Methylisothiazolinone.

Methylisothiazolinone (MI) is a preservative that causes allergic contact dermatitis in about 6% of people, but standard allergy tests miss roughly 40% of MI allergies because the test concentration is too low. European data shows MI allergy rates around 1.5%, but U.S. rates are largely unknown—meaning many people with MI sensitivity may go undiagnosed.