Methylchloroisothiazolinone
In plain terms
A strong preservative almost always sold blended with a close relative, methylisothiazolinone, under names like Kathon CG. It kills bacteria and mold well, and it is also one of the most common causes of preservative skin allergy, which is why Europe banned it from leave-on products.
What the evidence shows
Methylchloroisothiazolinone, usually paired three-to-one with methylisothiazolinone as the blend Kathon CG, is a potent preservative and a potent skin sensitizer. Between about 2010 and 2015 it drove a measured rise in contact allergy across Europe and North America: patch-test clinics saw positive reactions climb into the high single digits and, in North America, above 10% of tested patients by 2017 to 2018, most often as hand or face eczema from wipes, cleansers, and other rinse-off and leave-on products. The reactivity is high even at a few parts per million, which is why standard patch testing had to raise its test concentration to catch cases. Dermatology made the point plainly by naming the co-preservative Allergen of the Year for 2013. Regulators followed the evidence: the European Union banned methylisothiazolinone from leave-on cosmetics in 2017, treating wet wipes as leave-on, and restricted the chloro blend to 15 parts per million in rinse-off products. Because gentler preservatives do the same job, and because a sensitized person then reacts to the same chemistry in household cleaners and paints, this is one to skip rather than manage.
On the label
Listed as methylchloroisothiazolinone, or as the pair 'methylchloroisothiazolinone (and) methylisothiazolinone,' or under a trade name like Kathon CG, usually near the preservative end of the list. Once patch testing confirms this allergy, the same chemistry turns up in household cleaners and wall paint, common triggers to check as well.