There are two types of cosmetic ingredients: Maleic acid is firmly in Category #2. It sounds like it should come with a hazmat suit and a warning label, yet it’s quietly hanging out in some of the most popular hair products on the market, doing the cosmetic equivalent of unpaid overtime. So let’s talk about maleic acid. Where it came from, what it does, why your hair might secretly love it, and whether you should panic when you see it on an ingredient list (spoiler: probably not). What Is Maleic Acid? Maleic acid is an organic acid used primarily in hair care products, especially