What is Methylisothiazolinone?
Methylisothiazolinone, usually abbreviated to MI on ingredient lists, is a synthetic preservative from a chemical family called isothiazolinones. Its job is straightforward: it kills or inhibits the bacteria, yeasts and moulds that would otherwise grow in water-based cosmetic products like shampoos, conditioners and body washes.
You will also sometimes see MI paired with another preservative called methylchloroisothiazolinone (MCI). When they appear together, the blend is often listed under the trade name Kathon CG. The two are related but distinct compounds, and regulators have assessed them both separately and as a combination.
MI became widely used in the early 2000s as formulators looked for alternatives to older preservative systems. It is effective at low concentrations and works across a broad range of pH levels, which makes it practically convenient for cosmetic chemists. For a long time, it was considered a modern, efficient choice.
What the Science Says
The scientific story around MI has developed significantly since the mid-2000s, and it is worth walking through the key findings clearly.
Skin sensitisation is the central concern. Sensitisation is a two-step process. First, a person's immune system is exposed to a substance and "learns" to recognise it as a threat, without any visible reaction at that stage. Then, on a later exposure, the immune system mounts a response, producing symptoms like redness, itching, swelling or blistering. Once someone is sensitised, even tiny amounts of the substance can trigger a reaction for the rest of their life.
The European Scientific Committee on Consumer Safety (SCCS) has reviewed MI multiple times. In its 2014 opinion (SCCS/1521/13), the committee concluded that MI at 0.0015 percent (15 parts per million) in leave-on cosmetics was not safe for consumers. For rinse-off products including shampoos, the committee stated that a safe concentration could not be established based on the data available at the time, meaning they could not rule out a risk even for brief-contact products.
A follow-up SCCS opinion in 2016 (SCCS/1570/15) reinforced these conclusions. The committee found no new data that would change its position on leave-on products, and it noted that MI is a strong skin sensitiser in its own right.
Contact allergy rates to MI rose sharply in Europe during the period when its use expanded. Research published by Uter et al. (2013) in the journal Contact Dermatitis documented a marked increase in positive patch test results across European dermatology clinics, correlating with the broader introduction of MI into cosmetic formulations. Patch testing is the standard clinical method for identifying contact allergies: small amounts of a substance are applied to the skin under a patch for 48 hours and reactions are assessed.
In response to this evidence, the EU moved to restrict MI. Under EU Cosmetics Regulation 1223/2009, MI is now prohibited in leave-on cosmetics. For rinse-off cosmetics, the permitted maximum concentration is 0.0015 percent. This limit remains in place, but scientific debate about whether it is genuinely protective for all consumers has continued.
It is also worth noting that MI has been identified as an allergen of the year. The American Contact Dermatitis Society named MI its Allergen of the Year in 2013, reflecting the clinical significance of its sensitisation potential at a population level.
Why It Matters for Your Hair
You might reasonably think that a shampoo, which is rinsed off quickly, poses little risk compared with a moisturiser that sits on skin all day. That intuition is sensible, and it is broadly how regulators have approached the distinction too.
However, a few points complicate the picture for rinse-off products.
First, people vary enormously in how long shampoo actually sits on the scalp. Someone who massages shampoo in, leaves it while they wash the rest of their body, and then rinses has considerably more exposure time than someone who rinses immediately. Scalp skin can also be more permeable than skin on other parts of the body.
Second, once a person is already sensitised to MI, even very brief contact can be enough to trigger a response. Sensitisation often happens through leave-on products, but the resulting allergy does not distinguish between product types. A sensitised person using a shampoo containing MI may well react.
Third, the scalp is not a standalone area. Rinse-off shampoo runs down the face, neck and back, bringing any actives into contact with a larger skin surface area than the scalp alone.
None of this means that every person using MI-containing shampoo will experience a problem. The majority of people are not sensitised to MI. But the science is clear that MI is a potent sensitiser and that a meaningful proportion of the population has become reactive to it, largely through cosmetic product exposure.
Our Position at MyHolyBody
We do not use methylisothiazolinone in any of our products, and we want to be straightforward about why.
