Beginning Sept. 1, the European Union has banned an ingredient commonly found in gel nail polish. The banned substance, TPO (trimethylbenzoyl diphenylphosphine oxide), gives nails a super glossy sheen and a quick dry time under UV or LED light. However, it has been deemed toxic in the EU after several studies have shown that it can cause fertility issues and adversely affect reproductive health. Gel nail polish, which has gained popularity since the 2000s, is cured under UV or LED light, resulting in a hard, durable manicure.