What Is Methylchloroisothiazolinone (CMIT/MI)?
In plain English: Methylchloroisothiazolinone is a synthetic preservative, usually blended with methylisothiazolinone in a mix known as Kathon CG, used to keep water-based cleaners, detergents, and personal care products from spoiling. It is a well-documented cause of preservative skin allergy.
Also listed as: MCI, CMIT, MCI/MI, Kathon CG, methylchloroisothiazolinone/methylisothiazolinone
The honest science
Methylchloroisothiazolinone (MCI) is rarely used alone. It is almost always paired with methylisothiazolinone in a roughly 3-to-1 blend marketed as Kathon CG, one of the most widely used preservative systems in cosmetic, household, and industrial products 1.
The MCI/MI mix is a recognized skin sensitizer. After formaldehyde-releasing preservatives, it has been described as the second most common cause of preservative contact allergy in Europe 1. Because of that, in 2014 the EU's Scientific Committee on Consumer Safety supported restricting the MCI/MI mix in leave-on products such as body creams, while still allowing limited use in rinse-off products like shampoos at very low concentration 1.
Honestly framed, the issue here is allergy, not acute poisoning at the trace levels used in finished products. The mix is a strong contact sensitizer, so the people most affected are those who have already become allergic through repeated exposure 2. If a cleaner leaves your hands red or itchy, MCI/MI is one of the ingredients worth ruling out, alongside its relatives MI and benzisothiazolinone 2.
Where you'll find it
- dish soap
- all-purpose cleaners
- laundry detergent
- shampoos and shower gels
- industrial and janitorial products
The safer-swap angle: People sensitized to isothiazolinones do best with formulas that skip them altogether. Ecolosophy's small-batch, plant-based concentrate is made with care and uses no MCI/MI blend.
Frequently asked questions
What is Kathon CG?
Kathon CG is a common trade name for the methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) preservative blend. When you see any of those names, you are looking at the same isothiazolinone family.
Is MCI/MI banned?
It is not banned outright, but the EU restricted the mix in leave-on products in 2014 and limits it to low levels in rinse-off products, specifically to reduce skin-allergy cases. Rules differ by country.
Can it cause a rash the first time I use it?
Usually not. Allergic contact dermatitis develops after your immune system becomes sensitized through repeated exposure. Once sensitized, even small amounts can trigger a reaction.
Sources
- Update on Allergic Contact Dermatitis Due to Methylchloroisothiazolinone/Methylisothiazolinone and Methylisothiazolinone — Actas Dermo-Sifiliograficas (ScienceDirect)
- Methylchloroisothiazolinone (MCI) — British Society for Cutaneous Allergy
Ingredient safety data changes as new research is published, and product formulas change over time. Always read the current label and check primary sources.
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