Methylchloroisothiazolinone

Methylchloroisothiazolinone (MCI) is a potent preservative used at very low concentrations (often in the ppm range, historically in combination with methylisothiazolinone) yet is strongly associated with both irritant dermatitis and exceptionally high rates of allergic contact dermatitis on patch testing. Even rinse-off exposure can trigger significant reactions in sensitized individuals, and leave-on use has been restricted/banned in multiple regions due to the high clinical burden. Given its well-documented sensitization potential and risk to eczema-prone and compromised skin, it warrants a very high irritancy score for patient safety. Safety Notes: In consumer cosmetics, methylchloroisothiazolinone (MCI) is almost always used in combination with methylisothiazolinone (MI) as the legacy preservative blend (e.g., Kathon CG), and real-world formulas typically deliver MCI in the low-ppm range; very low-end usage is seen in well-preserved rinse-off products around ~0.0002% (2 ppm) MCI. The upper end observed in OTC products aligns with the EU maximum for the MCI/MI mixture in rinse-off products (0.0015% total), which corresponds to ~0.00044% MCI if using the common 3:1 MI:MCI ratio; however, market documentation and labeling practices often quote the blend limit, so high-strength rinse-off products can effectively represent up to ~0.0015% as the preservative system concentration, while leave-on use is banned in the EU and largely avoided in modern skincare due to sensitization concerns.