Methylisothiazolinone (MI): The Allergen of the Decade

Key takeaways

• Methylisothiazolinone (MI) is a preservative used in water based products like shampoos, liquid soaps and wipes.
• Climbing allergy rates led the American Contact Dermatitis Society to name MI its Contact Allergen of the Year in 2013.
• The EU banned MI in leave on cosmetics and set a low limit for rinse off products.
• Once someone is allergic, the sensitivity is usually lifelong, so avoidance is the main strategy.
• MI also appears in paints and household cleaners, which can cause reactions through the air.

Sometimes an ingredient goes from obscure to infamous in just a few years. Methylisothiazolinone, usually shortened to MI, is one of those. Introduced widely in cosmetics as a replacement for other preservatives, it triggered such a wave of allergic rashes that dermatologists began calling it an epidemic. If you have ever developed an itchy, stubborn rash from a shampoo or wipe and never figured out why, MI is worth knowing about.

What Is Methylisothiazolinone?

MI is a synthetic preservative from a group called isothiazolinones. It is very effective at stopping bacteria from growing in water based products, which is why manufacturers like it. For years it was used mainly in a mixture with a related compound, methylchloroisothiazolinone (MCI), sold under trade names such as Kathon CG. In the mid 2000s, regulators allowed MI to be used on its own at much higher concentrations than it had been in the mix. As parabens fell out of favor with shoppers, many brands reached for MI as a "paraben free" alternative.

The result was a sharp rise in allergic contact dermatitis, a delayed immune reaction that appears as red, itchy, sometimes blistering skin where the product touched, and sometimes beyond it.

Where MI Shows Up

MI needs water to do its job, so it is found almost entirely in water based products:

• Shampoos, conditioners and liquid hand soaps
• Body washes and bubble baths
• Baby wipes and moist toilet wipes, a major source of reactions in the 2010s
• Lotions and sunscreens, in markets where leave on use is still allowed
• Household cleaners, dish soaps and laundry products
• Water based wall paints, which can release MI into the air for days after painting

Names to Look For

• Methylisothiazolinone (MI or MIT)
• Methylchloroisothiazolinone (MCI or CMIT), often paired with MI
• Benzisothiazolinone (BIT), common in household products
• Octylisothiazolinone (OIT), used in some leather and industrial goods

Cosmetics must list MI by name, but household cleaners and paints are not held to the same ingredient labeling rules, so those can be harder to check.

What Research and Regulators Say

The American Contact Dermatitis Society named methylisothiazolinone its Contact Allergen of the Year in 2013, a title meant to draw attention to an allergen that is either newly common or under recognized. Patch testing clinics in Europe and North America reported rates of MI allergy that climbed steeply through the early 2010s.

Europe responded with some of the firmest action on any cosmetic preservative. The European Union banned MI in leave on cosmetics, effective in 2017, and limited it in rinse off products to 0.0015 percent (15 parts per million) from 2018. The MCI and MI mixture had already been barred from leave on products there. Several studies since have reported that MI allergy rates in Europe began to fall after these limits took hold, which suggests the restrictions worked.

In the US, there is no federal limit on MI in cosmetics. The industry funded Cosmetic Ingredient Review panel has concluded that MI is safe in rinse off products at limited concentrations and in leave on products when formulated to be non sensitizing, which places responsibility on each manufacturer. The difference is a good example of the broader gap covered in US vs EU cosmetic rules.

Why Allergies Rose So Quickly

The timing of the MI wave makes sense once you look at the numbers. In the older MCI and MI blend, MI made up only a small share of a mixture capped at 15 parts per million, so people were exposed to just a few parts per million of MI. When MI was approved for use on its own, manufacturers could use up to 100 parts per million, many times more than before. At the same time, it went into leave on products like lotions and wet wipes, which stay on the skin for hours instead of being rinsed off. Higher concentration plus longer contact is a classic recipe for sensitization. Dermatologists began reporting clusters of hand, face and diaper area rashes within a few years, and many cases were traced back to products that people used every day without suspicion.

