The study was led by Dr Shanna H Swan of the University of Rochester Medical Center (URMC) in New York, and is about to be published in the International Journal of Andrology.
Swan is professor of Obstetrics and Gynecology and director of the URMC Center for Reproductive Epidemiology.
Made from oil, phthalates are a family of organic chemicals used to make plastics such as polyvinyl chloride (PVC) softer and more flexible, They have been around for about 50 years and are the most commonly used plasticisers in the world.
Phthalates are present in vinyl and plastic tubing, household products, and personal care products such as soaps and lotions. They are considered controversial because more and more studies are linking them with genital defects, metabolic abnormalities, and reduced testosterone in babies and adults.
In the US, a federal law passed in 2008 banned 6 phthalates from being used to make children's items such as teethers, soft books, bath toys, dolls and plastic figures.
Swan and her team have previously shown that phthalate exposure during pregnancy might affect the development of genitals of both male rodents and baby boys, and scientists call this cluster of genital alterations the "phthalate syndrome".
Recent studies have highlighted particular concerns about human exposure to di(2-ethylhexyl) phthalate (DEHP), and dibutyl phthalate (DBP) via the food chain. DEHP and DBP are used primarily in the manufacture of PVC and exposure can occur when food is processed, packaged, stored, or heated.
Swan, who is interested in environmental causes of reproductive health problems and an expert in phthalates, told the press that researchers are concerned because phthalates are anti-androgens and have the potential to alter the development of the masculine brain, which depends on the presence of the androgen testosterone.
Since 1998 Swan, has been leading the federally funded, multi-center Study for Future Families (SFF), which is collecting a large database of information to help scientists investigate toxins.
For this study, Swan and colleagues used data from a subset of SFF mothers who gave birth between 2000 and 2003 and who around the 28th week of pregnancy gave urine samples that were analyzed for phthalate metabolites by the US Centers for Disease Control and Prevention (CDC).
The researchers established contact with mothers from the SFF subsample and asked them to complete a standard research questionnaire, called the Preschool Activities Inventory (PSAI), for their 145 children aged from 3.5 to 6.5 years.
The PSAI differentiates play behavior within and between the sexes, and past studies have shown it reflects the endocrine-disrupting properties of other toxins, such as PCBs and dioxins, said the authors. The inventory addresses three aspects of play: types of toys children choose (eg trucks versus dolls), activities (eg rough-and-tumble play), and child characteristics.
To overcome the possibility that the results might be skewed by the choice of toys available to the children, the researchers also asked additional questions about the parents' views on atypical play, for instance if they had a boy who chose to play with toys normally picked by girls, to what extent would they encourage or discourage him to do so.
The final scores were arranged on a scale where higher scores equated to more male-typical play and lower scores more female-typical.
Swan and colleagues then looked at how the phthalate metabolite analysis of the mothers' urine in pregnancy related to the play scores of their children.
The results showed that:
- Higher levels of metabolites of two phthalates, DEHP and DBP, were linked to less male-typical behavior in boys.
- No other phthalate metabolites measured in their mother's urine linked to the less-masculine behavior.
- Girls' play behavior was not linked with phthalate levels in their mothers' urine.
"Not only are they consistent with our prior findings that link phthalates to altered male genital development, but they also are compatible with current knowledge about how hormones mold sex differences in the brain, and thus behavior," said Swan.
"We have more work to do, but the implications are potentially profound," she added.
Speculating on the possible biological mechanisms underlying these results, Swan suggested that phthalates may lower fetal testosterone production during a critical window of development, somewhere between weeks 8 and 24 of gestation, when the testes begin to function, and this affects brain sexual differentiation.
With previous studies suggesting that hormone disrupters like phthalates can impair genital development and hormones in the body, and this study linking play-behaviour with fetal phthalate exposure, there is now a need for research that explores more deeply how these chemicals potentially disrupt the sexual development of the brain, said the researchers.
Swan's co-authors include researchers from URMC and other institutions such as the University of Iowa Hospitals and Clinics, UCLA School of Medicine, University of Minnesota School of Medicine, University of Missouri School of Medicine and the University of Cambridge in the UK.
International Journal of Andrology
Sources: University of Rochester Medical Center, Phthalates Information Centre Europe.
Phthalate studies point to several health problemsResearchers from Brigham and Women's Hospital published a report in Environmental Health Perspectives which revealed that women with high phthalate levels in their urine have a considerably higher risk of developing diabetes, compared to those with the lowest levels. (Link to article)
Researchers from the Children's Environmental Health Center at The Mount Sinai Medical Center found a link between obesity in young children and exposure to phthalates. (Link to article)
Phthalate concentrations were found in infants' urine by researchers from the CDC in Atlanta and the University of Rochester School of Medicine and Dentistry. The investigators said phthalates are widely used in baby care products, such as lotion, talc and shampoo. (Link to article)
A Finnish study found a link between phthalates and diabetes risk among elderly people; even when circulating phthalate levels were only moderately elevated their risk doubled. (Link to article)
At Columbia University's Mailman School of Public Health, scientists found "prenatal exposure to phthalates may adversely affect child mental, motor and behavioral development during the preschool years". (Link to article)
Scientists from the University of Michigan, in a large-scale study, confirmed a link between phthalate and BPA concentrations and thyroid hormone levels. Thyroid hormones play a key role in reproduction, metabolism, energy balance and other body functions. (Link to article)
Written by: Catharine Paddock, PhD