Preservatives are not optional in water-based cosmetics. Without them, products become breeding grounds for microorganisms that can genuinely harm people. Every brand making water-based formulations has to solve the preservation question, and there is no solution that is entirely without trade-offs. We have simply concluded, based on the current evidence, that MI is not the trade-off we are willing to make.
The SCCS's finding that a safe concentration for rinse-off products cannot be established is meaningful to us. "We cannot establish a safe level" is a different statement from "this level is safe." The regulatory maximum for shampoos exists as a practical limit within a functioning market, not as a scientific endorsement of safety for all users.
We also think about cumulative exposure. A person uses shampoo, conditioner, body wash, face wash and potentially other rinse-off products every day. If multiple products in that routine contain MI at the permitted limit, the effective daily exposure adds up, even if each individual product technically complies with the rules.
The founder's view has always been that when credible scientific doubt exists, the right response is to look for alternatives rather than wait until regulatory action forces a change. There are effective preservative systems that do not rely on MI. We use them. We test our formulations for both preservation efficacy (whether harmful microorganisms are actually controlled) and skin compatibility.
We are not telling you that every product containing MI will harm you. What we are telling you is what we chose and why.
Frequently Asked Questions
Is methylisothiazolinone banned in Europe? MI is banned in leave-on cosmetics across the EU under Regulation 1223/2009. In rinse-off products such as shampoos, it is still permitted up to a maximum concentration of 0.0015 percent (15 parts per million). The SCCS has stated that it cannot establish a safe concentration even for rinse-off use based on available data.
What is the difference between MI and MCI? Methylisothiazolinone (MI) and methylchloroisothiazolinone (MCI) are related synthetic preservatives in the isothiazolinone chemical family. They are often used together in a blend. Both have been identified as skin sensitisers. MCI has been subject to stricter restrictions for a longer period. MI received increased regulatory attention from around 2013 onward as its use as a standalone ingredient expanded.
Can MI cause a reaction from shampoo even if I rinse it off quickly? If you are already sensitised to MI, yes, even brief contact can trigger a reaction. Sensitisation typically builds up over time through repeated exposures, often via leave-on products. Once your immune system has learned to recognise MI as a threat, it responds to very small amounts regardless of contact time. People who are not yet sensitised face a lower risk from brief-contact products, though repeated exposure through multiple products increases sensitisation risk over time.
What preservatives does MyHolyBody use instead? We use alternative preservative systems that are supported by strong safety data and that pass the same efficacy testing requirements. We publish full ingredient lists for all our products. If you have a specific sensitivity or allergy, we recommend reviewing the full INCI list (the standardised ingredient list you find on every cosmetic product in the EU) for any product you are considering.
Is MI the same as MIT? Yes. MIT is simply a common abbreviation for methylisothiazolinone, used interchangeably with MI. You may see either abbreviation in scientific literature and consumer discussions. On product labels, it will appear as "methylisothiazolinone" in the INCI (International Nomenclature of Cosmetic Ingredients) list.
Sources
- European Scientific Committee on Consumer Safety (SCCS). Opinion on Methylisothiazolinone (P94) in leave-on cosmetic products. SCCS/1521/13. Adopted 27 March 2014. - European Scientific Committee on Consumer Safety (SCCS). Opinion on Methylisothiazolinone (MI) in cosmetic products. SCCS/1570/15. Adopted 15 December 2016. - European Parliament and Council. Regulation (EC) No 1223/2009 on cosmetic products. Annex V (list of permitted preservatives), as amended by Commission Regulation (EU) 2016/1198. - Uter W, Gefeller O, John SM, Schnuch A, Geier J. Contact sensitisation to methylisothiazolinone in Europe: a multi-centre study. Contact Dermatitis. 2014;70(5):269-275. - Lundov MD, Moesby L, Zachariae C, Johansen JD. Contamination versus preservation of cosmetics: a review on legislation, usage, infections, and contact allergy. Contact Dermatitis. 2009;60(2):70-78. - American Contact Dermatitis Society. Allergen of the Year 2013: Methylisothiazolinone. Dermatitis. 2013;24(1):2-4.
