"Trimethylbenzoyl diphenylphosphine oxide is a photoinitiator that, when exposed to light (UV or LED), triggers a chemical reaction that causes gel polish to harden, giving it durability and shine," explains Michelle Henry,

While many gel nail polishes use a chemical called TPO now banned in Europe, manufacturers are already selling TPO-free versions of their polishes.

A chemical in gel nail polish has been banned in Europe, prompting nail salons and customers to think twice about gel manicures. Trimethylbenzoyl diphenylphosphine oxide, known as TPO, is an ingredient that makes wet gel polish harden under a UV light.