Children of women, who were exposed to air pollution while they were pregnant, are five times as likely to develop symptoms of attention deficit hyperactivity disorder (ADHD), researchers report.

Scientists at Columbia University say a risk of symptoms appearing in children by age 9 goes up dramatically if their mothers were exposed to elevated levels of air pollutants known as polycyclic aromatic hydrocarbons, or PAHs, during pregnancy.

Writing in the journal PLOS One, the researchers report monitoring 233 nonsmoking pregnant women and their children in New York City from pregnancy into childhood. The team found a higher-than-usual number and degree of symptoms that characterize ADHD in children of mothers with high PAH exposure, compared with children whose mothers did not experience high exposure.

While researchers think genetics plays a large role in ADHD, they say various environmental factors are known or suspected to be involved as well.

"Our research suggests that environmental factors may be contributing to attention problems in a significant way," says lead study author Frederica Perera at the Columbia Center for Children's Environmental Health at the Mailman School of Public Health.

ADHD affects about 11 percent of children aged 4 to 17 years, data from the Centers for Disease Control and Prevention (CDC) show.

PAHs are toxic air pollutants that can be emitted by a number of sources, including traffic, residential heating boilers and power plants that burn fossil fuel.

"This study suggests that exposure to PAH encountered in New York City air may play a role in childhood ADHD," Perera says. "The findings are concerning because attention problems are known to impact school performance, social relationships, and occupational performance."

PAHs can continue to circulate within the body for a long period of time, Perera says, so even brief exposures within the womb may contribute to changes in a child's body as they grow.

To reduce the risks, she says, families can take measures to ensure cooking areas in the home are properly ventilated and avoid burning incense, candles or other sources of PAHs.

Most importantly, Perera says, one should avoid exposure to tobacco smoke.

Pregnant women can also increase their intake of antioxidants from fresh vegetable or fruit to help counteract the potential oxidative damage on fetal cells, she says.

Previous studies conducted by the Columbia researchers have established links between prenatal PAHs and reduced IQs, depression and anxiety, and development delays and attention problems in young children between the ages of 3 and 7.