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Methylchloroisothiazolinone in skincare
Potent isothiazolinone preservative; EU-banned in leave-on products; rinse-off only at ≤15 ppm; more sensitizing than MI alone.
EU-banned in leave-on cosmetics; permitted in rinse-off products (shampoos, shower gels) only at ≤0.0015% as part of the MCI/MI 3:1 mixture. Do not use in any leave-on product. In rinse-off products, minimize to lowest effective concentration. Individuals sensitized to MCI/MI will react to trace amounts. Always check ingredient lists.
What it is
Methylchloroisothiazolinone (MCI, CMIT, 5-Chloro-2-methyl-4-isothiazolin-3-one) is the chlorinated, more potent sibling of methylisothiazolinone, typically used in a 3:1 MCI:MI mixture (commercial product Kathon CG). MCI is considered more irritating and sensitizing than MI alone. Can cause chemical burns at high concentrations. EU banned the MCI/MI mixture from all leave-on cosmetics in 2014 (one year before MI alone was banned). Permitted only in rinse-off products at a maximum of 0.0015% (15 ppm) for the combined MCI/MI mixture. FDA: the MCI/MI mixture is a standardized chemical allergen; US allows up to 15 ppm in rinse-off and 7.5 ppm in other cosmetics. North American Contact Dermatitis Group found ~10.9% positive patch test reactions to MCI/MI, making it among the most clinically significant cosmetic allergens. Sensitization once established creates lifelong reactivity to trace amounts in any product.
The facts
| Category | Preservative (Isothiazolinone) |
| Typical use | Morning and evening |
| Concentration YARAI treats as a full dose | 0.0015% |
| EU maximum permitted concentration | 0.0015% (0.0015% (of a mixture in the ratio 3:1 of 5 -Chloro-2-methyl-isothiazol-3(2H)-one and 2-Methylisothiazol-3(2H)-one) |
| EU Cosmetics Regulation annex | V |
| EU status | restricted |
Why the amount matters, not just the presence
Most ingredient checkers tell you whether something is in the bottle. That is a smaller question than it sounds. Methylchloroisothiazolinone at 0.0015% and methylchloroisothiazolinone at a trace level are not the same ingredient in any way that affects your skin.
An ingredient list is ordered by quantity, so position carries real information. YARAI reads that position and scores accordingly, which is why the same ingredient can land differently in two different products.
The honest limit: a list gives order, not percentages. Estimating concentration from position is an estimate. It is a better estimate than ignoring position, and it is still an estimate. We would rather tell you that than let a number look more certain than it is.
Regulatory status
EU CosIng Annex V restricted substance. Max: 0.0015% (of a mixture in the ratio 3:1 of 5 -Chloro-2-methyl-isothiazol-3(2H)-one and 2-Methylisothiazol-3(2H)-one
Common questions
What does methylchloroisothiazolinone do in a skincare product?
Potent isothiazolinone preservative; EU-banned in leave-on products; rinse-off only at ≤15 ppm; more sensitizing than MI alone.
How much methylchloroisothiazolinone should a product contain?
YARAI treats 0.0015% as a full dose for this ingredient. Below that, the ingredient is present but is scored as contributing less. An ingredient list gives you the order of ingredients, not their percentages, so this is an estimate read from position rather than a measurement.
Is methylchloroisothiazolinone restricted in the EU?
The EU Cosmetics Regulation permits this ingredient up to 0.0015% in leave-on cosmetics, listed under V. 0.0015% (of a mixture in the ratio 3:1 of 5 -Chloro-2-methyl-isothiazol-3(2H)-one and 2-Methylisothiazol-3(2H)-one
When should I use methylchloroisothiazolinone?
EU-banned in leave-on cosmetics; permitted in rinse-off products (shampoos, shower gels) only at ≤0.0015% as part of the MCI/MI 3:1 mixture. Do not use in any leave-on product. In rinse-off products, minimize to lowest effective concentration. Individuals sensitized to MCI/MI will react to trace amounts. Always check ingredient lists.
Check your own products
Point your camera at a product. YARAI identifies it from the label, finds that product's full ingredient list from published sources, and scores it. No barcode and no box needed, which is how it handles imported and small-brand products.
Concern level, category and thresholds on this page are read directly from the same data the YARAI app scores with, so this page and the app cannot disagree. EU figures cite Regulation (EC) No 1223/2009. Last built 2026-09-09. Nothing here is medical advice.