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Exposure of expectant mothers to phthalates, a common ingredient in many plastics, has been linked to smaller penis size and incomplete descent of testicles in their baby boys, according to a new research paper that found the chemical also appears to make the overall genital tracts of boys slightly more feminine.

The findings are sure to add more controversy to phthalates, a chemical that is added to polyvinyl chloride plastic to make it less brittle, and to many types of personal care products including fragrances, hair sprays and nail polish.

The research was conducted on children from three different areas of the United States, and found a strong statistical correlation between expectant mothers who had above-average levels of the chemical in their urine while pregnant and the feminizing effect on their sons.

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Phthalates are "probably reproductive toxins and should be eliminated from products gradually because we don't need them," said Shanna Swan, director of the Center for Reproductive Epidemiology at the University of Rochester's school of medicine, who led the team of scientists who examined the boys.

The paper is published in the current issue of the journal Environmental Research.

The Virginia-based American Chemistry Council, which represents the makers of the chemical (Exxon Mobil, BASF, Ferro Corp., and Eastman Chemical), issued a statement saying it "cautioned against over-interpreting any individual study."

Scientists have been investigating the possible effects on boys of phthalates because rodent studies have shown the chemical has the peculiar ability to shorten the space between the anus and the genitalia in male mice exposed during fetal development. This space, known as anogenital distance or AGD, is normally about twice as long in young male mice than in females. For mice, AGD is considered a measure of masculinity and a way to determine the sex of the pups. Scientists are so confident of the effect that they've given the impact of the chemical on male rodents a name - phthalate syndrome.

Surveys of children have also found that there is a marked sexual difference for this trait in humans, too, with the length in boys about 50 per cent more than in girls.

Dr. Swan's research, conducted on 106 boys from Los Angeles, Columbus, Missouri and Minnesota, is among the first to raise the possibility that phthalate syndrome may also be at work in humans, because it found pregnant women with the highest amount of phthalates were markedly more likely to give birth to boys who had shorter anogenital distances.

When the boys were compared, none of the 29 with a shorter AGD were born to women who had low amounts of phthalates, while among the boys with a long space, only one was born to a mother with a high amount of the chemical.

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The difference in the genital distance between the high-exposure and low-exposure boys was slight - around 3 to 4 per cent.

The paper also showed that incomplete descent of the testicles was "significantly" associated with mothers having more of the type of phthalate used in polyvinyl chloride plastic.

This phthalate, known as DEHP, has been listed as a toxic substance in Canada, and Health Canada has proposed but not implemented a prohibition limiting the chemical to no more than 0.1 per cent of the weight of toys used by young children.

Phthalates may have adverse effects because they are able to reduce testosterone synthesis by interfering with an enzyme needed to produce the male hormone. This raises worries that they may alter any process dependent on the hormone that choreographs male development. Phthalates can easily leach out of products, enabling humans to absorb them through diet, skin and inhalation.

Dr. Swan cautioned that the research was conducted on a relatively small number of boys, and the findings need to be independently verified by other investigators. It also isn't known what effect, if any, the chemical might have on the fertility of the boys, later in life, because the group would need to be followed into adulthood.

Nonetheless, Dr. Swan said she believes labelling laws need to be strengthened to allow consumers to choose whether to buy products or packaging that contain phthalates.

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Cosmetics often contain phthalates, but the chemical isn't specifically mentioned because it is included in other listed items, such as fragrances.

Dr. Swan says she tries to buy phthalate-free cosmetics and doesn't store or microwave food in plastic containers, among other steps, to minimize her own exposure.

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