Even levels of synthetic oestrogen* as low as one part per billion in water are sufficient to turn male fish female. Entire fish stocks in some parts of the UK river system have been affected.

One UK Environment Agency study on male roach in ten rivers found that 10% were sterile, another 25% had damaged sperm and nearly 50% had eggs in their testes. The agency is worried that UK drinking water, and therefore humans, may also be being affected. The oestrogen levels are below those detected by current water quality testing.

* a female hormone found, for instance, in the Pill