Methylisothiazolinone: Preservative or allergy trigger?
Arin Editorial · how this is made
You will see 'Methylisothiazolinone' or simply 'MI' on labels. It is a preservative that prevents water-based cosmetics from spoiling. But allergy debates have made it one of the most talked-about ingredients in recent years.
What is this ingredient?
Methylisothiazolinone is a synthetic preservative that stops bacteria, yeast and mold from growing in water-based products. It is common in rinse-off products such as shampoo, conditioner and body wash. It works at very low concentrations, so only small amounts are added to formulas.
What does it do?
Water is a good breeding ground for microbes. Without preservatives, products spoil over time: they change color and smell, and harmful microorganisms can multiply. MI stops that spoilage. It can be used alone or with methylchloroisothiazolinone (MCI); together they provide broader protection.
Where does the allergy debate come from?
The real issue is allergy. MI became one of the most common causes of contact dermatitis, a skin allergy. Allergy rates rose especially in leave-on products like wet wipes and lotions. The European Union therefore banned MI in leave-on products and allowed up to 0.0015% (15 ppm) in rinse-off products. The CIR panel, on the other hand, says up to 100 ppm is safe in rinse-off products. Those two different limits show the balance between allergy risk and the need for preservation.
What do safety data say?
In its neat form, MI is considered corrosive to the skin. But the levels used in cosmetics are very low. According to the CIR compilation, lotion, shampoo and sunscreen formulas containing 100 ppm did not irritate rabbit eyes. In a human eye study, a 100 ppm solution caused brief mild redness and tearing that resolved within an hour.
In the EU it is limited to 0.0015% (15 ppm) in rinse-off products and banned in leave-on products; the CIR panel considers up to 100 ppm safe in rinse-off products.
If you see 'Methylisothiazolinone' on a label, first check whether the product is rinse-off or leave-on. In rinse-off products such as shampoo and body wash, the risk is low at permitted doses. Avoid it in leave-on products, especially moisturizers, sunscreens and wet wipes. If you have had allergic contact dermatitis, you may need to avoid this ingredient entirely. Be careful if you have sensitive skin.