Preservatives are a necessary additive in water-based products in order to limit degradation. The isothiazolinones are a group of antimicrobials preservative compounds, derivatives of which are used to control bacteria, fungi, and algae in cooling water systems, fuel storage tanks, pulp and paper mill water systems, oil extraction systems, ship hull paint, and personal care products. Some derivatives are potent allergens and can cause hypersensitivity by direct contact or via the air.

Included in this group are:
   ° Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI/MIT)
   ° Chloromethylisothiazolinone (CMIT)
   ° Benzisothiazolinone (BIT)
   ° Octylisothiazolinone (OIT)
   ° Dichlorooctylisothiazolinone (DCOIT)

Both methylisothiazolinone and methylchloroisothiazolinone are powerful biocides and preservatives that control or kill harmful microorganisms in personal care and cleaning products such as shampoos, conditioners, hair colors, body washes, bubble bath, laundry detergents, liquid hand soaps, and hand dishwashing soaps. They are commonly used under the trade names Kathon CG (PDF about Kathon CG from The Dow Chemical Company) and Euxyl K 100 (among others) in low concentrations in "rinse-off" products. Even products claiming to be gentle, hypoallergenic, sensitive, or eco-friendly can contain these preservatives.

It was introduced in the US in 1980 (some sources give the date as the mid-1970s). It was used in rinse-off products at a higher concentration than for leave-on products and was responsible for an epidemic of contact sensitivity in the 1980s and early 1990s. Today MCI/MI/MIT is still used, mainly in in "rinse-off" products, although at much lower concentrations, instead of another group of preservatives, the parabens, despite the fact that parabens have a lower incidence of causing a sensitizing reaction. MCI/MI/MIT has been banned in Canada but is still popularly used in the US.

In 2013, methylisothiazolinone was declared the 2013 Contact Allergen of the Year by the American Contact Dermatitis Society.

The long-term consequences of low-level chronic exposure to isothiazolinones on the central nervous system have not been thoroughly investigated. source

Among the products in which isothiazolinones may be found are:
   ° cosmetics: foundation, lip gloss, mascara, nail varnish, makeup remover
   ° hair products: shampoo, conditioner, hair color, hairspray, baby shampoo
   ° personal care: liquid soaps, body wash, shaving cream, deodorants, moist wipes
   ° dermatological products: lotion, sunscreen, baby lotion
   ° household cleaning products, including detergents
   ° paint
   ° moist toilet paper
   ° metalworking fluids
   ° products stated to be antibacterial or anti-mold

What to look for on labels:
   ° Methylisothiazolinone (MIT):
      ° 2-methyl-4-isothiazoline-3-one
      ° Neolone 950 preservative
      ° MI
      ° OriStar MIT
      ° Microcare MT
   ° Methylchloroisothiazolinone (CMIT):
      ° 5-Chloro-2-methyl-4-isothiazolin-3-one and MCI.

Other alternative names:
Alternative names: 2-Methyl-4-isothiazolin-3-one, 5-Chloro-2-methyl-4-isothiazolin-3-one, Methylisothiazolinone; 3(2H)-Isothiazolone, 2-methyl-; 2-Methyl-2H-isothiazol-3-one; 2-Methyl-4-isothiazolin-3-one; 2-Methyl-3(2H)-isothiazolone, Acticide, Algucid, Amerstat 250, Benzisothiazolinone (BIT), Caswell # 572A, Euxyl K 100, Fennosan IT 21, Grotan, Grotan TK2, Isothiazolinone (Kathon), Kathon CG, Kordek, Methylchloroisothiazolinone (MCI), Methylisothiazolinone (MIT or MI), Mergal K7, Metatin GT, Mitco CC 32 L, Microcare, Neolone, Optiphen MIT, OriStar MIT, ProClin, SPX, Zonen MT

Household products database list of products that contain Methylisothiazolinone

Sources (in alphabetical order):
Chemical Irritant In Cosmetics Banned In Europe: MIT Leads To Epidemic In Skin Allergies, Eczema
Harsh Preservatives Found in Many Cosmetics and Household Products
Increasing trend of sensitization to Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) (from U.S. National Library of Medicine)
Ingredient Watch List: Methylisothiazolinone, the Toxic Ingredient That Could Cause Nerve Damage
Methylchloroisothiazolinone (and) methylisothiazolinone
Methylisothiazolinone: An Emergent Allergen in Common Pediatric Skin Care Products
Methylisothiazolinone and Methylchloroisothiazolinone