Unless you have been living under a rock, you probably already know that the preservative methylisothiazolinone (MI) has caused an epidemic of allergic contact dermatitis (ACD) and was named the 2013 American Contact Dermatitis Society Allergen of the Year.1 Methylisothiazolinone is not new on the market, but its solo use as a preservative is relatively new. In this article, we review the emergence of MI as a common allergen, discuss North American MI patch test results, and describe common and uncommon sources of MI exposure. We also explore the related isothiazolinones, benzisothiazolinone (BIT) and octylisothiazolinone (OIT).
Methylchloroisothiazolinone (MCI) and MI have been utilized as a preservative in a 3:1 ratio since the 1980s. In 2005, MI was first used alone as a preservative in personal care products in concentrations of up to 100 ppm, which represented a 25-fold increase in exposure to MI in personal care products and thus unleashed an epidemic of ACD.1 In the 2015 to 2016 cycle of the North American Contact Dermatitis Group (NACDG) patch testing results, MI was found to be positive in 13.4% of patch tested patients (N=5597) and also had the highest significance-prevalence index number, a calculation that represents the relevance of positive reactions in relationship to prevalence.2 In Europe, MI is banned in leave-on products and is allowed in rinse-off products in concentrations of up to 15 ppm. In the United States, the Cosmetic Ingredient Review panel concluded that MI is safe at a maximum concentration up to 100 ppm in rinse-off products and safe in leave-on products when formulated to be nonsensitizing, which may be determined based on a quantitative risk assessment.3
It is recommended that MI be patch tested at a concentration of 2000 ppm (0.2% aqueous).4 Testing at lower concentrations may result in missed positives. In addition, it should be noted that MCI/MI is present in the T.R.U.E. Test (SmartPractice), but MI alone is not.
Sources of MI Exposure
The first few case reports of MI contact allergy were associated with occupational exposures. In 2004, Isaksson et al5 reported 2 cases of MI allergy following exposure to wallpaper glue and a chemical burn from a biocide, respectively. Soon after, Thyssen et al6 reported 4 occupational cases of MI allergy at a paint manufacturing plant.
An early case series of MI contact allergy associated with personal care products was published in 2010 in which the authors described adults with ACD from wet wipes and a makeup remover that contained MI.7 A more recent report indicated that MI is now an infrequent ingredient in wet wipes but is still found in a wide variety of household and personal care products.8 A 2017 query of the American Contact Dermatitis Society’s Contact Allergy Management Program (CAMP) database revealed that 12.9% of all products contained MI. Furthermore, CAMP data revealed that MI was the most commonly found preservative in both hair care and household products.9 An additional CAMP database study revealed that 53% of shampoos and 45% of conditioners contained MI, and it also was commonly found in hair dyes, soaps and cleansers, hand cleaners and sanitizers, vaginal hygiene products, sunscreens, and moisturizers.10