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CAS number 26172-55-4 YesY
PubChem 33344
ChemSpider 30800 YesY
ChEBI CHEBI:53621 YesY
Jmol-3D images Image 1
Molecular formula C4H4ClNOS
Molar mass 149.60 g mol−1
Appearance colorless liquid
Density 1.02 g/cm³, liquid
Solubility in water miscible with
R-phrases R23/24/25 R34 R43 R50/53
S-phrases (S2) S26 S28 S36/37/39 S45 S60 S61
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Methylchloroisothiazolinone (5-chloro-2-methyl-4-isothiazolin-3-one), also referred to as MCI, is a preservative with antibacterial and antifungal effects within the group of isothiazolinones. It is effective against gram-positive and gram-negative bacteria, yeast, and fungi.

Methylchloroisothiazolinone is found in many water-based personal care products and cosmetics.[1] Methylchloroisothiazolinone was first used in cosmetics in the 1970s. It is also used in glue production, detergents, paints, fuels, and other industrial processes. Methylchloroisothiazolinone is known by the registered tradename Kathon CG when used in combination with methylisothiazolinone.[2]

Methylchloroisothiazolinone may be used in combination with other preservatives including ethylparaben, benzalkonium chloride, or 2-bromo-2-nitropropane-1,3-diol.


In pure form or in high concentrations, methylchloroisothiazolinone is a skin and membrane irritant and causes chemical burns. In the United States, accepted concentrations are 15 ppm in rinse-offs and 8 ppm in other cosmetics.[citation needed]

The International Agency for Research on Cancer (IARC), does not currently list methylchloroisothiazolinone as a known, probable, or possible human carcinogen,[3] nor have in vivo tests found evidence of carcinogenic activity.

Allergic contact dermatitis[edit]

Methylchloroisothiazolinone is an allergen for a small but growing portion of individuals. The first publication of the preservative as a contact allergen was in 1988.[4] In 2013, the American Contact Dermatitis Society named methylchloroisothiazolinone as the Contact Allergen of the Year.[5] A common indication of an allergic reaction is eczema-like symptoms including redness and itching, and upon longer exposure also burning sensations and blisters on the part of the skin that is exposed to the allergen. Continued exposure can lead to high sensitization which will be triggered each time the individual comes in contact with the allergen due to the memory T-cells that will remain in the local skin area.


  1. ^ Reinhard et al. (2001). "Preservation of products with MCI/MI in Switzerland". Contact Dermatitis 45 (5): 257–264. PMID 11722483. 
  2. ^ Knudsen BB, Menne T (1990). "Kathon CG--a new contact sensitizing preservative". Ugeskr Laeger 152 (10): 656–657. PMID 2321281. 
  3. ^ "International Agency for Research on Cancer". IARC (updated November 2007). Retrieved 7 January 2008. 
  4. ^ De Groot, A. C.; Weyland, J. W. (1988). "Kathon CG: A review". Journal of the American Academy of Dermatology 18 (2 Pt 1): 350–358. PMID 3279090.  edit
  5. ^ Castanedo-Tardana, M. P.; Zug, K. A. (2013). "Methylisothiazolinone". Dermatitis : contact, atopic, occupational, drug 24 (1): 2–6. doi:10.1097/DER.0b013e31827edc73. PMID 23340392.  edit

External links[edit]

  • Record in the Household Products Database of NLM