For years, researchers have known that air pollution can worsen such respiratory conditions as asthma in children. But a recently released study has shed new light on how exposure to airborne pollutants can also affect the developing brains of the very young.

Researchers have found that toddlers exposed to particulate matter score lower on IQ tests—losing as many as 2.63 points on those exams for every 2 micrograms per cubic meter of pollution exposure.

And the harm from pollution, researchers found, can begin long before birth: Children of pregnant women who were exposed to air pollution from fossil fuel exhaust and particulate matter in utero are more likely to experience behavioral problems and poor cognitive performance, they report.

The findings, part of a peer-reviewed study in the June issue of the journal Environmental Health Perspectives, builds on a growing body of research examining the effects of pre- and postnatal exposure to pollution on children. A previous study had found, for example, that toddlers have a 6% increased risk of behavioral problems for every 2 parts per billion of nitrogen dioxide to which they are exposed. Nitrogen dioxide is commonly found in vehicle exhaust and emissions from industrial plants.

Taken together, researchers said, the findings bolster earlier hypotheses about the broad impacts of airborne pollutants on childhood health and development.

One of the most concerning aspects of the study for parents of young children is the ubiquity of air pollutants, researchers said.

“Air pollutants are something that everybody is exposed to every day of our lives,” said the study’s lead author, Yu Ni, an epidemiologist at the University of Washington. “It is cumulative, and it is indoor and outdoor. You just cannot avoid it at all. You can do something to prevent and to protect yourself, but you just cannot say that I have zero exposure to air pollution at all.” 

The exposure risk is particularly acute for young children, Ni said: Because of their small stature, they are closer to the ground and consequently have a greater chance of breathing in higher amounts of dust and other particulate matter. Children also typically breathe at a faster rate than adults, which may cause them to take in more particles in a shorter amount of time than those who are older.

Ni also noted that for some children, the risks begin while they are still in utero and their mothers are breathing in nitrogen dioxide. The study found that children born to pregnant people who were exposed to high levels of nitrogen dioxide, particularly in the first and second trimesters of pregnancy, had more behavioral problems than other children.

Catherine Karr, a pediatrician and environmental epidemiologist at the University of Washington and the senior author of the study, said that continued exposure to pollutants, both in utero and after birth, increases the potential of long-term adverse health effects.

The study’s authors emphasize that they adjusted the results for sociodemographic, behavioral and psychological factors to zero in on the role played by air contaminants. “What contributes to a child’s overall health and in this case, healthy development of brain and behavior?” said Karr, who is a professor at the university’s School of Public Health and School of Medicine. “Healthy nutrition, a good night’s sleep, feeling safe, feeling loved. Air pollution is just part of that formula. 

“It’s just another potential insult. And it might be in the context of any one child—it might be the thing that kind of tips them into a direction where it really becomes a clinical diagnosis. It becomes something that is really problematic in their everyday life.”

Researchers studied nearly 2,000 pregnant people in six different cities before and after they gave birth. Once the children were born and grew into toddlerhood, researchers conducted IQ tests to measure cognitive abilities and evaluated them with the Child Behavior Checklist, a tool commonly used by therapists and other caregivers to evaluate behavior problems.

One of the researchers, Kaja LeWinn, a social epidemiologist at University of California, San Francisco, said that while the results of the study are compelling on their own, they also add to a growing body of research on the effects of air pollution on children, especially those from low-income households and marginalized communities.

“We’re seeing this consistently,” LeWinn said, citing a study that found that people of color are exposed to higher levels of particulate matter pollution than other Americans. “These disparities or these social inequities are along the lines of race, ethnicity, as well as of economic status. That is something I feel like we need to pay attention to.”

Nicholas Newman, a pediatrician who was not involved in the study, said the research underscored the importance of continued research into how pollutants affect parts of the body that are not related to breathing ailments.

“As we’re finding out that there are effects on the body beyond the respiratory system in the air, what is probably of most interest right now is how it affects the developing brain,” said Newman, an associate professor of pediatrics and environmental and public health sciences at the University of Cincinnati. “Some of the things that we’re concerned about are how these particularly small particles may either get into the brain directly and affect it or cause another problem there.”

Ni said that researchers hope their work can inform public policy decisions around the mitigation of hazardous particles in the air. “The U.S. has gone a long way under the Clean Air Act to reduce air pollution, so compared to many countries, the air pollution levels inside the U.S. are relatively low,” she said. “So we feel like maybe in the future policy can have an even lower threshold level.”

Ni said she also hopes that steps can be taken to protect especially vulnerable populations, including high-risk pregnant women and children who live with respiratory ailments, from the harmful effects of air pollution.

Karr underlines that the research also has implications for efforts to address climate change.

“Air pollution and climate change are inextricably linked,” Karr said. “So if we start to really get serious about addressing climate change, we’re going to have a reduction in some of these air pollutants that also have direct health consequences.”