Methylisothiazolinone (MI) is a synthetic preservative used to kill bacteria and fungi in water-based products. You’ll find it in everything from shampoo and liquid dish soap to interior house paint and industrial cooling systems. It works by breaking down essential proteins inside microbial cells, which prevents the growth of organisms that would otherwise spoil a product or create health hazards. Despite its effectiveness, MI has gained notoriety as a potent skin allergen, and regulations around its use have tightened significantly since the early 2010s.
How MI Works as a Preservative
MI belongs to a chemical family called isothiazolinones. These compounds kill microorganisms by reacting with and oxidizing sulfur-containing proteins (called thiols) that are critical to a cell’s survival. Without functioning thiols, bacteria and fungi can’t maintain their basic metabolic processes and die off. This broad-spectrum action is what makes MI attractive to manufacturers: a small amount can protect a product from bacterial contamination, fungal growth, and even algae.
MI is frequently paired with a related chemical, methylchloroisothiazolinone (MCI), in a 1:3 ratio. This combination is sold under the trade name Kathon and has been a standard preservative system in the cosmetics and personal care industry for decades. MI can also be used on its own at higher concentrations, a practice that became common as regulators placed limits on the MCI/MI blend.
Consumer Products That Contain MI
MI shows up in a surprisingly wide range of household and personal care items. In the bathroom, it’s used in shampoos, conditioners, body washes, hand soaps, moisturizers, sunscreens, and hair dyes. In the kitchen and laundry room, it preserves liquid dish soap, liquid laundry detergent, all-purpose spray cleaners, and window cleaning solutions. It’s also found in air fresheners, including products like Glade Premium Room Spray.
Paint is another major category. MI and its companion MCI are added to water-based latex paints, lacquers, and coatings to prevent bacterial growth in the can and on the painted surface. The preservative slowly leaches from painted walls and exterior surfaces over time, particularly when exposed to rainfall. Sealants, adhesives, floor finishes, and car wash soaps also commonly contain MI.
One detail worth noting: products labeled “natural” or “green” may still contain MI. Because it replaced formaldehyde-releasing preservatives in many formulations, it was initially marketed as a safer alternative. The ingredient can appear on labels under its full name, methylisothiazolinone, or abbreviated as MI or MIT.
Industrial and Commercial Uses
Beyond consumer products, MI plays a significant role in industrial settings. Commercial mixtures of MI and MCI are used as biocides in industrial circulating cooling water systems, including large air-conditioning plants and cooling towers. In these environments, the chemical inhibits the growth of potentially harmful microbes, including Legionella bacteria, which cause Legionnaires’ disease.
MI is also used as a preservative in papermaking, leather treatment, cutting fluids, and fuel storage systems, where it prevents microbial spoilage. Other industrial applications include corrosion inhibitors, construction materials, photochemicals, and lubricants. Essentially, any water-based industrial product that sits in storage or circulates in a system is a candidate for MI preservation.
The Allergy Problem
MI’s effectiveness as a preservative came with a significant downside. As its use expanded through the 2000s, rates of allergic contact dermatitis linked to MI surged. The American Contact Dermatitis Society named MI the Allergen of the Year in 2013, recognizing what researchers described as an epidemic of skin reactions. The pattern mirrored an earlier wave of allergies to the MCI/MI combination in the 1980s. When regulators restricted MCI/MI, manufacturers shifted to MI alone at higher concentrations, which triggered a second peak of sensitization between 2013 and 2014.
Allergic reactions to MI typically appear as red, raised patches on the skin that can progress to blisters, crusting, and even ulceration in severe cases. The reaction isn’t limited to the area where a product was applied. In one clinical study of 23 patients with confirmed MI allergy, 90% had rashes that spread beyond the original contact site, with the legs affected in 90% of cases, the trunk in 60%, and the hands, scalp, and face in 40-50%. Women made up over 91% of the affected patients, with a mean age of 48. This likely reflects higher exposure through cosmetics and cleaning products rather than a biological difference in susceptibility.
What makes MI allergy tricky is that the reaction can develop after months or years of uneventful use. Once sensitized, even trace amounts can trigger a flare. If you develop unexplained rashes, particularly on the hands, face, or scalp, MI exposure through personal care products or household cleaners is worth investigating with a dermatologist through patch testing.
Regulatory Limits on Concentration
In response to the allergy surge, regulators in Europe took decisive action. The European Union’s Scientific Committee on Consumer Safety reviewed the evidence and concluded that MI is only safe in rinse-off cosmetic products (like shampoo or body wash) at a maximum concentration of 15 parts per million, or 0.0015%. That’s roughly one-seventh of the 100 ppm limit that was previously allowed.
For leave-on products like moisturizers, lotions, and sunscreens, the committee found that no concentration of MI could be considered safe from the standpoint of allergy risk. The EU subsequently banned MI from leave-on cosmetics entirely. In the United States, regulation has been less aggressive, though many manufacturers have voluntarily removed MI from leave-on formulations in response to consumer pressure and the weight of clinical evidence.
How to Check Your Products
If you suspect MI sensitivity or simply want to avoid the ingredient, scan the ingredient list for “methylisothiazolinone,” “MI,” or “MIT.” Also look for “methylchloroisothiazolinone” or “MCI,” since the two are frequently used together. The trade name Kathon on industrial or professional products indicates an isothiazolinone-based preservative system.
Keep in mind that MI isn’t limited to products you apply to your skin. Household cleaners, laundry detergent, and paint can all cause reactions through skin contact or airborne exposure. People with confirmed MI allergy often need to audit their entire household, not just their bathroom cabinet, to identify and eliminate sources of exposure.