Methylchloroisothiazolinone — answers to MCI (often paired with MI, sold together as Kathon CG)
Appears in 3 scanned products.
A synthetic preservative from the isothiazolinone family, almost always used in combination with a related compound, methylisothiazolinone. Together they are added in very small amounts to stop bacteria and fungi from growing in water-based products.
Why it’s used
It is an effective, broad-spectrum preservative that works at low concentrations, which made it popular in shampoos, conditioners, and other water-heavy formulas that would otherwise spoil.
What the evidence says
This is one of the better-documented cases of a cosmetic ingredient causing contact allergy. Rising rates of allergic skin reactions through the 2000s and 2010s led the EU Scientific Committee on Consumer Safety to review the evidence and recommend restrictions; the EU subsequently banned it from leave-on products entirely and capped its concentration in rinse-off products. The concern is sensitization and allergic contact dermatitis, not systemic toxicity.
Context
Risk is strongly tied to contact time: leave-on use (lotions, creams) carries meaningfully higher risk of sensitization than rinse-off use (shampoo, body wash), which is why regulators drew the line exactly there. Where still permitted, it is capped at low parts-per-million concentrations.
Educational information, not medical or safety advice. Consult a professional for allergies or medical concerns.