Diazolidinyl Urea

Diazolidinyl urea is a water-soluble preservative. This preservative is considered safe for use at concentrations up to 0.5%, although it’s usually present at lower concentrations because it’s typically part of a blend with other preservatives (such as parabens). Diazolidinyl urea can be a formaldehyde-releasing preservative. Although that sounds scary, the amount of formaldehyde released is well below the recommended limits of exposure. Moreover, other ingredients (such as proteins) in a product cause the free formaldehyde to evaporate and become inactive before it could possibly harm skin. See formaldehyde-releasing preservative

See: Formaldehyde-Releasing Preservative