Diazolidinyl urea description
Diazolidinyl urea is a water-soluble preservative. This preservative is considered safe in concentrations up to 0.5%, although it is usually present in lower concentrations because it is often part of a mixture with other preservatives such as parabens. Diazolidinyl ureas may be formaldehyde-releasing preservatives. While that sounds scary, the amount of formaldehyde released is well below the recommended exposure limit. Additionally, other ingredients in the product, such as proteins, can cause free formaldehyde to evaporate and become inactive before it can harm the skin. See formaldehyde-releasing preservatives
An antimicrobial preservative that helps keep your products from going wrong too quickly. It is especially effective against bacteria, especially Gram-negative bacteria, yeasts and molds.
Somewhat controversially, it belongs to the notorious family of formaldehyde releasers. That is, when it is added to a formula, it slowly breaks down to form formaldehyde. We write more here at Dmdm Hydantoin about formaldehyde releasing preservatives and the concerns surrounding them, but don't get too scared, these are more theories than proven facts.
As for diazolidinyl urea itself, a 1990 study wrote that it was a mild cumulative skin irritant at concentrations up to 0.4%, but it was reviewed by CIR (Cosmetic Ingredient Review) in 2006. The review found that when the concentration is 0.5%, it is safe to use because the amount of formaldehyde released will be lower than the recommended limit (less than 0.2%).
All in all, it's up to your personal decision and skin sensitivity.