Imidazolidinyl UreaUpdated a month ago
An antimicrobial preservative that slowly releases carcinogenic formaldehyde into a product over time (see Formaldehyde). People may develop a contact allergy to formaldehyde or to the ingredient itself. The American Society of Contact Dermatitis lists it as one of its core allergens, even in concentrations as low as 2%. Longer storage times and higher temperatures increase the amount of formaldehyde released from the formaldehyde-releaser, which can lead to a higher risk of skin reaction.
Sources:
https://online.personalcarecouncil.org/ctfa-static/online/lists/cir-pdfs/pr254.pdf
https://www.cancer.org/cancer/cancer-causes/formaldehyde.html
https://www.contactderm.org/i4a/pages/index.cfm?pageID=3563
http://www.safecosmetics.org/get-the-facts/chemicals-of-concern/formaldehyde/
https://ntp.niehs.nih.gov/ntp/htdocs/chem_background/exsumpdf/imidazolidinylurea_508.pdf