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METHYLCHLOROISOTHIAZOLINONE

Last update on Mar 19, 2026

Methylchloroisothiazolinone (MCI or CMIT) is a potent, broad-spectrum isothiazolinone preservative effective against bacteria, yeast, and mold. This clear to pale yellow liquid provides excellent preservation at very low concentrations. It has become highly restricted due to significant sensitization and allergy risks. It is now rarely used in leave-on products and is mostly limited to rinse-off formulations in some regions.

CAS Number: 26172-55-4Chem/IUPAC Name: 5-Chloro-2-methyl-2H-isothiazol-3-oneEINECS/ELINCS No: 247-500-7COSING REF No: 35333

Products with METHYLCHLOROISOTHIAZOLINONE

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BIOPOL® C 30

by Brenntag Beauty & Personal Care

INCI: METHYLCHLOROISOTHIAZOLINONE | METHYLISOTHIAZOLINONE | BENZYL ALCOHOL

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BIOPOL® C 20 N

by Brenntag Beauty & Personal Care

INCI: METHYLCHLOROISOTHIAZOLINONE | METHYLISOTHIAZOLINONE

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JAKILL KATHON

by Hangzhou Jarsin Chemical Technology

INCI: METHYLCHLOROISOTHIAZOLINONE | METHYLISOTHIAZOLINONE

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What is METHYLCHLOROISOTHIAZOLINONE used for?

Preservative / Antimicrobial: Protects water-based products from microbial contamination. Historically used in shampoos, conditioners, body washes, cleansers, wet wipes, and some leave-on products (now heavily restricted).
Preservative booster: Enhances efficacy of other preservation systems at trace levels. Occasionally still found in rinse-off products where low concentrations are permitted.
Cost-effective broad-spectrum protection: Offers strong protection against gram-positive/negative bacteria, fungi, and yeast in aqueous systems.

Origin

Methylchloroisothiazolinone is produced synthetically through chlorination and cyclization of N-methyl-3-mercaptopropionamide or related precursors. It is fully petrochemical-derived and manufactured to high purity standards for industrial and cosmetic applications. Commercial forms are usually supplied as aqueous solutions.

What does METHYLCHLOROISOTHIAZOLINONE do in a formulation?

PRESERVATIVE|

ANTIMICROBIAL|

SKIN PROTECTING

Safety Profile

Methylchloroisothiazolinone is a known strong sensitizer and one of the most frequent causes of allergic contact dermatitis in recent decades. It has a high hazard rating from EWG (typically 6-8) and is heavily restricted or banned in many regions:

EU: Banned in leave-on products since 2017; maximum 0.0015% (15 ppm) in rinse-off products (Annex V).
US: No federal ban, but CIR recommends avoiding in leave-on products >0.00075% and advises caution even in rinse-off.
Japan / Canada / many brands: Self-restricted or avoided in leave-on; rinse-off limits similar to EU.
Primary concern: Contact allergy, sensitization, and cross-reactivity with other isothiazolinones (MIT, BIT, OIT).
Not suitable for sensitive, damaged, or baby skin; many “clean beauty” and hypoallergenic brands have completely phased it out.

Patch testing is strongly recommended; many dermatologists advise complete avoidance in leave-on products.