Pregnant women who were exposed to high levels of common household chemicals found in plastics, cosmetics and air fresheners had children whose intelligence suffered years later, a US study said Wednesday.

Kids whose moms had elevated traces of di-n-butyl phthalate (DnBP) and di-isobutyl phthalate (DiBP) had an average IQ that was around six points below their peers whose mothers had lower levels of chemical exposure.

Based on the findings, researchers urged pregnant women to limit their exposure to scented products including air fresheners and dryer sheets, avoid microwaving food in plastics, and steer clear of recyclable plastics labeled as 3, 6, or 7 in order to reduce risks to their offspring.

"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," says lead author Pam Factor-Litvak, PhD, associate professor of Epidemiology at the Mailman School of Public Health at Columbia University in New York.

"While there has been some regulation to ban phthalates from toys of young children, there is no legislation governing exposure during pregnancy, which is likely the most sensitive period for brain development."

The study in the journal PLOS ONE is the first to report a link between prenatal exposure to phthalates and IQ in school-age children.

The research is based on 328 low-income New York City women and their children.

They measured the women's exposure to four phthalates -- DnBP, DiBP, di-2-ethylhexyl phthalate, and diethyl phthalate -- in the third trimester of pregnancy by taking urine samples.

Their children were given IQ tests when they turned seven years of age.

"Children of mothers exposed during pregnancy to the highest 25 percent of concentrations of DnBP and DiBP had IQs 6.6 and 7.6 points lower, respectively, than children of mothers exposed to the lowest 25 percent of concentrations after controlling for factors like maternal IQ, maternal education, and quality of the home environment that are known to influence child IQ scores," said the study.

The other two phthalates studied showed no link to lower IQ.

The level of exposure found in the women was well within what the US Centers for Disease Control and Prevention observed in a national sample.

"The magnitude of these IQ differences is troubling," said senior author Robin Whyatt, professor of environmental health sciences and deputy director of the Columbia Center for Children's Environmental Health at the Mailman School.

"A six- or seven-point decline in IQ may have substantial consequences for academic achievement and occupational potential."

DnBP and DiBP are found in dryer sheets, vinyl fabrics, lipstick, hairspray, nail polish, and some soaps. Products in the United States rarely list on the label whether they contain phthalates or not.