Diazolidinyl Urea (Formaldehyde)
Commonly found in: sunscreen, lotion, conditioner, and shampoo.
Why you should avoid it: "This is a commonly used preservative that releases formaldehyde into products. Formaldehyde is a carcinogen," Harper warned. This ingredient is known an "in situ" reaction, she explained, meaning the original chemical turns in to something else once it's in the formula. So be warned: formaldehyde won't appear on the ingredient list, but diazolidinyl urea will.