New York City children exposed in the womb to high levels of pollutants in vehicle exhaust had a five times higher risk of attention problems at age 9, according to research by Columbia University scientists published Wednesday.

The study adds to earlier evidence that mothers' exposures to polycyclic aromatic hydrocarbons (PAHs), which are emitted by the burning of fossil fuels and other organic materials, are linked to children's behavioral problems associated with Attention Deficit Hyperactivity Disorder (ADHD).

“Our research suggests that environmental factors may be contributing to attention problems in a significant way,” said Frederica Perera, an environmental health scientist at Columbia’s Mailman School of Public Health who was the study's lead author.

About one in 10 U.S. kids is diagnosed with ADHD, according to the Centers for Disease Control and Prevention. Children with ADHD are at greater risk of poor academic performance, risky behaviors and lower earnings in adulthood, the researchers wrote.

“Air pollution has been linked to adverse effects on attention span, behavior and cognitive functioning in research from around the globe. There is little question that air pollutants may pose a variety of potential health risks to children of all ages, possibly beginning in the womb,” said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Steven & Alexandra Cohen Children’s Medical Center of New York. He did not participate in the new study.

Previous studies by the Columbia University researchers linked prenatal PAHs to reduced IQs, anxiety and depression, attention problems and developmental delays in younger children, between the ages of 3 and 7.

In addition to PAHs, a variety of other pollutants have been linked to ADHD or ADHD-like behaviors.

Included are organophosphate pesticides, polychlorinated biphenyls (PCBs), phthalates and perfluorinated compounds.

For the new study, the researchers followed the children of 233 African-American and Dominican women in New York City. They measured the amount of benzo[a]pyrene bound to DNA – a biological marker for PAHs – in the mothers' blood at the time of birth. Forty-two percent had detectable levels in their blood.

When the children were about 9 years old, parents filled out a questionnaire commonly used to screen for ADHD behavior problems. The researchers found that children whose mothers had the highest amounts of the PAH at the time of birth were five times more likely to show more behaviors associated with inattention than children whose mothers had the lowest levels. They were three times more likely to exhibit more total behaviors (inattention, hyperactivity and impulsivity) associated with ADHD.

The levels of PAHs were “quite typical of other urban areas that have been monitored,” Perera said.

It’s not known how many of the children had an ADHD diagnosis.

The questionnaires indicated some behaviors involved in ADHD, but they do not mean that the child necessarily has ADHD, said David Bellinger, a Harvard professor of neurology who studies effects of environmental exposures on kids’ developing brains.

“The diagnosis of ADHD involves much more extensive information-gathering and ruling out of other diagnoses,” he said.

The researchers were the first to measure PAHs directly in people’s bodies rather than using air pollution levels as a proxy for their exposures.

“Having these individual measures of exposure reinforces the possibility that there is a causal connection,” said Jonathan Chevrier, an environmental health scientist at McGill University in Montreal.

The scientists tested the children’s urine samples to distinguish prenatal from postnatal air pollution exposures. They also controlled for other factors in the child’s genetics and early life experiences, such as tobacco smoke and stress, that could contribute to ADHD-like symptoms.

The researchers, however, did not account for children’s exposures to lead or mercury, two contaminants that also have been linked to attention problems. “It’s possible that exposure to one of these substances is highly correlated with PAH exposure,” Bellinger said.

It’s not clear how PAHs might affect developing brains. “We know that these chemicals damage DNA. They also mimic natural hormones and may interfere with placental growth,” which could deplete oxygen and nutrients for the developing fetus, Perera said.

Since the Columbia team started tracking air pollution levels in 1998, PAHs have declined, which Perera attributes to stricter anti-idling laws and phaseout of older diesel buses in New York City.

“Air pollution knows no boundaries. These involuntary exposures are largely the provenance of policy makers,” Perera said.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.