2023 Top Story in Dermatology: Prevalence of Methylchloroisothiazolinone/Methylisothiazolinone Contact Allergy in the US
Methylisothiazolinone (MI) and methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) are well-known contact allergens. They are preservatives that are part of the class of isothiazolinones, which are commonly found in personal care products, cosmetics, and household and industrial products. MI and MCI/MI currently rank second and sixth as contact allergens in the most recent North American Contact Dermatitis Group (NACDG) patch test results from 2019 to 2020.1 They also rank first and fourth, respectively, in significance-prevalence index number, which is a weighted calculation depending on prevalence and degree of certainty ascribed to relevance.1-2
MI was named allergen of the year in 2013 to highlight the rise in exposure to MI and subsequent allergy.3 MI was approved as a stand-alone preservative in cosmetics in 2005. The MCI/MI mix can miss up to 40% of allergy to MI due to the low concentration of MI in the MCI/MI mix.3 In the most recent data from NACDG, allergy to MI remains high at 13.8% of those patch tested, second only to nickel.1 There was a statistically significant decrease in positivity to MI in Canada but not in the US.1
A study of recent trends in the prevalence of MCI/MI contact allergy in North America and Europe suggests that isothiazolinone allergy is decreasing in Europe but remains very high in North America. This may be due to stricter government regulation of MI in Europe.4 Allowable concentrations and restrictions of MI in personal care products vary across the globe. Restrictions regarding MI concentrations in consumer products in both Canada and Europe have resulted in decreases in MI positivity. Similar limitations have not been implemented in the US, and we continue to see higher rates of allergy to MI in the US.
As clinicians managing patients with dermatitis, we must remember to utilize the patch test, which is the criterion standard tool to diagnose allergic contact dermatitis. Patch testing to standard FDA-recognized allergens may miss allergens, including MI, which is not in the standard tray. Expanded testing to MI in addition to MCI/MI is needed in many cases to identify MI allergy, which remains very prevalent as a cause of allergic contact dermatitis in those patch tested in the US.
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Additional Info
- DeKoven JG, Warshaw EM, Reeder MJ, et al. North American Contact Dermatitis Group patch test results: 2019-2020. Dermatitis. 2023;34(2):90-104.
- Maouad M, Fleischer Jr AB, Sherertz EF, et al. Significance-prevalence index number: a reinterpretation and enhancement of data from the North American Contact Dermatitis Group. J Am Acad Dermatol. 1999;41(4):573-576.
- Castanedo-Tardana MP, Zug KA. Methylisothiazolinone. Dermatitis. 2013;24(1):2-6.
- Reeder MJ, Warshaw E, Aravamuthan S, et al. Trends in the prevalence of Methylchloroisothiazolinone/Methylisothiazolinone contact allergy in North America and Europe. JAMA Dermatol. 2023;159(3):267-274.
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