Methylchloroisothiazolinone
Methylchloroisothiazolinone (MCI) is a preservative, usually blended with methylisothiazolinone as "MCI/MI" (Kathon CG). It is a potent contact sensitiser restricted to rinse-off products in the EU, so it is flagged for sensitive and eczema-prone skin.
MCI is almost always used together with MI in a fixed ratio to preserve rinse-off products such as shampoos and shower gels.
Because the MCI/MI mix is a strong sensitiser, EU Regulation 1223/2009 permits it only in rinse-off products at up to 15 ppm and bans it in leave-on products. People with a known isothiazolinone allergy, sensitive skin, or eczema should check for it.
Who this affects
Caution
- Sensitive skin & eczema: A potent preservative contact allergen; EU-restricted to rinse-off products and best avoided on reactive skin.
Frequently asked questions
MCI/MI is a preservative made of methylchloroisothiazolinone and methylisothiazolinone combined in a 3:1 ratio, best known commercially as Kathon CG. It is related to, but not the same as, methylisothiazolinone used alone — MCI/MI contains the additional chlorinated compound. Both are potent contact allergens used to protect water-based cosmetics from microbial growth.
MCI/MI is one of the most frequent preservative causes of allergic contact dermatitis, and the SCCS advised it was not safe in leave-on products. Commission Regulation (EU) No 1003/2014 therefore restricted the 3:1 mixture to rinse-off products only, at a maximum of 0.0015% (15 ppm), applicable from 16 July 2015.
Yes — MCI/MI is a strong, well-established contact allergen and one of the most common preservative causes of allergic contact dermatitis. It was recognised as a sensitiser decades before the later methylisothiazolinone epidemic. Allergy is diagnosed by patch testing, and the physical product label plus a dermatologist’s assessment are the authoritative guide.
In the EU, MCI/MI is permitted only in rinse-off products such as shampoos, shower gels and cleansers, at a maximum of 0.0015% (15 ppm) of the 3:1 mixture. It is banned in leave-on products like creams and lotions. It appears on labels as "Methylchloroisothiazolinone (and) Methylisothiazolinone."
People with a confirmed isothiazolinone allergy are generally advised to avoid MCI/MI entirely, as even permitted rinse-off levels can cause reactions in sensitised individuals. Those with sensitive or eczema-prone skin often avoid it as a precaution, but patch testing by a dermatologist is the authoritative way to confirm sensitisation. Note that being allergic to the MCI/MI mix does not always predict allergy to methylisothiazolinone alone, and vice versa.