What it is
DMDM hydantoin is a member of the imidazolidinyl class of antimicrobial preservatives that work by slowly releasing small amounts of formaldehyde in the water phase of a product. It provides broad-spectrum protection against bacteria and, to a lesser degree, yeast and mold, and is typically used at low concentrations. It is commonly paired with other preservatives to give a balanced antimicrobial system.
Safety & warnings
DMDM hydantoin is a formaldehyde releaser, and the formaldehyde it liberates is a recognized contact allergen; people who are sensitized to formaldehyde or to formaldehyde-releasing preservatives may react to it. It is generally well tolerated by non-sensitized skin at the low levels used for preservation, but it is a common consideration in patch testing for cosmetic dermatitis. It is not regarded as comedogenic.
Origin & sourcing
Synthetic. It is produced industrially by reacting dimethylhydantoin with formaldehyde, and it is not derived from natural sources.
Questions everyone asks
Why is DMDM hydantoin used in sunscreen?
It preserves the water phase of the formula, preventing microbial growth so the product stays safe to use over its shelf life.
Does DMDM hydantoin release formaldehyde?
Yes. It is a formaldehyde-releasing preservative that liberates small amounts of formaldehyde, which is why people sensitized to formaldehyde may wish to avoid it.