Methylisothiazolinone

Methylisothiazolinone is a preservative strongly associated with high rates of allergic contact dermatitis in patch testing and real-world outbreaks, even at low leave-on concentrations (historically up to ~100 ppm) and in rinse-off products. Because sensitization can be delayed and severe—particularly in eczema-prone or barrier-impaired skin—its presence meaningfully elevates the risk of significant irritation/inflammation and warrants a very high irritancy score for patient safety. Safety Notes: In modern consumer skincare, methylisothiazolinone (MI) is most often encountered in rinse-off products at trace levels around 1–5 ppm (0.0001–0.0005%), used as part of preservation systems and sometimes residual from supplier blends; MI is largely absent from leave-on products in regulated markets due to sensitization concerns. The highest OTC levels historically and still occasionally seen in some rinse-off products are around the EU-permitted maximum of 0.01% (100 ppm) for rinse-off; MI is not permitted as a preservative in leave-on cosmetics in the EU, and many brands globally have voluntarily reduced or eliminated it from leave-on formats.