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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.

The fish in the study were in water with a concentration of 11.6 micrograms of nonyl phenol per liter, a concentration detected in some drainage canals. A microgram is one-millionth of a gram.

The research also found the estrogen receptors of the medaka are highly susceptible to the chemical. Human estrogen receptors are much less receptive to nonyl phenol, meaning it is less harmful to people, ministry officials said.

About 16,500 tons of nonyl phenol were produced in fiscal 2000 in Japan, mainly for use in industrial detergents used in the textile and metal processing industries. It is also used to make resins and plasticizers.

The ministry plans to consult with industries about the possibility of replacing the chemical with other substances.

Nonyl phenol was detected in 617 of 1,574 drainage canals, rivers and lakes in Japan surveyed between fiscal 1998 and 1999 by the Environment Agency, the ministry’s predecessor.

The substance does not affect the endocrine system in concentrations less than 0.6 microgram per liter, according to the ministry survey. Nonyl phenol concentrations exceeded 0.6 microgram per liter at 71 of the locations.

Since fiscal 2000, the ministry has researched the impact on health and the ecosystem of suspected endocrine disrupters.

Taisen Iguchi, a professor at the Okazaki National Research Institutes, said nonyl phenol is certainly an endocrine disrupter as the ministry’s study had proved two vital points.

These are that the effects of the substance were recorded at concentrations found in the environment and that the receptors of the medaka were highly susceptible to it.

He added that authorities should swiftly devise measures to prevent the chemical from being released into the environment.

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