Methylisothiazolinone, or MI for short, is a common ingredient in many skin care products that can cause itchy rashes. A preservative that kills bacteria and prevents fungus from forming, MI can be found in shampoos, conditioners, dishwashing liquids, detergents, beauty products and baby wipes. It can also be found in some water-based paints.  

Methylchloroisothiazolinone, or MCI for short, is a similar ingredient that can also trigger rashes. Over the years, MI and MCI have been used in combination. But increasingly, MI is being used on its own and in greater concentrations.

Because of increasing exposure and use of higher concentrations of MI in skin care products, dermatologists named MI the Contact Allergen of the Year in 2013 to increase awareness.

A rash, called allergic contact dermatitis, appears when a person who is sensitive to these chemicals are exposed, resulting in itchy red bumps that can sometimes sting or burn.  The location of the rash depends on the exposure area, and often begins after using the product(s) for days or weeks without issue.  

Treatment of allergic contact dermatitis involves avoiding the offending agent and applying a topical steroid to reduce itching and inflammation.

Identifying the cause of allergic contact dermatitis can be difficult. Your doctor may want you to bring in your skin care products to review their ingredient lists. Some products may need to be discontinued if the skin improves after non-use. Your doctor might recommend patch testing, which involves exposing the skin on your back to multiple chemicals, and evaluating the results 4-5 days later.  

The Good Guide provides a list of products that contain MI. If you are experiencing an unexplained rash, talk with your doctor.