A nationwide survey of 28 rivers found that effluent discharges were rich in chemicals that mimic the effect of female oestrogens. The rivers affected are believed to include the Aire in Yorkshire and the Dart and Ex in Devon.
The scientists, from Brunel University in Middlesex and the Ministry of Agriculture, Fisheries and Food, detected the effects of the synthetic oestrogens along the entire length of some of the rivers.
Professor John Sumpter, a fish physiologist at Brunel, said the findings showed that oestrogenic pollutants are a widespread problem and probably affect almost every river in the country.
His research on polluting sewage effluent may explain why the sperm counts and overall fertility of Western men have declined in recent decades, as reported in last week's Independent on Sunday.
Professor Sumpter is studying fish because their reproductive systems have much in common with humans. Fish and men, for example, both have two testes - the sperm-producing organs - and anything that can disrupt semen-making in fish is likely to have a similar impact on men.
The results of the river survey led Professor Sumpter to suspect that man-made chemicals in sewage effluent are acting as female oestrogens. He believes they can cause male fish to develop smaller testes and become 'feminised' in that they produce the female yolk protein found in eggs.
'Every chemical that is an oestrogen in fish is equally an oestrogen in mammals and vice versa,' Professor Sumpter said. 'In the past few years more and more man-made chemicals have been shown to be weak oestrogens, but because we use them in large amounts there are large amounts of them in the aquatic environment.'
Scientists investigating falling sperm counts in humans are collaborating with him to test the hypothesis that man-made chemicals in the environment may be behind the drop in male fertility. They are interested in sewage effluent because that it is where most of the estimated 60,000 man-made chemicals end up being discharged into the environment.
Drinking water is suspected as one route by which the oestrogenic chemicals enter the body, because of the volume of sewage effluent being discharged into British rivers. Professor Sumpter is sceptical that drinking water may be to blame because, if it is, it must be in extremely low concentrations, far lower than in other products that can come into direct contact with the skin.
'Knowing the route of exposure is very, very tricky,' he said. 'There may have been a bit in your coffee for example. It might have come through the water in the tap, or through the detergent I used to clean the cup. Or it might have come from the detergent used to clean the glass of the coffee jar beforehand.'
One of Professor Sumpter's collaborators, Richard Sharpe, an expert on male fertility at the Medical Research Council's reproductive biology unit in Edinburgh, has so far found that about 30 of the chemicals found in effluent have oestrogenic properties. The number is almost certain to increase as more pollutants are tested.
The contraceptive pill is not thought to be responsible for the oestrogens in sewage effluent because women excrete its hormones in a biologically inactive form that has no effect on fish.
Professor Sumpter became involved after the Department of the Environment asked him to investigate reports of fish found with both male and female sexual characteristics in the river Lee, a tributary of the Thames.
The scientists have been told by the department, which funded the research, not to name the affected rivers for fear of causing public alarm.
Fish are extremely sensitive to pollution. Professor Sumpter said: 'If you put male fish in sewage effluent channels, they make extraordinary amounts of yolk. They behave as though they are female. Physiologically they are behaving as if they are exposed to oestrogens.'
In every river where the fish survived - some 15 in all throughout mainland Britain - he has found that male trout make female yolk. 'Essentially it is the same wherever you go. There's either a real oestrogen or a chemical that mimics the effect of oestrogen present in effluent.'
Identifying the chemical is a hugely difficult undertaking. One method relies on a process of elimination. The effluent soup can be broken down into its broad constituents and tested for oestrogenic properties. But this takes time and can only give Professor Sumpter a rough idea of the culprit.
Research in the US has shown that nonylphenol, which is found in plastic wrappings, furniture polishes, many toiletries, skin creams, herbicides and pesticides, could behave as an oestrogen. Further research has confirmed that nonylphenol can cause male trout to produce the female yolk protein. Unfortunately, this does not yet constitute proof that it is nonylphenol in the sewage effluent that causes hermaphrodite fish to develop - and it falls far short of proof that it can be responsible for falling sperm counts in humans.
The Drinking Water Inspectorate - the Government's watchdog on water quality - says that Professor Sumpter has no evidence that nonylphenols are present in drinking water. But the professor warned: 'What one's got is a potential problem that could be very serious. If there really are oestrogenic effects on male fertility then this is a major issue.'
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