The rise in male infertility and the decline in human sperm counts could be linked with chemicals in the environment known as anti-androgens which block the action of the male sex-hormone testosterone, a study has found.

Scientists have identified a group of river pollutants that are able to stop testosterone from working. These anti-androgens have been linked with the feminisation of fish in British rivers and could be affecting the development of male reproductive organs in humans, it found.

The study has established a link between anti-androgens released into rivers from sewage outflows and abnormalities in wild fish where males develop female reproductive organs. It is the first time that anti-androgens and hermaphrodite fish have been linked in this way.

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Until now it was thought another class of chemicals, which mimic the effect of the female sex-hormone oestrogen, were responsible for sex-changed fish. However the latest study indicates that the cause may be the result of a rather more complicated interaction taking place between different pollutants.

Dr Susan Jobling of Brunel University, is one of the authors of the study carried out with colleagues from Exeter and Reading universities and the Centre of Ecology and Hydrology. She said: "We have identified a new group of chemicals in our study on fish, but we do not know where they are coming from or what they are. We've only been able to measure their testosterone-blocking potential."

For the study, published in thejournal Environmental Health Perspectives, the scientists analysed anti-androgenic activity in samples of river water taken near 30 sewage outflows. They were able to demonstratestatistically that this activity could be linked with hermaphrodite fish found in the same rivers.

Dr Jobling said that there are several chemicals in widely-used pharmaceuticals and pesticides that are known to have anti-androgenic activity. They included flutamide and cyproterone, used to treat prostate cancer, and several compounds found in agricultural pesticides.

The scientists detected relatively high levels of anti-androgenic chemicals near sewage outflows – suggesting they came from domestic sources. One possibility is that drugs excreted from the body may end up in rivers. However the scientists have not discounted the idea that anti-androgens may also be seeping into rivers as run-off from agricultural land.

Scientists first detected sex-change fish in British rivers more than 20 years ago. During the same period, medical researchers found that human sperm counts have been falling in several countries over a period of 30 years or more. This has been matched by a corresponding rise in other male reproductive problems, such as the congenital condition testicular dysgenesis, which can affect fertility.