Methylisothiazolinone

What it is

Methylisothiazolinone is a small, highly reactive isothiazolinone that kills microbes at very low concentrations. When EU rules allowed it in leave-on cosmetics at up to 0.01% from 2005, use expanded rapidly and so did allergy: patch-test clinics across Europe recorded a sharp rise in sensitisation, peaking around 2013. Regulators responded with two successive restrictions, and it is no longer permitted in any leave-on product, including face masks.

What it does well

• Extremely effective at very low concentrations

  It controls a broad spectrum of bacteria and fungi at parts-per-million levels, which is why it was adopted so widely. This is a technical advantage, not a reason for consumers to seek it out.

  Strong evidence

Worth knowing

• One of the most significant contact allergens of the last two decades

  Prevalence of MI allergy in European patch-test clinics rose from about 2.0% to 3.7% between 2010 and 2012, and researchers called for urgent action. It typically presents as facial or hand eczema.

  Matters most toAnyone with unexplained facial or hand eczema; anyone already sensitised must avoid it in every product type.

• Still permitted in rinse-off products, and still present in household products

  Once someone is sensitised, exposure from shampoos, washing-up liquids, paints and wipes can keep the dermatitis going even after cosmetics are changed.

  Matters most toPeople with confirmed MI allergy.

• Regulation demonstrably worked

  Prevalence of MI allergy in Europe fell by about 50% between 2015 and 2017 after the restrictions, while rates in North America, where no comparable limits applied, kept climbing. This is a case where an EU restriction visibly reduced harm.

  Matters most toNobody needs to worry about MI in an EU-compliant leave-on face mask; it cannot legally be there.

Where the EU stands

Sources (3)

Every link goes to the original: a journal, a PubMed record or an EU document.

1. [1]
   Methylisothiazolinone contact allergy--growing epidemic

   Contact Dermatitis · 2013 · doi:10.1111/cod.12149

   Documented a rise in MI contact allergy from 2.0% to 3.7% of patch-tested patients between 2010 and 2012 and called for urgent regulatory action, identifying cosmetics as the main source.

2. [2]
   The epidemic of methylisothiazolinone contact allergy in Europe: follow-up on changing exposures

   Journal of the European Academy of Dermatology and Venereology · 2020 · doi:10.1111/jdv.15875

   After the EU restrictions, MI contact allergy prevalence fell by about 50% between 2015 and 2017, with far fewer cases traced to leave-on cosmetics.

3. [3]
   Trends in the Prevalence of Methylchloroisothiazolinone/Methylisothiazolinone Contact Allergy in North America and Europe

   JAMA Dermatology · 2023 · doi:10.1001/jamadermatol.2022.5991

   European positivity peaked in 2013-2014 and then fell, while North American rates rose to 15.0% for MI alone by 2017-2018, a divergence the authors attribute to earlier and stricter European regulation.