Imidazolidinyl Urea
In plain terms
A very common formaldehyde-releasing preservative, milder than diazolidinyl urea but the same kind of concern: the released formaldehyde is a skin sensitizer. It is not the same as plain urea. Formaldehyde-free options exist.
What the evidence shows
Imidazolidinyl urea is one of the most widely used preservatives of its type, and like its close relative diazolidinyl urea it works by gradually releasing formaldehyde to hold back microbes. It releases less formaldehyde than diazolidinyl urea does, so it sits a step lower on the scale, but the same basic concern applies: the released formaldehyde is a skin sensitizer, and people who become allergic to it commonly cross-react with formaldehyde and the other releasers. It is often paired with parabens to cover a broad range of bacteria and mold. The concern on skin is allergic contact dermatitis rather than the inhaled-formaldehyde cancer question, which is a separate exposure route, and the safety panel permits it at low concentrations where the amount of formaldehyde given off is small. Its name causes the same confusion diazolidinyl urea does: it shares the word urea with the plain humectant but is a different, synthetic preservative that does release formaldehyde, while plain urea does not. Because formaldehyde-free preservatives are readily available and do the same work, this is an easy one to pass on, especially for sensitive or reactive skin.
On the label
Listed as imidazolidinyl urea, often alongside parabens. It is a formaldehyde-releasing preservative, milder than diazolidinyl urea but the same kind of concern, and not the same as plain urea. Formaldehyde-free options are easy to find.