Methylisothiazolinone (MI): The Preservative That Became 'Allergen of the Year'

What methylisothiazolinone is

Methylisothiazolinone (CAS 2682-20-4) is a synthetic biocide — a chemical whose job is to kill microbes. In cosmetics it plays the role of preservative, the ingredient that stops bacteria, yeasts and moulds from growing in water-based products both on the shelf and during use.

MI has relatives. For years the best-known version was the 3:1 mixture of methylchloroisothiazolinone (MCI) and MI, sold under trade names like Kathon CG and used in everything from cosmetics to industrial cooling water. In the early 2000s MI was released as a standalone preservative — first for industrial products, then, in 2005, permitted for use in cosmetics on its own. A peer-reviewed review of the MI story notes that MI was soon found in a small but meaningful share of cosmetic products — about 1.5% of over 1,200 products surveyed, mostly rinse-off ones — at concentrations ranging from 2 to 100 ppm.

The naming on labels is straightforward: it appears as methylisothiazolinone, and the mixture as methylchloroisothiazolinone (and) methylisothiazolinone. There is no umbrella-word trickery here, unlike fragrance — if it is in the formula, the word is on the list. The problem was never hidden; it was how many people reacted to it.

Why brands reach for MI

Water is the enemy of an unpreserved cosmetic. Any product that contains water — shampoo, lotion, liquid soap, wet wipes — is an inviting home for microbes, and those microbes can cause spoilage (discolouration, bad smells, separation) or, at the extreme, grow to levels that harm the user. The industry's own position, set out by the Personal Care Products Council's Cosmetics Info programme, is blunt: preserving water-based products is critical to their safety and quality, and MI earned its place because it works.

MI is broad-spectrum and effective at very low doses — tens of parts per million rather than the percentages some older preservatives need. It is cheap, colourless and stable across a wide pH range, which made it an attractive replacement as formulators moved away from older preservative systems. That efficiency is precisely what made the allergy story so consequential: a preservative used in tiny amounts still managed to sensitise a remarkable number of people.

Where you will still find MI

In the EU, MI's cosmetic territory has shrunk dramatically — leave-on products are off-limits entirely, and rinse-off products are capped at 15 ppm. But MI has not disappeared from the world of water-based products:

• Rinse-off cosmetics — shampoos, shower gels, liquid hand soaps, bubble bath and dish soap remain the most common cosmetic source
• Wet wipes and leave-on products outside the EU — where the EU bans do not apply, MI can still appear in these categories
• Household and industrial products — paints, adhesives, detergents and cleaning products, where MI's use is not restricted the way cosmetics are; painters are one of the classic occupational cases of MI allergy
• Older products on shelves — a 2022 European audit of patch-tested patients found that exposure to older cosmetic products containing MI may still be happening, years after the restrictions

Tip: because the EU restricts only cosmetics, a bathroom shelf can be fully compliant while a tin of wall paint or a bottle of surface spray in the same house still carries MI. Occupational and household exposures are a documented blind spot for sensitised people.

The allergy epidemic that made MI infamous

Few cosmetic ingredients have a documented epidemic curve. MI has one. After its standalone cosmetic approval in 2005, reports of allergic contact dermatitis to MI began climbing — and then kept climbing. Dermatology clinics across Europe reported a surge in MI-positive patch tests through the early 2010s, peaking around 2013–14. The rise was described in the scientific literature as unprecedented in Europe, with repeated warnings published from 2013 onward.

The peak numbers were startling. In 2013 the American Contact Dermatitis Society named methylisothiazolinone its "Allergen of the Year" — the dermatology equivalent of a fire alarm. In Turkey, a 24-year clinic series found MI/MCI sensitisation peaking at 16.4% of patch-tested dermatitis patients in 2015, then gradually falling — though it was still 6.5% in 2019, years after the EU restrictions. The signature exposures were painfully ordinary: wet wipes and moist toilet paper (including in young children, with perianal dermatitis cases documented), shampoos and shower gels, liquid soaps, and — outside the bathroom — wall paints and hair gels in occupational cases.

The long view comes from a 2026 systematic review and meta-analysis in Contact Dermatitis that pooled 115 studies covering 1,514,781 dermatitis patients from 2000 to 2025. Its numbers: pooled contact-allergy prevalence of 5.48% for MI and 4.58% for the MCI/MI mixture, with about 55–60% of positive patch tests judged clinically relevant. Europe's rates were lower than Asia's and the Americas' — a pattern the authors attribute to the EU's proactive restrictions — and there is, the review says, "an indication of a decline" following regulation. But it also warns that isothiazolinones in non-cosmetic consumer products may be the next chapter.

Related: the story of another preservative class regulators eventually pushed out — formaldehyde and formaldehyde-releasing preservatives.

