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Methylisothiazolones are chemical compounds used as highly effective biocidal agents against fungal and bacterial species. They are widely used as a preservative in cosmetic industry, and as a pesticide, e.g. in paints and water tanks.

There are known bacterial species which exhibit increased resistance to the compound.

Most common compounds are:

  • MCI (Methylchloroisothiazolinone, 5-Chlor-2-methyl-2,3-dihydroisothiazol-3-on, CAS 26172-55-4)
  • MI (methylisothiazolone, 2-Methyl-2,3-dihydroisothiazol-3-on, CAS 2682-20-4).


Methylisothiazolone is highly corrosive and causes burns if use undiluted. Common usage rates in this preservative are .01%. The general public would not come into contact with undiluted MIT. Causes irritation of skin by contact if undiluted, and it is highly destructive of mucous membranes if applied undiluted.

It is lethal in low doses if absorbed via oral ingestion, inhalation or dermal contact (see references). Known allergen in concentrations as low as 20 ppm, with prevalence in population between 3% and 8%.[1] Suggested usage rate is .01% for lotions and usage rates.

Maximum permissible dose for use in cosmetics was set to 15 ppm in year 1997. It is approved for use in over 25 countries and has a long history of safety data.

Teratogenic risk: class D in Germany.


  1. ^ Jacob, Pontoppidan Thyssen; Jeanne Duus Johansen; Torkil Menné (2007). "Contact allergy epidemics and their controls". Contact Dermatitis. pp. 185–195. Retrieved 2007-08-21.