Methylchloroisothiazolinone (MCI)
Also known as: MCI, CMIT, 5-chloro-2-methyl-4-isothiazolin-3-one, Kathon CG (MCI/MI blend)
A potent synthetic biocide and preservative, almost always used in combination with methylisothiazolinone (MI) as the blend 'Kathon CG.' MCI is an even more potent sensitizer than MI alone and is one of the most common causes of occupational contact dermatitis.
1 = low concern, 10 = avoid
Risk by Usage Frequency
How risk changes depending on how often you use products containing Methylchloroisothiazolinone (MCI).
Even a single exposure can trigger lifelong sensitization in susceptible individuals.
Daily exposure dramatically increases risk of developing contact allergy. Strongly avoid.
Unacceptable risk. High probability of sensitization and chronic dermatitis.
Health Risks
One of the most potent contact sensitizers in cosmetics. Causes severe allergic contact dermatitis even at very low concentrations.
Contact Dermatitis, 2014 — MCI/MI sensitization epidemic in Europe
Neurotoxic effects documented in in vitro studies at concentrations used in cosmetics.
Neurotoxicology, 2006
Can cause chemical burns and severe skin reactions in sensitized individuals on re-exposure.
Cross-sensitization with MI means individuals allergic to one are often allergic to both.
Global Regulatory Status
How methylchloroisothiazolinone (mci) is regulated in cosmetics and personal care products around the world.
90% of countries with data ban or restrict this ingredient
Details
Permitted only in rinse-off at max 0.0015% as 3:1 MCI/MI mixture; banned in leave-on since 2016.
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Mirrors EU; rinse-off only at max 0.0015%.
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Rinse-off only at max 0.0015% as MCI/MI mixture.
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Permitted with limits under MHLW positive list.
Details
Banned in leave-on; max 0.0015% in rinse-off.
Details
Restricted; concentration limits apply.
Details
Rinse-off only at max 0.0015% per Safety Technical Standards.
Details
ANVISA: rinse-off only at max 0.0015%.
Details
Rinse-off only at max 0.0015%, aligned with EU.
Why Brands Use Methylchloroisothiazolinone (MCI)
Extremely effective broad-spectrum preservative at very low concentrations. The MCI/MI blend (Kathon CG) is one of the most cost-effective preservation systems available.
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Better alternatives exist. Brands choose methylchloroisothiazolinone (mci) because it's cheap and effective, but safer options like potassium sorbate, sodium benzoate, ethylhexylglycerin deliver similar results without the health concerns.
Methylchloroisothiazolinone (MCI) in Product Categories
Click a category to see every product containing methylchloroisothiazolinone (mci) in that category, with full ingredient breakdowns.
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Safe Alternatives
What Numbrrrz Uses Instead
Numbrrrz never uses isothiazolinone preservatives in any formulation. Our lip balms are preserved naturally by vitamin E, and our simple four-ingredient formula eliminates the need for synthetic antimicrobial preservatives.
