Methylisothiazolinone (MI/MIT)
What it is
Methylisothiazolinone (MI, also written MIT) is a synthetic preservative that kills bacteria, yeast, and mold so water-based products don't spoil while they sit on a shelf or in a bathroom for months. It's used alone or blended with a close chemical cousin, methylchloroisothiazolinone (MCI), in a roughly 3:1 mix originally sold industrially as "Kathon CG." You'll find it in baby wipes and other wet wipes, shampoo, conditioner, liquid laundry detergent, fabric softener, dish soap, all-purpose cleaners, and paints — essentially anywhere a manufacturer needs a cheap, broad-spectrum preservative for a water-based formula. Its use expanded sharply starting in the mid-2000s, partly as manufacturers moved away from older formaldehyde-releasing preservatives.
Why it's flagged
MI is one of the most thoroughly documented contact allergens in modern dermatology. As its use spread through cosmetics and household products in the 2000s and 2010s, dermatology clinics in North America and Europe tracked a sharp, sustained rise in patch-test-confirmed allergic reactions, which led the American Contact Dermatitis Society to name it "Allergen of the Year" in 2013. Under the EU's CLP chemical classification system, MI carries a harmonized classification as a Category 1A skin sensitizer (H317) — the strongest sensitization category — a classification that became mandatory across the EU in 2020. Based on sensitization data, the EU's Scientific Committee on Consumer Safety (SCCS) concluded in a 2015 opinion that MI is not shown safe at any concentration tested in leave-on cosmetic products, and that the previous 100 ppm limit was too high even for rinse-off products. The European Commission acted on that opinion with two regulations: one in 2016 banning MI outright from leave-on cosmetics (effective February 2017), and one in 2017 cutting the allowed concentration in rinse-off cosmetics from 100 ppm down to 15 ppm / 0.0015% (effective 2018). Health Canada made nearly identical changes to its Cosmetic Ingredient Hotlist in 2015–2016.
How concerned to be
This is a genuinely "context-dependent" ingredient, and the line that matters is rinse-off versus leave-on exposure — but it's also an ingredient where US and EU/Canadian regulators do not fully agree, which is worth being upfront about. The EU and Canada have taken the stricter position: MI is banned outright from leave-on cosmetics, and capped at 15 ppm (0.0015%) in rinse-off products like shampoo, hand soap, and most liquid detergents, based on SCCS's conclusion that no tested concentration was shown safe for leave-on use. Patch-test positivity has been trending down since those mid-2010s restrictions took effect — North American Contact Dermatitis Group data show 13.8% positivity in 2019–2020 and 11.5% in 2021–2022, still meaningful but easing from the peak that made MI "Allergen of the Year." The US Cosmetic Ingredient Review (CIR) panel, by contrast, is notably more permissive: its 2020 safety assessment concluded MI is safe in rinse-off products at concentrations up to 100 ppm — nearly seven times the EU/Canada limit — and that leave-on products can also be safe if formulated to be non-sensitizing, as demonstrated through a quantitative risk assessment, rather than banning leave-on use outright as the EU and Canada did. So this isn't a case of every regulator agreeing on one safe threshold; the US scientific position is meaningfully looser than the EU/Canada one, and there's no US legal cap at all — American manufacturers have largely followed the EU's lead voluntarily on leave-on products, but it isn't required or guaranteed. Leave-on exposure — moisturizers, leave-on cosmetics, and baby/wet wipes (treated as leave-on since they're wiped on and not rinsed off) — is where the vast majority of the documented allergy epidemic originated, which is exactly why the EU and Canada banned MI outright from that category rather than just lowering a limit. Bottom line: fine at the low, regulated levels in rinse-off products; worth avoiding in leave-on products, particularly wipes and infant skincare — especially in the US, where there's no legal backstop and the reference safety threshold manufacturers may be working from is looser than in the EU or Canada.
What it's called on a label
Look for any of these on an ingredient list: MI, MIT, Methylchloroisothiazolinone and Methylisothiazolinone, MCI/MI, Kathon CG, 2-Methyl-4-isothiazolin-3-one, 2-Methyl-3(2H)-isothiazolone, Neolone 950, Methylisothiazolinone, Methylchloroisothiazolinone (and) Methylisothiazolinone.
Sources
- Methylisothiazolinone, Dermatitis (journal of the American Contact Dermatitis Society) (2013)
- Opinion of the Scientific Committee on Consumer Safety (SCCS) on Methylisothiazolinone (MI) (P94), in cosmetic products (sensitisation only), European Commission Scientific Committee on Consumer Safety (SCCS) (2015)
- Commission Regulation (EU) 2016/1198 of 22 July 2016 amending Annex V to Regulation (EC) No 1223/2009 on cosmetic products, European Commission / EUR-Lex (2016)
- Commission Regulation (EU) 2017/1224 of 6 July 2017 amending Annex V to Regulation (EC) No 1223/2009 on cosmetic products, European Commission / EUR-Lex (2017)
- Substance Information: 2-methyl-2H-isothiazol-3-one (harmonised classification, Skin Sens. 1A, H317), European Chemicals Agency (ECHA) (2020)
- Final Amended Safety Assessment of Methylisothiazolinone as Used in Cosmetics, Cosmetic Ingredient Review (CIR) Expert Panel (2020)
- North American Contact Dermatitis Group Patch Test Results: 2021-2022, Dermatitis (journal) (2025)
- Health Canada informing Canadians of risks related to certain preservatives (MI/MCI) in cosmetic, non-prescription and natural health products, Health Canada (2016)
- Allergens in Cosmetics, U.S. Food and Drug Administration (FDA) (2024)