Attention-deficit hyperactivity disorder (ADHD) is one of the most common childhood conditions, affecting 11 percent of U.S. kids in 2012. Diagnoses of the condition have increased 42 percent since 2004, according to the Centers for Disease Control and Prevention (CDC), and by 4 years old, some kids are already taking prescription drugs. While many cases of ADHD may be diagnosed incorrectly, researchers looking at why there’s been such an increase have another suggestion: air pollution.

In a study published in the journal PLOS One, researchers at Columbia University’s Mailman School of Public Health found that air pollution throughout its hometown, New York City, may have contributed to a rise in ADHD diagnoses. The findings are particularly important for low-income groups, who are at an increased risk of exposure to the pollutants studied, which are known as polycyclic aromatic hydrocarbons (PAHs).

“This study suggests that exposure to PAH encountered in New York City air may play a role in childhood ADHD,” said lead author Dr. Frederica Perera, director of the school’s Center for Children’s Environmental Health, in a press release. “The findings are concerning because attention problems are known to impact school performance, social relationships, and occupational performance.”

Aside from possibly contributing to the development of ADHD, exposure to PAHs has been linked to birth defects, behavioral problems in kids, kidney and liver damage, cataracts, and various cancers. They’re organic compounds found in almost every aspect of our lives, from vehicle exhaust and tobacco smoke to grilled meat (it’s a product of charring) and medicines. Obviously, there’s no getting away from them — only minimizing exposure.

For the study, the researchers followed 233 pregnant NYC moms over the course of their pregnancies, and throughout the majority of their children’s early lives. During this time, they also took blood and cord samples from the mothers (shortly after birth) and urine samples from the kids at ages 3 and 5 (to test for PAH metabolites).

After testing the kids for ADHD, the researchers found that children whose mothers were exposed to higher levels of PAHs during pregnancy were five times more likely to develop ADHD. Of the 33 exposed to high levels, 13 developed the hyperactive-impulsive subtype of ADHD, while seven had the inattentive subtype, and 13 had both, The Washington Times reported.

Though it’s not entirely clear how pollution contributes to the condition, the researchers suggested in their paper that it could be due to oxidative stress, a disruption in the endocrine system, DNA damage, or issues with placental growth, which could have led to reduced oxygen and nutrient intake. “During the fetal period and early childhood years, the brain is rapidly developing and vulnerable to neurotoxic insults that may manifest as adverse outcomes in childhood and adulthood,” the researchers wrote.

Not all exposure to PAHs is the same, however, and low-income groups are at an increased risk. A study from earlier this year found that throughout the country, even in the most rural states and cleanest cities, low-income minorities were more likely to live around high levels of PAHs. In all, there were 38 percent higher levels of the pollutant nitrogen dioxide in minority neighborhoods than in white neighborhoods. It makes sense. After all, low-income individuals are more likely to smoke cigarettes, and live near toxic waste sites — the Bronx’s Hunts Point is the poorest congressional district in the nation; it also has a sewage treatment plant — and congested highways.

The researchers said that more research is needed to determine how much pollution contributes to ADHD. If they’re able to determine that environmental factors are to blame, even partially, they may be able to work toward prevention methods. Until then, however, we’re going to have to rely on “currently available pollution controls, greater energy efficiency, the use of alternative energy sources, and regulatory intervention,” they wrote.

Source: Perera F, Chang H-W, Tang D, et al. Early-Life Exposure to Polycyclic Aromatic Hydrocarbons and ADHD Behavior Problems. PLOS One. 2014.