Methylchloroisothiazolinone (MCI/CMIT)
Methylchloroisothiazolinone (MCI) is a highly effective preservative used in shampoos and detergents, usually in a mixture with methylisothiazolinone (MCI/MI). It is one of the strongest skin sensitisers among the isothiazolinones. In the EU it is banned in leave-on cosmetics, and in rinse-off products it is permitted only at a very low limit.
Risk range
1 is the lowest and 10 the highest risk. The range depends on the amount used, whether the product is rinsed off and how it is used. The Temiz İçerik app uses the same ranges.
The lower end of the range (about 5.5–6.5) is for rinse-off products that comply with the EU limit; the upper end (about 7.5–8.5) is for leave-on products, in which it is banned in the EU.
In detergents and cleaners it is a sensitiser even at ppm levels; frequent hand contact and concentrated products increase the risk.
What is MCI and what is it used for?
Methylchloroisothiazolinone is a chlorine-containing synthetic biocide from the isothiazolinone family. In practice it is almost always used as a commercial mixture with methylisothiazolinone in a ratio of about 3 to 1. This is why the two substances usually appear side by side on labels.
At very low concentrations, this mixture provides strong protection against bacteria, yeasts and moulds. Because it is cheap and effective, it has been used for decades in cosmetics, detergents, paints and industrial fluids.
Which products contain it?
- Shampoos, shower gels, conditioners and liquid soaps (in the EU, only in rinse-off products)
- Washing-up liquids, laundry detergents and fabric softeners
- Surface and floor cleaners
- Water-based paints, adhesives and metalworking fluids
Is it harmful? The scientific assessment
The only significant known problem with MCI is allergic contact dermatitis, but this problem is serious. MCI is an even stronger sensitiser than MIT. After the MCI/MI mixture began to be used in cosmetics, it became a common cause of contact allergy in Europe and one of the substances routinely included in patch test series.
For this reason, the EU has banned the MCI/MI mixture in leave-on cosmetics and limited it to a total of 0.0015% (15 ppm) in rinse-off products. On the EU list, this mixture appears in its form containing magnesium chloride and magnesium nitrate as stabilisers. Turkish legislation is aligned with the EU.
In cleaning products, MCI is subject to chemicals and biocides legislation and is classified as a strong skin sensitiser; products above a certain threshold must carry a label warning. In pure form it is corrosive and toxic, but the concentration in products is very low. It is not classified as a carcinogen.
Who should be careful?
- People diagnosed with an MCI/MI or MIT allergy: Cross-sensitivity between the two substances is common; they are advised to look for both on labels.
- People with hand eczema and those who work in cleaning: Wearing gloves is important during prolonged contact with detergents.
- People with recurring eczema on the scalp, face or hands: They can have a patch test done by a dermatologist to find the trigger.
What to do when you see it on a label
If you don't have an allergy and the product is a rinse-off product, a shampoo containing MCI/MI is acceptable within the legal limits. If you see it in a leave-on product or a wet wipe, the product may not comply with EU/Turkish rules. For sensitive skin, you can choose products made with preservatives of lower allergy potential, such as phenoxyethanol.
Frequently asked questions
Are MCI and MIT the same thing?
No, they are different substances from the same family. MCI contains chlorine and is a stronger allergen; in commercial products the two are mostly used as a mixture.
How does it appear on the label?
In cosmetics it is written side by side as “Methylchloroisothiazolinone, Methylisothiazolinone”. In cleaning products, the phrase “reaction mass of CMIT/MIT” may also be seen.
Is MCI allergy dangerous?
It is not a life-threatening type of allergy; but it can cause itchy, persistent eczema, and the sensitivity is usually permanent. See a dermatologist about symptoms.
Sources
Updated: 2026-10-11 · How we score
This page is for general information and is not personal medical advice. Warnings on the product label and the advice of a health professional always come first.