Methylisothiazolinone: the preservative behind an allergy epidemic

Most people have never heard of methylisothiazolinone. Yet in the 2010s this preservative caused so many new skin allergies that dermatologists called it an epidemic. The story is a good example of how an ingredient can look harmless for years, then cause trouble once it’s in everything, and how regulation can turn things around.

What methylisothiazolinone is and why it’s used

Methylisothiazolinone, or MI for short, is a preservative: an ingredient added to water-based products so that bacteria and mould can’t grow in them. It works at very low concentrations, which is why it usually sits near the end of an ingredient list.

It has a close relative, methylchloroisothiazolinone, or MCI. According to DermNet, the two were first used together as a mix in industrial products like paints, adhesives and cleaning products. Around 2005, MI on its own started being used widely in cosmetics and household products.

How a preservative became an epidemic

MI turned out to be a strong allergen, meaning it’s good at teaching the immune system to react to it. The result is allergic contact dermatitis: an itchy, red, sometimes blistering rash that appears a day or more after contact, wherever the ingredient touched.

Within a few years, patch-test clinics (where doctors test people for skin allergies) were seeing positive results to MI far more often. The American Contact Dermatitis Society named it Contact Allergen of the Year in 2013. DermNet reports that in Australia and New Zealand in 2015–16, 20 to 25% of people who were patch tested reacted to MI.

The rash showed up in telling places. DermNet lists the hands, the face and eyelids, the scalp, the diaper area in babies, and the skin around the bottom and genitals in adults. Those last ones point straight at wet wipes, one of the big sources at the time, along with shampoos, liquid soaps, body washes, moisturizers, sunscreens and makeup.

Leave-on products were a big part of the problem. A cream or a wipe leaves the preservative sitting on your skin for hours, while a shampoo is rinsed away within minutes.

What Canada and Europe did about it

In 2013, the European Union’s Scientific Committee on Consumer Safety recommended against using MI in leave-on products, according to a 2023 study in JAMA Dermatology. European authorities then banned MI from leave-on cosmetics in 2017 and limited it in rinse-off products in 2018.

Canada moved in two steps through the Cosmetic Ingredient Hotlist, Health Canada’s list of ingredients that are prohibited or restricted in cosmetics:

• December 2015: the MCI/MI mix was prohibited in leave-on cosmetics and limited to 0.0015% (15 parts per million) in rinse-off products.
• June 2018: MI on its own was prohibited in leave-on cosmetics, and the total of MI plus MCI in rinse-off products was capped at 0.0015%. Health Canada gave the reason plainly: the “risk of sensitization”.

The United States took a different path. The JAMA Dermatology authors note there are no formal government restrictions there, only voluntary industry recommendations.

Did the rules work?

In Europe, yes. A study across 14 clinics in 11 European countries found that the rate of MI allergy among people patch tested dropped by half between 2015 and 2017, from about 6% to 3%. Leave-on cosmetics also made up a shrinking share of the products causing reactions. A Belgian study across five hospitals saw the rate fall from 7.9% in 2014 to 3.1% in 2019, which the authors called a return to pre-epidemic levels.

North America looked different. The 2023 JAMA Dermatology study compared large patch-test databases. In Europe, MI allergy peaked around 2013–2014 and then declined. In North America, it kept climbing through 2017–2018, when 15% of people tested reacted to MI. The authors suggest Europe’s earlier and stricter regulation is the likely reason. Canadian rules for MI alone only came into force in 2018, near the end of that data.

Where MI still turns up

The rules cover cosmetics, and only leave-on ones fully. That leaves several places MI can still be found:

• Rinse-off products: shampoo, conditioner, body wash and liquid hand soap can still contain small amounts.
• Household products: the Belgian researchers found rinse-off cosmetics, paints and detergents were the most common sources in recent years. Cleaning products, dish soap and laundry products aren’t cosmetics, so the Hotlist doesn’t apply to them.
• Paint and glue: MI evaporates, so DermNet notes it can cause airborne contact dermatitis, a rash on exposed skin like the face and eyelids, after painting or spending time in a freshly painted room. DermNet suggests highly sensitive people avoid freshly painted rooms for up to four weeks.

For babies, this is part of why wipes have changed. A wipe is a leave-on product, so in Canada it shouldn’t contain MI anymore. If you’re shopping for your little one, you can browse baby-care products and still check every label.

How to read labels if you’re allergic

If a patch test shows you’re allergic to MI, the ingredient list is your best tool.

• Look for the full names. On cosmetics, you’ll see “methylisothiazolinone” and “methylchloroisothiazolinone”, usually near the end.
• Watch for the cousins. DermNet says all chemicals in the isothiazolinone group may cross-react. The Belgian study found people allergic to MI often also reacted to octylisothiazolinone and, increasingly, benzisothiazolinone. Any word ending in “-isothiazolinone” deserves a second look.
• Check every time. DermNet warns that products with similar names from the same company don’t necessarily use the same preservatives, and formulas change.
• Household products need extra care. The Hotlist doesn’t apply to them, so if a label isn’t clear, contact the manufacturer or wear gloves for cleaning.

If you have a rash that keeps coming back on your hands, face or around your bottom and you can’t find the cause, see your doctor and ask about patch testing. MI is part of the standard series in North America and Europe, according to DermNet, so it will be checked.

To sum up: MI is an effective preservative that turned out to sensitize a lot of people once it was used widely in leave-on products. Canada now bans it from leave-on cosmetics and caps it in rinse-off ones, and in Europe similar rules cut allergy rates in half. It still lives in shampoos, soaps, cleaners and paint, so if you’re allergic, read labels for anything ending in “-isothiazolinone”.