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Men who are exposed to high levels of the plastic-making compound bisphenol A on the job are more likely to experience sexual-performance difficulties, including problems maintaining erections and ejaculating.

The finding, based on a study of men who worked in Chinese factories where bisphenol A was either made or used in products, comes from the first research to look at the effects of the estrogen-mimicking chemical on the male reproductive system in humans and is raising more questions about the controversial compound, which is used in everything from tin cans to water bottles.

The men exposed to BPA, as the chemical is commonly known, had four times the risk of erectile dysfunction and seven times the risk of ejaculation problems when compared with men who did not have workplace exposure to the compound. Those working with BPA also reported reduced libido and less satisfaction with their sex lives.

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Until now, not much has been known about the impact on men of exposures to BPA because research into the possible health effects of the chemical has been confined mainly to laboratory animals. Some animal studies have shown impacts such as a reduced ability to mate, along with biological changes such as an increase in the size of the prostate, the organ in males that produces seminal fluid.

"The effects that have been shown in animal studies now are showing in humans," said De-Kun Li, lead author of the study and a reproductive epidemiologist based in Oakland, Calif., at Kaiser Permanente, a U.S. non-profit health-care and insurance provider.

The men who were exposed to BPA in the workplace had extremely high levels of the chemical in their urine - about 50 times higher than readings found in people who do not work with the chemical.

But Dr. Li said further research is needed to determine whether exposures more typical of the general population also place men at an increased risk of having impaired sexual performance. The finding of sexual problems in those with high residues "doesn't mean the lower exposed group is safe. It just means we don't know," he says.

Details of the study were published this week in the journal Human Reproduction.

Researchers have been curious about the impact of BPA on men because the chemical, which is not found in nature, has a shape able to fool cells into viewing it as estrogen, the main female hormone.

Surveys in the United States have found that more than 90 per cent of the population carries traces of BPA in their urine, with concentrations in the low parts per billion. Although this is an extremely tiny amount, it is still about 1,000 times higher than the concentrations of estrogen naturally found in people, and indicates that almost everyone is getting a large extra dollop of a synthetic chemical able to act like a female hormone.

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It is not known for sure where the BPA found in people is coming from, but a prime suspect is food. The chemical is used to make the epoxy liner inside almost all food and beverage cans, and is able to leach into the contents. It is also used in some dental sealants, carbonless cash-register receipts, and polycarbonate plastic bottles.

Some of the Chinese workers exposed to BPA were involved in the manufacture of epoxy resins and were exposed by inhaling or ingesting dust containing the chemical.

The researchers looked at Chinese workers because a large amount of BPA manufacturing has moved to developing countries. They also needed to find companies willing to allow access to their workers, who were asked about their sex lives through questionnaires. The responses of the BPA-exposed group were compared with those of a control group of workers who do not use the chemical. Some of the workers also gave semen samples, which are being evaluated for any abnormalities.

Reacting to worries over BPA, Health Canada announced a ban last year on the sale of polycarbonate baby bottles and has asked infant-food makers to reduce residues in their cans as a precaution, but the health agency said adult exposures are not a concern.

In the past decade, bisphenol A has emerged as one of the most controversy chemicals in use. Although all 11 industry-funded studies into the chemical have found it to be safe in laboratory animal experiments, about 90 per cent of the approximately 200 peer-reviewed scientific papers conducted by independent scientists have found that it has impacts at low doses, including lower age of puberty in females and skewed mammary-gland development

Dr. Li says that, given the controversy, "the prudent approach probably is trying to avoid BPA as much as you can" because reducing exposures "doesn't seem to have any downside."

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Kaiser Permanente has adopted this approach in purchasing for its hospitals, placing BPA on a list of chemicals of concern. It is asking suppliers to divulge whether products contain it and is seeking substitutes, if available, according to Kathy Gerwig, Kaiser's vice-president for workplace safety.

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