The adaptability of humans over the course of our evolution has allowed us to thrive in otherwise inhabitable environments. People of the Andes are a good example of this; they’ve become more tolerant of the poison arsenic, which is found in their drinking water, than the rest of us. But in that same vein, the environment also has a drastic effect on our health, and various studies have suggested as much. Now, a new study published in JAMA Psychiatry finds a common airborne pollutant may affect the development of the growing baby in utero.

A wealth of research has looked at the effects of fine particulate matter, minuscule remnants produced in construction sites, power plants, and automobile engines that have gone airborne. But for this study, the researchers were interested in the effects of polycyclic aromatic hydrocarbons (PAHs). These pollutants are typically released through combustion — coal, wood, diesel fuel, fat (such as that steak you love), tobacco, and incense are common sources. Pregnant women who expose themselves to these could be putting their children at risk of not only neurodevelopmental delays but lower intelligence and psychiatric problems like anxiety and depression as well, the study found.

“This is the largest MRI study to date of how early life exposure to air pollutants, specifically PAH, affect the developing mind,” said lead author Dr. Bradley Peterson, director of the Institute for the Developing Mind at Children’s Hospital Los Angeles’ (CHLA) Saban Institute.

Researchers from CHLA and Columbia University’s Mailman School of Public Health conducted MRI scans in 40 children out of a cohort of over 600 mother-child pairs living in New York City. These children were chosen for their low exposure to other pollutants besides PAHs. All of the mothers were assessed for PAH exposure during their last trimester of pregnancy, and then the children’s postnatal exposure was assessed in follow-ups until the ages of 7 to 9.

“The effects were extraordinarily powerful,” Peterson told Time. “The more prenatal exposure to PAH, the bigger the white matter problems the kids had. And the bigger the white matter problems, the more severe symptoms of ADHD, aggression, and slow processing they had on cognitive tasks.”

White matter is found throughout the brain, and creates the network of nerve fibers that allow for effective signaling. When there’s a problem with these fibers signals can’t be transmitted as quickly — the children in this study had lower volumes of white matter. Most peculiarly though, Peterson’s team found only the left hemisphere’s white matter was affected, suggesting the toxins target developing areas of the brain. The left hemisphere, where language capabilities come from, may undergo especially intense structural changes just before birth, Time notes.

While Peterson admits it would be hard for moms to eliminate exposure, they can take steps to minimize it. These steps include avoiding inhaling secondhand smoke and automobile exhaust, as well as visiting parks and other smoke-free areas as often as possible. “Even if you reduce your exposure from moderately high to moderate levels, it’s going to have a beneficial effect on the developing fetus,” Peterson said.

Source: Peterson B, Rauh V, Perera F, et al. JAMA Psychiatry. 2015.