Synthetic, formaldehyde-releasing preservative that has mixed research on its potential to aggravate skin. Despite its connection to formaldehyde, the Cosmetic Ingredient Review Board has confirmed the safety of this preservative in three separate reviews, each spaced several years apart.
References for this information:
Medocyna Pracy, Volume 66, Issue 3, 2015, pages 327-332
Contact Dermatitis, January 2010, pages 18-31