At a glance
| Chemical family | Isothiazolinone — chlorinated biocide preservative (component of Kathon CG at 3:1 with MIT) |
| CAS number | 26172-55-4 |
| Classification | Not IARC classified. Potent contact sensitizer (stronger than MIT alone). EU banned in leave-on cosmetics (2016). EU restricted to 15 ppm in rinse-off cosmetics. Contact Allergen of the Year (MIT, closely related, 2013) |
| Where you encounter it | Laundry detergents; household cleaning products; water-based paints; industrial cooling water; some rinse-off cosmetics (shampoo, body wash). Not permitted in leave-on cosmetics (EU) |
| Sleep micro-environment relevance | Residues from laundry detergent on sheets and pillowcases. Cleaning product residues on bedroom surfaces. Water-based paint on bedroom walls. Multiple indirect contact pathways during sleep |
Regulatory & certification status
| European Union | Banned in leave-on cosmetics (2016). Restricted to 15 ppm MCI/MIT (3:1) in rinse-off cosmetics. Authorised as a biocide under the Biocidal Products Regulation for in-can preservation. CLP Acute Tox. 3 (H301, H311, H331), Skin Sens. 1A (H317). Regulatory |
| United States | FDA permits in cosmetics without a specific concentration limit. CIR (Cosmetic Ingredient Review) assessed MCI/MIT. No federal ban on leave-on use, though the market has largely shifted away. Regulatory |
| Canada | Health Canada Cosmetic Ingredient Hotlist limits MCI/MIT to 15 ppm in rinse-off products. Not permitted in leave-on cosmetics. Regulatory |
| International | Not IARC classified. EU banned in leave-on cosmetics. One of the most potent contact allergens in consumer products worldwide. Regulatory |
What it is
Methylchloroisothiazolinone is a chlorinated isothiazolinone biocide developed by Rohm & Haas in the 1970s and marketed as Kathon CG (in combination with MIT at a 3:1 ratio). It is one of the most effective broad-spectrum preservatives available — active against bacteria, fungi, and algae at very low concentrations. However, this potency extends to skin sensitisation: MCI is one of the most potent contact allergens in common consumer products. The epidemic of isothiazolinone contact allergy that swept Europe in the 2010s was primarily driven by MIT in leave-on cosmetics, but MCI/MIT combinations in cleaning products and paints contributed. The EU banned MCI/MIT in leave-on cosmetics in 2016 and restricted the combination to 15 ppm in rinse-off products. MCI is not used alone in consumer products — it is always in the 3:1 combination with MIT.
Where it shows up in bedding
MCI enters the bedroom through multiple indirect pathways. Laundry detergents preserved with Kathon CG leave trace residues on sheets, pillowcases, and sleepwear. Cleaning products containing MCI/MIT used on nightstands, headboards, and floors deposit residues on touched surfaces. Water-based paints on bedroom walls may contain isothiazolinones as in-can preservatives. For MCI-sensitised individuals, these combined residue pathways can trigger or maintain contact dermatitis during the 6-8 hours of nightly skin contact with bedding. Switching to isothiazolinone-free laundry and cleaning products is the primary recommendation for sensitised patients.
Citations
- Schwensen, J.F. et al. (2017). Contact allergy to methylchloroisothiazolinone/methylisothiazolinone. Contact Dermatitis, 76(2): 67-75. Source Peer-reviewed
- EU Commission Regulation (EU) 2016/1198 amending the Cosmetics Regulation re: MCI/MIT. Source Regulatory
- ECHA. 5-Chloro-2-methyl-2H-isothiazol-3-one — Substance Information. Source Regulatory
Frequently asked questions
Can my laundry detergent cause a skin reaction from MCI?
Yes. If you are sensitised to MCI (or the MCI/MIT combination), trace residues from preserved laundry detergent on sheets, pillowcases, and clothing can trigger or maintain contact dermatitis. The reaction pattern — generalised or at contact sites with bedding — may not be immediately obvious as laundry-related. A dermatologist can diagnose MCI allergy through patch testing. Switching to an isothiazolinone-free detergent resolves the exposure.
What is Kathon CG?
Kathon CG is the trade name for the 3:1 mixture of methylchloroisothiazolinone (MCI) and methylisothiazolinone (MIT) developed by Rohm & Haas (now Dow). It is one of the most widely used preservative systems in water-based products — effective at very low concentrations against bacteria, fungi, and algae. The 'CG' stands for 'cosmetic grade.' Despite its effectiveness, the combination is a potent contact sensitizer, which has led to progressive regulatory restrictions.
Related compounds
Embr is a sleep environment company researching and addressing the chemistry of the bedroom. Research and product development in progress.
Last reviewed 2026-07-08. If you find a factual error, contact us.