cosmetics and personal care products [8]. Cosmetic applications of MCI have narrowed over time. Allergic reactions to the ingredient were mostly associated with leave-in products in the 1980s and 1990s, according to the FDA. Since MCI was largely removed from most cosmetic products and is now used primarily in rinse-off products at lower concentrations, rates of allergic and irritant reactions have declined, agency officials stated.

Regulatory Limits and International Restrictions

In the United States, MCI combined with MI is permitted in concentrations of up to 15 parts per million (ppm) in rinse-off products and 8 ppm in other cosmetics, according to the FDA. The Cosmetic Ingredient Review considers MCI acceptable for use in cosmetics, the agency stated [8]. In 2014, the European Commission Scientific Committee on Consumer Safety issued a voluntary ban on "the mixture of methylchloroisothiazolinone and methylisothiazolinone" in leave-on products such as body creams. The measure was aimed at reducing the risk of and the incidence of skin allergies. The preservative remains permitted 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 to MI [8]. Canada's Cosmetic Ingredient Hotlist permits MCI only in combination with MI, and only for rinse-off products. 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. MCI/MI is not permitted for leave-in products in Canada, officials said.

Side Effects and Allergic Reactions

According to the FDA, MCI is a standardized chemical allergen and can cause chemical burns at high concentrations [8]. Since MCI was largely removed from most cosmetic products and is now used primarily in rinse-off products at lower concentrations, rates of allergic and irritant reactions are lower, officials said. The rate of contact allergy is about 8%, according to the FDA. A 1996 study in Environmental and Molecular Mutagenesis reported that Kathon CG and Kathon 886 have been identified as sensitizing agents following occupational exposure and the use of cosmetics containing the biocide [1]. The study also found that methylchloroisothiazolinone tested positive for mutagenicity in the Salmonella typhimurium TA100 assay without metabolic activation, while methylisothiazolinone did not [1]. The International Agency for Research on Cancer does not list MCI as a known, probable, or possible human carcinogen, according to the FDA. According to the Environmental Working Group, these common preservatives are found in many liquid personal care products and have been linked to lung toxicity, allergic reactions, and possible neurotoxicity [2]. A 2018 study published in Toxicological Research found that the mixture of CMIT/MIT causes allergic contact dermatitis and is no longer permitted for use as a preservative in leave-on cosmetics [3]. DrugBank reports that MCI is a known dermatological sensitizer and allergen, with side effects including flaky or scaly skin, breakouts, redness, itchiness, and moderate irritation [4].

Labeling and Alternative Perspectives

Consumers can identify MCI on ingredient lists under names including 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, and methylchloroisothiazolinone, according to the FDA. Natural health advocates recommend that consumers choose natural personal care products to avoid synthetic preservatives such as MCI. Imelda Burke, author of "The Nature of Beauty," wrote that she chooses to avoid certain ingredients and that she seeks out minimally processed, organic and natural products, applying the same values to products she puts on her body as to foods she consumes [7]. She advised that natural skincare is often made locally in small batches with sustainability and the environment in mind, making them a great option for those who prefer products without harsh chemicals [7]. According to the FDA, methylchloroisothiazolinone, in high concentrations, can be a skin irritant and cause chemical burns. Because of this, many countries, including the United States, have restricted concentration levels of MCI/MI in products, officials said. Consumers seeking to avoid MCI can read product labels and look for the chemical under any of its listed names, or choose natural personal care alternatives, according to natural health advocates.

References

Mutagenicity of Cosmetic Products Containing Kathon by Acrobat 5.0 Paper Capture Plug-in for Windows. Environmental and Molecular Mutagenesis 28: 127-132 (1996)
Methylisothiazolinone and Methylchloroisothiazolinone. https://www.safecosmetics.org/chemicals/methylisothiazolinone/
Mixture Toxicity of Methylisothiazolinone and Propylene .... https://pmc.ncbi.nlm.nih.gov/articles/PMC6195885/
Methylchloroisothiazolinone. https://go.drugbank.com/drugs/DB14197
Alliance for Natural Health, September 08, 2022. ANH-USA.org
Beyond Soap by Sandy Skotnicki-2
The nature of beauty organic skincare botanical beauty rituals and clean cosmetics by Burke Imelda author
The Uses, Benefits, and Side Effects of Methylchloroisothiazolinone. https://www.healthline.com/health/methylchloroisothiazolinone

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