Methylisothiazolinone

Methylisothiazolinone is a preservative with a clear, well-documented problem: it is a strong skin sensitizer that set off one of the largest contact-allergy epidemics dermatology has seen. The trouble began in 2005, when it started being used on its own at up to twenty-five times the previous concentration, and allergic contact dermatitis climbed steeply enough that the American Contact Dermatitis Society named it Allergen of the Year in 2013. In North American patch testing it became the single most common preservative people react to, ahead of formaldehyde. Regulators responded: Europe and Canada banned it from leave-on products like creams and lotions and capped it at a low level in rinse-offs, after which sensitization fell but stayed high. The United States is more permissive, allowing leave-on use if a formulator can show the level is low enough not to sensitize, which is why the label is where you catch it. Two things keep this in proportion. The concern is allergic rashes in people who have become sensitized, not cancer or systemic poisoning, so it is not a hazard to everyone equally. But because the sensitizer is potent, the sensitized group is large, and effective alternative preservatives exist, avoiding it is the low-cost call. It is also worth knowing beyond cosmetics: this preservative is volatile and common in water-based wall paint, where it can cause a rash just from being in a freshly painted room.