What it is
Methylisothiazolinone is a broad-spectrum biocide that prevents microbial spoilage in rinse-off and leave-on products, often replacing or supplementing other preservative systems. It is effective at very low concentrations and is frequently paired with methylchloroisothiazolinone in the blend commonly listed as Kathon. In sunscreens it appears in the water phase to keep emulsions microbiologically stable over their shelf life.
Safety & warnings
Methylisothiazolinone is a recognized contact allergen and a notable cause of allergic contact dermatitis, particularly from leave-on products, which led regulators in several markets to restrict its concentration and limit its use to rinse-off applications. People with known isothiazolinone sensitivity should avoid it. It is used at trace preservative levels and is not considered comedogenic.
Origin & sourcing
Synthetic; manufactured industrially rather than derived from natural sources.
Questions everyone asks
Why is methylisothiazolinone used in sunscreen?
It acts as a preservative, keeping the water-based emulsion free of bacteria and mold so the product stays safe and stable throughout its shelf life.
Can methylisothiazolinone cause skin reactions?
Yes. It is a known contact allergen and can trigger allergic contact dermatitis in sensitized individuals, which is why its use in leave-on products is restricted in some regions.