The Environment Ministry announced Friday that nonyl phenol, an organic chemical used in cleaning products, disrupts the endocrine system of "medaka" killifish and causes males to assume female reproductive traits.

The study, believed to be the first to prove the chemical is an endocrine disrupter, found that male medaka in water with relatively low concentrations of nonyl phenol began developing eggs inside their testes, the ministry said.

While many chemical substances are suspected of affecting human hormones, this development marks the first time for any government to officially categorize a substance as an endocrine disrupter, according to ministry officials.