Methylisothiazolinone (MI or MIT) belongs to a group of chemicals often referred to as isothiazolinones, along with chloromethylisothiazolinone (CMIT), benzisothiazolinone (BIT), octylisothiazolinone (OIT), and dichlorooctylisothiazolinone (DCOIT). It’s sometimes listed as neolone 950, kathon CG, 2-methyl- 4 isothiazolin-3-one, and 2-methyl-3 (2h) –isothiazolone. It was registered with the US Environmental Protection Agency in 1977 as an antimicrobial agent and as a pesticide.

Since the 1980s, MIT has been one of the cheapest and most effective ways of controlling slime-forming bacteria, fungi, and algae in industrial cooling water systems, pulp and paper mills, air washer systems, mining, and energy and oil production. It’s also been useful in controlling mold and mildew on wooden items, and as an antifungal agent in many industrial and household products including latex adhesives, lacquers, paints, fuels, and resin emulsions.

MIT has crept over time into natural and mainstream deodorants and many personal care products as a cheap but powerful preservative for suppressing bacterial growth in water-based (aqueous) solutions. However, not much work has been done to comprehensively ascertain the risks associated with incorporating MIT in these products.

Exposure to MIT via inhalation, skin contact, and the eye can be acutely toxic. Inhalation can result in nasal lesions. Indeed, reports indicate there’s a staggering increase in the rate at which people are developing allergic reactions to the chemical. Some people are able to use MIT products as a stay-on for a long time without reacting; however many use them a few times and develop sensitization. One strange thing about MIT is that once you develop an allergic reaction, it becomes awfully difficult, if not impossible, to reverse the reaction. The best you can do once this happens will be to avoid products containing MIT completely. But this can be somewhat daunting given that it’s present in so many personal care products.

There’s a close consensus among health experts on the widespread nature of allergic reactions emanating from the use of MIT products. The only point of contention, however, is whether it’s the leading or the second leading cause of allergic contact dermatitis and not formaldehyde releasers (see diazolidinyl urea). Health experts, including Ana Rodrigues-Barata and Luis Conde-Salazar, won’t hesitate to tell you that the increased use of MIT is culminating in an epidemic of contact sensitization.

It’s difficult to understand how this chemical has, to date, managed to stay under the radar in many countries. It needs mentioning, though, that The American Contact Dermatitis Society has done its part to highlight MIT’s adverse reactions by ‘crowning’ the chemical as its Contact Allergen of the Year for 2013.

Japan and other countries have banned MIT even from rinse-off products. The European Commission has also taken some action, albeit very miniscule, by recommending that MIT be precluded from stay-on products. At this point, no such MIT-specific restrictions are available in the US and Australia: the US has only set a general standard of 7.5 ppm and 15 ppm for all stay-on and rinse-off products, respectively.

The rising skin sensitization has prompted calls for the maximum allowable concentration of MIT in personal care products to be revised dramatically downward as a matter of urgency.