It is worth being clear about what kind of risk this is. MI is not considered a carcinogen or hormone disruptor. Its problem is allergy. For most people it causes no reaction at all. For those who become sensitized, though, even small amounts can set off a rash, and the sensitivity is typically lifelong.

Who Should Be Most Careful

• Anyone already diagnosed with MI or MCI allergy. Avoidance in personal care, household and paint products is the main approach.
• Parents, since wipes used many times a day on a baby or toddler are a well documented route of sensitization. See our guide to non toxic skin care for kids.
• People with eczema or sensitive skin, whose weakened barrier may let allergens in more easily. Our article on sensitive skin irritants lists other common triggers.
• Hairdressers, cleaners and painters, who handle products containing MI all day.

If you have a rash that keeps coming back, especially on the hands, face, scalp or around the bottom, a dermatologist can perform patch testing to identify the cause. Guessing can take years. Testing often takes about a week.

A positive patch test result usually comes with a list of products and names to avoid. Keep a photo of it on your phone so you can check labels while shopping, and share it with anyone who buys household products for you.

How to Spot and Avoid MI

1. Read every rinse off and leave on label for the names above. Our guide to reading ingredient labels explains where preservatives usually appear in the list.
2. Swap wipes for a soft cloth and water when practical, or choose wipes that disclose an MI free preservative system.
3. Choose solid and anhydrous products. Bar soaps, balms and oils contain little or no water, so they do not need water soluble preservatives like MI.
4. Ventilate after painting and consider low isothiazolinone paints if you are sensitive.
5. Watch the replacements. Some brands moved from MI to phenoxyethanol or formaldehyde releasers, each with its own considerations.

Why Solid Products Sidestep the Problem

Water is what makes most products need strong preservatives. Take the water out, and the need largely goes with it. That is one quiet advantage of the way we make things at Lucky Lady Ranch. Our cold process Body Bar and Shampoo and Conditioner Bar are solid soaps that stay dry between uses, and our balms and oils are made without water at all. There is no place for MI to hide because there is no reason to add it. For many people with preservative allergies, switching to bars and balms simplifies label reading considerably. As always, if you have a known allergy, check every ingredient list, including ours, and talk with your dermatologist.

From the ranch

Made in small batches, without the ingredients discussed above.

Questions people ask

What products contain methylisothiazolinone?

MI is most common in water based products: shampoos, conditioners, liquid soaps, body washes and wet wipes. It is also used in household cleaners, laundry products and water based paints.

What does a methylisothiazolinone allergy look like?

It usually appears as a red, itchy, sometimes blistering rash where the product touched the skin, often on the hands, face, scalp or diaper area. Reactions can be delayed by a day or two, which makes the cause hard to spot without patch testing.

Is methylisothiazolinone banned?

The EU bans MI in leave on cosmetics and limits it to 0.0015 percent in rinse off products. The US has no federal limit, though many brands have voluntarily reduced or removed it.

Can you outgrow an MI allergy?

Allergic contact dermatitis to MI is generally considered lifelong. Avoiding products that contain it is the main way to prevent flare ups. A dermatologist can confirm the allergy and suggest safe alternatives.

Is MCI the same as MI?

No, but they are closely related. Methylchloroisothiazolinone (MCI) is often used together with MI. People allergic to one often react to the mix, so many dermatologists advise avoiding both.

Sources and further reading

1. American Contact Dermatitis Society, Contact Allergen of the Year: Methylisothiazolinone (2013)
2. European Commission, Commission Regulation (EU) 2016/1198 on Methylisothiazolinone in Leave On Cosmetic Products
3. European Commission, Commission Regulation (EU) 2017/1224 on Methylisothiazolinone in Rinse Off Cosmetic Products
4. Cosmetic Ingredient Review (CIR), Safety Assessment of Methylisothiazolinone
5. American Academy of Dermatology (AAD), Contact Dermatitis: Diagnosis and Treatment

This guide is for general education and is not medical advice. If you have a skin condition, allergy, or are pregnant or nursing, talk with your doctor or a board certified dermatologist about the products that are right for you.