A water-soluble preservative that is very effective against a broad range of bacteria. Also has some anti-fungal ability. This preservative is considered safe for use in amounts up to 0.5%, though it is usually used below that amount due to it being part of a blend with other preservatives (such as the parabens). Diazolidinyl urea can be a formaldehyde-releasing preservative (Source: Contact Dermatitis
, December 2000, pages 339–343). Although that sound 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 formaldyde to evaporate and become inactive before it coud possibly harm skin.