Research has shown that the mice used in some lab experiments, because of their genetic make-up, may actually be quite tolerant of the pollutants to which they are exposed. As a result, it is possible that the testing is underestimating the dangers that come from these pollutants.

Of particular concern are the chemicals that mimic the female sex hormone oestrogen. They are sometimes referred to as endocrine disrupters or "gender benders" because of the way they appear to interfere with reproduction in fish and other wildlife by upsetting the normal function of the endocrine system.

The evidence is not wholly conclusive, but studies indicate that such chemicals can cause decreased sperm counts, deformed genitals, aberrant mating behaviour and sterility.

Lab tests

"Environmental oestrogens" have been found in products such as DDT and other pesticides, some plastics, industrial chemicals like PCBs, and even household detergents.

Government agencies and research labs test the chemicals on lab mice, but new research, published in the journal Science shows the wrong type of rodents may be being used.

A team from the University of California, Davis, analysed the effects of oestrogen doses given to several strains of laboratory mice developed under different selective breeding conditions.

Although all the strains of mice exposed to the oestrogen showed some signs of endocrine disruption - including smaller testes weight and inhibited sperm production - one strain of mice suffered far less than the others.

Testes function

This particular strain, known as CD-1, was 16 times more resistant to the damaging effects of the oestrogen on testes function.

CD-1 mice are frequently used in toxicological and pharmacology studies, and are specially selected to produce large and vigorous litters for the purpose. The researchers suggest that the genes controlling large litter size in these mice might also be affecting their sensitivity to oestrogen, making them a poor model for studying endocrine disruption.

"The use of laboratory animals that genetically are quite resistant to oestrogen for the evaluation of possible reproductive effects of various chemicals might be misleading and may mask our appreciation of how global exposure to oestrogen-like chemicals threatens wildlife, domestic animals and humans," said Jimmy Spearow, a reproductive geneticist, who worked on the UCDavis research.