What it is
Methylchloroisothiazolinone belongs to a group of compounds called Isothiazolinones.
What it does
Methylchloroisothiazolinone is a preservative: it protects products from the growth of fungi, bacteria and yeast, thus prolonging their shelf life.
Methylchloroisothiazolinone is an irritant and a strong allergen. Because of that, it is mostly used, in small concentrations, in rinse-off products so the chances of it causing negative reactions are much smaller.