Upgrades to a wastewater treatment plant in Colorado helped filter out gender-bending chemicals that were affecting fish, U.S. researchers reported on Monday.

They said male fish are now taking longer to be feminized by so-called hormone disrupters in one creek in Colorado after standard improvements to a wastewater treatment plant in Boulder in 2008.

David Norris of the University of Colorado at Boulder had earlier found ethinylestradiol, a female hormone used in contraceptives, in Boulder Creek. His team also had measured bisphenol A and phthalates, which are both used in plastics and which can mimic the effects of hormones, as well as pesticides and antidepressants in the water.

Most probably came from products flushed down toilets and drains, Norris told a meeting of the Endocrine Society in San Diego.

We excrete natural and synthetic estrogens and use shampoos, detergents and cosmetics containing a variety of hormone disrupters that wind up in waterways, he said in a statement.

Our bodies are being exposed every day to a variety of chemicals capable of altering our physiological development, including impacts on sensitive human fetuses.

Fish and frogs give early warning of these chemicals as their bodies are bathed in them all day.

For years people around the United States, Britain and elsewhere have reported strange changes in fish and amphibians -- for instance, male fish with female sex organs.

Norris and colleagues held fish in a tank filled with water from Boulder Creek and outflow from the wastewater treatment plant before it was was upgraded.

After seven days, adult male fathead minnows became feminized, looking and acting like females and with elevated levels of a protein known as vitellogenin that is normally produced by females.

After the upgrade, they repeated the experiment. It took a full month before they saw any effects on minnows in a tank containing 100 percent effluent water directly from the treatment plant.

The upgrade was nothing special, Norris said by email.

It was simply a planned change from a trickling filter process to an activated sludge process which is more efficient, he said.

Both systems are in use throughout the country, but the trickling filter process is not efficient enough for a city the size of Boulder (hence the need for the upgrade).

Norris said the upgrade was not prompted by reports about the fish. It is not clear yet how the chemical concentrations of the estrogenic hormones were reduced, only that both our chemical data and biological data indicate considerable reduction in the estrogenic nature of the effluent, he said.

Rainfall and snow melt may affect the process and do not necessarily help, Norris noted -- they may simply wash contaminants through the treatment plant faster and directly into the creek.

Our testing has been done at low flow in the summer when the system is rarely diluted by rainfall and is 50-80 percent effluent, he added.

(Editing by Mohammad Zargham)