Should we ban the suspects?

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The issue of linking falling sperm counts and environmental pollution is dogged by the difficulties of proving that a cause has a certain effect. There are some scientists who have even cast doubt on whether the effect in question - the failing reproductive health of men - is genuine.

Environmentalists, however, are firmly of the opinion that even with the limited evidence of the role played by environmental chemicals, the time to act is now rather than to wait until the effect becomes irreversible. "The Government should apply the precautionary principle to protect the public from gender-bending pollution,'' says Roger Lilley, pollution campaigner at Friends of the Earth.

At a press briefing yesterday at the Department of Environment, the case for caution was made by Professor Lewis Smith, director of the Institute for Environment and Health at Leicester University, who led the year-long government inquiry into the issue. Banning any single chemical that may be responsible would probably do more harm than good, he said.

Professor Smith said there is, as yet, only "a balance of probabilities" that sperm counts and sperm quality have fallen over the past few decades. Although two major studies have strongly suggested the effect is genuine, definitive proof could only come from a "prospective" study where a large sample of men are followed over a number of years. In this way scientists can positively eliminate all possible confounding factors - anything from tight underpants to changes in lifestyle - that could skew the data.

Despite this, scientists are working to the assumption that the effects are real and that there must be an underlying cause. But Professor Smith said identifying the cause is hampered by the complexity of the issue: some of the oestrogenic chemicals we are exposed to are present naturally in food and identifying any one that may be the cause of falling sperm counts and other reproductive disorders is an enormously complex task. "After all, you consume approximately 5,000 chemicals a day in your diet," he said.

Tests using cages of trout placed in reservoirs and rivers around Britain have tried to find evidence for oestrogenic chemicals in the environment. In the presence of relatively small amounts of oestrogenic chemicals in river water, male trout produce in their liver a protein normally only found in the egg yolks of female fish.

Using cages of male trout, a survey of 15 drinking water reservoirs failed to detect the presence of oestrogenic chemicals, allowing the Department of Environment yesterday to state boldly: "Fears about environmental oestrogens getting into drinking water supplies are unfounded."

However, when the same tests were carried out on half a dozen rivers near sewage outlets, it became clear that treated sewage does contain chemicals that mimic female hormones. In one stretch of river the male trout produce the egg yolk protein at a distance of five kilometres from the sewage outfall.

Clearly, there are some chemicals in domestic and industrial waste that have the potential to mimic oestrogen. The difficult question that remains is to identify which chemicals, out of a possible list of 100,000 on the market, could be responsible and whether humans are being exposed to levels that could have a medical effect.

Research on laboratory animals has also implicated specific chemicals involved in inhibiting normal development of male sex organs. For instance, certain members of a group of chemicals - phthalates - used in plastic wrapping have been shown to cause reproductive problems in rats, but there is no convincing evidence that they do the same in humans.

As Professor Smith said at yesterday's press briefing, by withdrawing certain chemicals based on limited scientific research you run the risk of "making the world a safer place for rats to live in". It is not an argument, however, that will convince the environmentalists.