Methylisothiazolinone is a synthetic chemical used for controlling microbial growth in water-containing solutions. Any product that contains water is particularly susceptible to being spoiled by microbial growth, causing problems such as discoloration, unpleasant odors or breakdown. Under certain conditions, microorganisms can even grow to potentially harmful levels. Preservatives are designed to help prevent these problems. Methylisothiazolinone is only one of a very limited number of broad spectrum preservatives - meaning it is effective against a variety of bacteria, yeasts and molds. .
Methylisothiazolinone and Methylchloroisothiazolinone were very common preservatives found in many liquid personal care and leave-in products throughout the 1980's and 90's - Shampoo, conditioner, hair color, body wash, lotion, sunscreen, mascara, shaving cream, baby lotion, baby shampoo, hairspray, makeup remover, liquid soaps and detergents. However, due to concerns about potentially increasing rates of skin sensitivity to MI, many cosmetic and personal care product manufacturers have begun removing it, especially from leave-on products such as lotions and wipes. According to the U.S. Food and Drug Administration (FDA), methylchloroisothiazolinone is a standardized chemical allergen. It has been banned in leave-on products in Europe since 2016.
If you read a label these ingredients may appear as Methylisothiazolinone (MIT): 2-methyl-4-isothiazoline-3-one, Neolone 950 preservative, MI, OriStar MIT and Microcare MT. Methylchloroisothiazolinone (CMIT): 5-Chloro-2-methyl-4-isothiazolin-3-one and MCI. All of these have one thing in common - they can cause an allergic reaction.