Kids whose moms had the highest levels of certain chemicals in their bodies during pregnancy had markedly lower IQs at age 7, researchers said Wednesday.
It's the latest in a series of studies linking the chemicals, called phthalates, with health effects ranging from behavioral disorders to deformations of the sex organs.
While the study doesn't show for sure that the phthalates damaged the kids' brains during development, the researchers say they did everything they could to filter out other possible effects and they still found the link between some — but not all — of the phthalates and IQ.
"Pregnant women across the United States are exposed to phthalates almost daily, many at levels similar to those that we found were associated with substantial reductions in the IQ of children," said Pam Factor-Litvak of the Mailman School of Public Health at Columbia University, who led the study.
Phthalates (pronounced "THAL-ates") are a group of chemical compounds used to keep other products flexible. They make plastics bendier and are used almost everywhere, from plastic toys and bottles to vinyl flooring and dryer sheets, cosmetics and air freshener.
More than 470 million pounds of phthalates are produced or imported in the United States each year.
They're also in just about everyone's bodies. Environmental groups have been worried about them for years, but there had been little evidence they caused any real harm to people. Large doses can affect animals, but the human body processes them out quickly.
Factor-Litvak's team has been trying to study their effects in kids, and they've been watching a group of inner-city New York women and their children since 1998.
The 328 moms gave urine samples when they were pregnant and the mothers and their children have been taking part in all sorts of tests since then.
For this study, the researchers of looked at levels of five common phthalates in urine taken from 328 of the women during the third trimester of pregnancy.
They included di-n-butyl phthalate (DnBP), butylbenzyl phthalate (BBzP), di-isobutyl phthalate (DiBP), di-2-ethylhexyl phthalate and diethyl phthalate.
Only two of them — DnBP and DiBP — were linked with a real difference in IQ, Factor-Litvak and colleagues wrote in their report, published in the journal PLOS ONE. The children of moms with the highest levels of those two chemicals scored on average four points lower on the IQ test than kids whose mothers had the lowest levels.
"We were very concerned that there might be another variable that might account for the results," Factor-Litvak told NBC News. "We controlled for many, many of those, including things like maternal education, maternal IQ … the child's sex, exposure to tobacco smoke in the home, prenatal tobacco use ... and nothing (else) accounts for the results."
There's valid biological evidence that could explain how a chemical like a phthalate might affect IQ. The phthalates as a whole can interfere with the action of hormones — they're called endocrine disruptors for that reason.
Hormones do a lot more than make the sexual organs develop. During the development of a fetus, they fire on and off at certain times to affect the brain and other organs.
"The developing brain relies on hormones," Factor-Litvak said. Thyroid hormones affect the development of neurons, for example. There might be a window of vulnerability during pregnancy when certain key portions of the brain are forming, she said, and kids whose moms take in a lot of the chemicals during those times might be at risk of having the process disrupted somehow.
The findings fit in with other work the team has done that showed an association between higher phthalate levels and behavioral differences in the same group of kids. They also found links with asthma.
Other research has found evidence that women who had higher levels of the compounds in their bodies were more likely to deliver preterm babies, and the American College of Obstetricians and Gynecologists (ACOG) and the American Society of Reproductive Medicine (ASRM) took a controversial stand last year saying doctors have an obligation to warn pregnant women about them.
South Korean studies have shown kids ages 8 to 11 who have higher evidence of phthalates in their urine were less attentive and more likely to be hyperactive. They also had measurably lower IQs. Another study found they might be associated with obesity in teenagers.
Factor-Litvak says other groups need to validate the findings before they could or should be accepted as real. "I think that there needs to be more research done in other populations," she said.
And it's not at all clear what people can do to lower their exposure to phthalates. The Columbia team says pregnant women can avoid microwaving food in plastics, avoid scented products as much as possible, including air fresheners and dryer sheets, and not using recyclable plastics labeled as 3, 6 or 7 — those numbers indicate that the specific phthalates were used in their manufacture.
The Consumer Product Safety Commission restricts some phthalates in kids' toys, but there are no restrictions on their use in most other consumer products.