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Study reports hints of phthalate threat to boys’ IQs

Researchers link lower IQs in children to a chemical found in some plastics and food packaging

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You may have a hard time spelling phthalates, but there’s no avoiding them. They’re in the air you breathe, water you drink and foods you eat. And this ubiquity may carry a price, particularly for young boys, emerging data suggest. Including a drop in their IQ.

A new study examines cognitive risks from phthalates. The study wasn't big — including just 667 third- and fourth-graders. But it does cover a broad and nationally representative cross-section of South Korea’s youngsters. Moreover, whatever changes occurred in these kids might well develop elsewhere. And that’s because residues of diethylhexyl phthalate, or DEHP — the phthalate that appeared most neurotoxic to these children — show up in people throughout the developed world, including the United States.

Soo-Churl Cho and Hee-Jung Yoo of Seoul National University College of Medicine and their colleagues recruited participants from nine grade schools. These drew from vastly different communities: Seoul, a metro area with 10 million people; Incheon, an industrial center with close to 3 million inhabitants; Ulsan, an industrial region about one-third Incheon’s size; Seongnam, a suburban home to some million residents; and a rural region covering land mass comparable to Seoul’s metro region, but inhabited by a mere 50,000 people.

Each child took the Korean version of a widely accepted IQ test known as the Wechsler Intelligence Scale for Children, or WISC. So did the kids’ moms, which offered the researchers — from four Korean medical schools — a gauge for the genetic component to each child’s IQ.

DEHP metabolites, or breakdown products, ranged from a minimum of 0.5 micrograms per liter to 445 µg/l. Two DEHP metabolites were measured and summed for each child. Then the kids were split into four groups on the basis of these metabolites. In an upcoming issue of Environmental Health Perspectives published online, ahead of print, the researchers report finding that as the amount of DEHP’s breakdown products in urine climbed, a child’s IQ fell a small amount.

And not uniformly across all components of the IQ testing. Sections where DEHP exposure seemed to play a role were the kids’ “full-scale” IQ, vocabulary score, verbal IQ and block-design tests. For instance, on the full-scale IQ, boys in the highest DEHP-exposure group scored 1 to 2 points lower than did those in the other three groups, depending on contamination levels in the lower groups and the range of possible confounding variables used to test for statistical validity. No such trend of some phthalate-linked drop in full-scale IQ emerged among girls.

In fact, the scientists suspected that they might observe some gender effect. Earlier studies by others have shown boys are more susceptible to adverse changes in reproductive development following prenatal exposure to phthalates. A recent study also showed a difference in gender-related play preferences betweem boys who were more heavily exposed in the womb to phthalates.

A more limited IQ risk emerged for dibutyl phthalate — a plasticizer and solvent used widely, from polyvinyl chloride and inks to adhesives and cosmetics. Here too, block-design scores fell as levels of DBP’s breakdown product in urine rose.

Although about a dozen other breakdown products of phthalates can also be measured, the researchers ignored them because none were very abundant in these kids.

For a little perspective, the U.S. Centers for Disease Control and Prevention notes that a 2005 study out of Korea found that children there tended to have mean DEHP metabolite values about triple those measured in U.S. children.

What to make of the findings? Well, the kids’ values were controlled on the basis of their moms’ IQ scores. That may or may not be the right thing to do, the researchers admit. For instance, the moms’ IQs might have influenced — through smart or not-so-smart diet and product choices — DEHP exposures in themselves and their children.

They also can’t say whether effects they measured in the children trace to recent exposures or ones that go back to the womb or infancy, when neural pathways were forming. Then again, recent studies have shown that at least when it comes to lead, another neurotoxic agent, the brains of even school-age children remain fairly plastic to its damage.

If follow-up studies do confirm a phthalate risk to cognition, Cho, Yoo and their colleagues speculate that it might be through a tinkering with thyroid hormone levels in the children. Thyroid hormones play a pivotal role in neural development. And the researchers point out, “Although human data are lacking, in vitro and in vivo studies suggest that phthalate exposure is associated with altered thyroid functioning.” They also cite papers suggesting additional possible brain impacts from low-dose exposures to phthalates.

It’s time that research identified how phthalates are getting into our children’s bodies so that policymakers might consider regulations to limit such tainting. After all, IQ is nothing to play with. And parents can’t be expected to make sound choices to protect their young unless they get solid data on sources of pollutants and possible risks from them.

Citations

Cho, S.-C., et al. 2010. Relationship Between Environmental Phthalate Exposure and the Intelligence of School-Aged Children. Environmental Health Perspectives (in press). DOI: 10.1289/ehp.0901376

Kim, B.-N., et al. 2009. Phthalates Exposure and Attention-Deficit/Hyperactivity Disorder in School-Age Children. Biological Psychiatry 66(Nov. 15):958. DOI: 10.1016/j.biopsych.2009.07.034

Engel, S.M., et al. 2009. Prenatal Phthalate Exposure and Performance on the Neonatal Behavioral Assessment Scale in a Multiethnic Birth Cohort. NeuroToxicology 30(July):522. DOI: 10.1016/j.neuro.1009.04.001

Ghisari, M. and E.C. Bonefeld-Jorgensen. 2009. Effects of Plasticizers and Their Mixtures on Estrogen Receptor and Thyroid Hormone Functions. Toxicology Letters 189:67. DOI: 10.1016/j.toxlet.2009.05.004
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