What regulators and the evidence say

The European Union moved in two steps, both driven by the EU's Scientific Committee on Consumer Safety (SCCS). In its 2013 opinion (SCCS/1521/13), the committee concluded that 100 ppm MI in cosmetics was not safe, that no safe concentration had been demonstrated for leave-on products — including wet wipes — and that 15 ppm was the safe level for rinse-off products from the point of view of allergy induction. Commission Regulation (EU) 2016/1198 then banned MI from all leave-on cosmetics from 12 February 2017 (keeping rinse-off at 100 ppm). A second regulation (EU 2017/1224) cut the rinse-off ceiling to 0.0015% (15 ppm), applying from early 2018. The mixture MCI/MI had been banned from leave-on products two years earlier, and the two entries are mutually exclusive: a product may use one or the other, never both.

The restrictions worked — partially. A European cross-sectional audit of 2,554 patch-tested patients across 10 departments in 8 countries found MI contact allergy at 2.9% in 2022, roughly half the 2015 level. But the same audit found rinse-off cosmetics still behind 73% of MI allergy cases, and leave-on products behind 13% — suggesting older or non-compliant products, plus non-cosmetic sources, still matter. Meanwhile benzisothiazolinone (BIT), a cousin not permitted in cosmetics at all, has overtaken MI as a cause of isothiazolinone allergy in Europe — a reminder that the chemistry keeps moving.

The United States took the softer path. The Cosmetic Ingredient Review (CIR) expert panel first judged MI safe at up to 100 ppm in 2010, reopened the assessment in 2013 as the European case reports mounted, and then reaffirmed safety for rinse-off products at up to 100 ppm — while concluding that leave-on use was safe only below 100 ppm and only when shown not to cause reactions. The panel explicitly cautions that people who know they are allergic to MI should avoid products containing it. There is no FDA ban; the US position rests on industry self-assessment rather than the hard limits the EU imposed.

The honest evidence summary: MI is one of the best-documented cosmetic sensitisers in modern dermatology, the EU's restrictions measurably reduced new cases, and rinse-off exposure can still provoke reactions in already-sensitised people. The open question is the one consumer groups raised back in 2016: whether 15 ppm in rinse-off products is low enough for everyone.

The debate

Who should avoid MI

MI is not an ingredient everyone must fear — most people use MI-preserved products without any problem. But avoidance is the right call for:

• Anyone with a positive patch test for MI or MCI/MI — for them, MI-containing products can trigger eczema even in rinse-off form; the CIR panel itself advises diagnosed individuals to avoid it
• People with unexplained hand, face or scalp eczema — MI is one of the first preservatives a dermatologist patch-tests for, alongside formaldehyde releasers; a positive result often explains years of mystery flares
• Parents of children with perianal or nappy-area dermatitis — moist wipes were a signature exposure route in the epidemic, and several clinicians singled out young children as a concern group
• People in high-exposure occupations — painters, cleaners and barbers face MI in products the cosmetic rules do not cover

If none of those describe you, MI in a compliant rinse-off product is a preference question rather than a health directive — and the spotting section below is how you exercise it either way.

How to spot MI on a label

• Methylisothiazolinone — the standalone preservative, usually near the end of the ingredient list
• Methylchloroisothiazolinone (and) methylisothiazolinone — the 3:1 mixture (Kathon CG); it and MI alone are mutually exclusive in EU products, so you will see one or the other, never both
• Check rinse-off products first — shampoos, shower gels, liquid soaps and bubble baths are where MI legally remains in the EU
• Check non-cosmetic products too — paints, adhesives, detergents and cleaning sprays carry isothiazolinones under the same names, without the cosmetic restrictions
• Older stock — products bought before 2018, or from markets without the EU rules, may still carry MI in leave-on formulas

Alternatives: preserving products without MI

MI's exit from leave-on products forced one of the largest reformulation waves in modern cosmetics — and it worked. Most mainstream brands now preserve with other systems, which means MI-free options are easy to find without hunting niche brands. The common replacements:

• Phenoxyethanol — now one of the most widely used cosmetic preservatives; it has its own flagged-ingredient debate (our explainer is next in this series), but no epidemic behind it
• Sodium benzoate and potassium sorbate — food-grade preservatives widely used in cosmetics, effective in acidic formulas
• Benzyl alcohol — a preservative that doubles as a solvent; low sensitisation rates in patch-test surveys
• Paraben systems — the old workhorses MI partly replaced; still among the least sensitising preservatives in patch-test data, which is why the MI story carries a certain irony — read our parabens explainer for the full picture

Also worth knowing: "preservative-free" water-based products are a red flag rather than a reassurance — water plus no preservative is a recipe for microbial growth. The goal is a product preserved with something your skin tolerates, not a product preserved with nothing.

Checked against EU regulations (2016/1198, 2017/1224), the SCCS opinion SCCS/1521/13, peer-reviewed literature (Isufi et al. 2026; Schwensen et al. 2022 audit), the US CIR assessment via Cosmetics Info, and the EWG Skin Deep database (an advocacy source) · Last updated 2026-10-23 · How we assess ingredients

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