| My doctor at first thought the eczema-like rash that appeared on my left arm was due to poison ivy. But after initially responding to a steroid injection, the rash returned on my arm and also appeared on my leg and back of my neck. By August, it was popping up all over my body, and various steroids did little to help. Sometimes I scratched so hard that it bled. Eventually, my doctors gave me something called a “patch test” that involves taping dozens of allergen patches to my back for 48 hours to see if you have a reaction. It’s as unpleasant as it sounds. To my surprise, it revealed I had an allergy to two preservatives that I had never heard of: methylisothiazolinone and methylchloroisothiazolinone. As I soon found out, these and other closely related preservatives are everywhere. They’re in shampoos, liquid dish detergents and soaps, laundry detergent, dishwasher rinse aid and cosmetics. They’re also in household paint and cleaners. And they’ve caused a worldwide epidemic of contact skin allergies in recent years, according to a 2023 study in JAMA Dermatology.
“In the last 15 years, we have seen a significant increase in the number of people who have contact allergies” to the preservatives, says Margo Reeder, a dermatologist at the University of Wisconsin School of Medicine and Public Health, and lead author on the study. “We need preservatives, but this class of preservatives is unfortunately causing a lot of contact allergies.” How common these allergies are in the general population isn’t known, as most people don’t have severe enough symptoms to get a patch test. But they are one of the most common findings from the tests. The JAMA Dermatology paper found 15% of people tested in the US and Canada in 2017 and 2018 had a contact allergy to methylisothiazolinone, or MI. A 2024 analysis that looked at a broader range of isothiazolinone preservatives found that 22% of people tested in North America had an allergy to one or more of them. The epidemic began after manufacturers started putting MI in various personal care products about 20 years ago, including baby wipes. Researchers at the time assumed, based on a flawed interpretation of animal data, that it would be less allergenic than the methylchloroisothiazolinone combination product they previously favored. Contact allergies to the preservatives soon started becoming more common in Europe. In the US, doctors in 2014 reported strange rashes on the bottoms and faces of young kids who’d been cleaned with baby wipes.
The European Union cracked down on MI starting in 2013, eventually banning it in leave-on products like wipes, and sharply restricting levels in rinse-off products like shampoo. Allergy rates there then declined. But in the US, it’s still allowed, though the independent Expert Panel for Cosmetic Ingredient Safety recommends limiting use to non-sensitizing levels. Columbia University dermatologist Donald Belsito, who is on the panel, told me that levels of the preservatives that remain in products here are unlikely to trigger contact allergies in people who don’t already have them. But for someone like me, who is already sensitized, even small amounts of MI can trigger a rash, he said. After the test result came in, my wife and I spent hours scanning the ingredient lists of every liquid in our medicine cabinet, laundry room and under the kitchen sink. Isothiazolinones were everywhere: in the shampoo, in liquid soaps, in dishwasher rinse aid, even our “hypoallergenic” laundry detergent. We got rid of all of them.
It’s still not clear whether MI was the main cause of my rash. I suffered from bad eczema as a child, and a recent biopsy was positive for eczema. So it may have been a middle-aged eczema flareup that was merely exacerbated by the preservatives. The good news: Between the allergen avoidance and a fancy new eczema drug I am taking, the rash is slowly subsiding. If you suspect you might have an MI allergy and don’t want to scan labels, you can buy brand products from multinational companies that do business in Europe. Columbia’s Belsito says big international companies such as Unilever Plc or Procter & Gamble Co. don’t like to reformulate products for each market, so their products may be less likely to contain high levels of isothiazolinones. — Robert Langreth | 
