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Air pollution might be linked to increasing cases of ADHD in children, a study of New York City women and children found. 

Exposure to pollution before birth might be the key, researchers from Columbia University's Mailman School of Public Health found, after a look at prenatal levels of polycyclic aromatic hydrocarbons (PAH), a component of air pollution, and ADHD symptoms in children later in life.

Mothers exposed to high levels of PAH during pregnancy had five times the odds of symptoms that characterize inattentive ADHD in their kids at age 9. The study is the first to examine prenatal PAH exposure and behavior in children over time. Although it shows a potential link between the two factors, it does not demonstrate that air pollution exposure causes ADHD. 

(MORE: All the Ways Air Pollution Hurts)

More than 10 percent of kids between 4 and 17 have one of the three forms of ADHD (including inattentive ADHD, the kind in the study), according to the U.S. Centers for Disease Control and Prevention. The CDC's figure comes from self-reported cases from parents, not verified doctor diagnoses. 

Still, there's no doubt incidence of ADHD is on the rise, in part thanks to increased awareness surrounding the disease. 

The new findings on ADHD and air pollution add to CDC studies linking prenatal PAH exposure to "developmental delays at age 3, reduced IQ at age 5 and symptoms of anxiety/depression and attention problems at ages 6 and 7," according to a Columbia University press release. 

Car exhaust, power plants and other sources generate PAHs, substances linked to heart disease, cancer and respiratory conditions.

As far as the mechanism for harm, it's likely that PAH causes DNA damage and oxidative stress, as well as possible endocrine disruption and decreased oxygen and nutrient levels in the placenta, researchers suggest. DNA damage and oxidative stress are also how air pollution damages the skin and other organs.

The study was published Nov. 5 in the peer-reviewed journal PLOS One.

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