Breathing polluted air is linked to lagging brain and cognitive growth in young teenagers

A new study published in Developmental Cognitive Neuroscience provides evidence that breathing high levels of neighborhood air pollution tends to slow down normal brain and mental development in young teenagers. Scientists discovered that children living in areas with high amounts of fine particulate matter or surface ozone showed less maturation in their brain structures and problem-solving skills over a two-year period compared to peers in cleaner environments.

The transition from childhood to early adolescence is a key phase for structural brain development and mental growth. During this time, the brain goes through physical changes and network reorganization to help young people maintain focus, process information, and regulate their behaviors. Scientists conducted this research to understand how specific environmental factors might disrupt these natural developmental processes.

“Environmental influences could shape many aspects of the brain and cognitive development,” said Omid Kardan, an assistant professor of psychiatry at the University of Michigan and the study’s lead author. “I am particularly interested in teasing apart the social and physical environmental exposures as they may provide different opportunities and scales for potential interventions. Air pollution is a physical environmental factor of particular interest, because research including animal models has shown multiple pathways by which it can impact the brain and its development.”

Previous studies looking at air pollution and young brains have yielded mixed results. Some past research suggested that pollution affects brain thickness, while other studies found no such connection. The authors noted that these inconsistencies might happen because earlier research often did not connect the brain imaging data with actual behavioral tests, like memory or attention tasks.

In addition, previous studies typically evaluated children’s brain networks using templates based entirely on adult brains. Because a preteen’s brain is still shifting from childhood patterns to adult patterns, using adult templates might miss subtle developmental steps. To address these gaps, the researchers designed a study that combined multiple types of brain scans with an array of cognitive performance tests.

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To gather their data, the scientists used information from the Adolescent Brain Cognitive Development Study. This is a massive, ongoing project tracking the biological and behavioral development of thousands of youths across the United States. “I’d like to acknowledge the Adolescent Brain Cognitive Development (ABCD) Study that has made these types of inquiries possible due to its large sample size and longitudinal neuroimaging and behavioral assessments,” Kardan said.

The scientists focused on a pool of 3,645 participants who completed brain scans and cognitive tests at age nine or ten, and then again two years later at age eleven or twelve. The researchers looked at three specific measures to track neurocognitive maturation over this period. First, they measured changes in the thickness of the cerebral cortex, which is the brain’s outer layer of gray matter.

As children grow into teenagers, this outer layer naturally thins out in a healthy process called synaptic pruning. During synaptic pruning, the brain eliminates unused connections to become more specialized and efficient. Second, the scientists mapped how the youths’ brain networks communicated with each other at rest.

They calculated a maturity score by comparing each child’s functional brain connectivity to both a typical infant brain and a typical young adult brain. A maturing brain should look increasingly different from the infant template and increasingly similar to the adult template over time. Third, the researchers measured overall cognitive performance using a combination of standardized computer tasks.

These tasks tested a wide range of mental abilities, including sustained attention, working memory, processing speed, and reading recognition. They also tested inhibitory control, which is the mental ability to ignore distractions, control impulses, and stay focused on a specific goal. Next, the researchers categorized the youths based on the average air quality of their home neighborhoods.

They used official thresholds for unhealthy air established by the United States Environmental Protection Agency. Fine particulate matter consists of tiny inhalable particles often produced by vehicle exhaust, wildfires, and power plants. Surface ozone is an invisible gas created when pollutants chemically react with sunlight.

The researchers created two separate comparisons to study each pollutant without the effects overlapping. In the first analysis, they identified 348 youths exposed to unhealthy levels of fine particulate matter. They matched these participants with 279 peers who lived in low-pollution areas.

For the second analysis, the researchers compared 355 youths exposed to high surface ozone with 324 peers from low-ozone environments. In both groups, the scientists paired the high-pollution and low-pollution youths based on household income, parental education, race, biological sex, and age. This matching process helped ensure that differences in brain development were associated with air quality rather than socioeconomic status or systemic inequalities.

“We used a powerful method that isolates the association of neural and cognitive development with exposure to air pollutants (rather than other variables such as socioeconomic factors),” Kardan told PsyPost. “Our results showed youth (9-12 years old) who live in high-pollution neighborhoods show a lower-than-expected growth in neural and cognitive measures compared to their demographically matched peers who live in low-pollution areas.”

Interestingly, the demographic profiles of the two pollution groups differed noticeably. “The youth exposed to high fine particulate matter (PM2.5) and those exposed to high Ozone (O3) pollutants were respectively of lower and higher socioeconomic status (SES) than the average SES in the sample,” Kardan noted. “But they both showed this delay-like pattern in neurocognitive development compared to their respective low-pollution peers.”

When looking at the youths over the two-year period, the researchers noticed distinct developmental differences between the exposure groups. The youths living in the low-pollution environments showed all the expected signs of healthy biological and mental maturation. Over the two years, these control groups experienced a normal thinning of the gray matter in their brain cortex.

The low-pollution youths also displayed an increase in adult-like brain network connectivity, moving further away from childhood brain patterns. At the same time, their average scores on the cognitive memory and attention tasks steadily increased. These results confirmed that the researchers’ measurements were accurately capturing the normal transition into adolescence.

In contrast, the young teenagers living in areas with high fine particulate matter or high surface ozone did not show these typical developmental milestones. Their brains showed less structural thinning and less network maturation than the brains of their unexposed peers. The brain networks of the youths in the high-pollution groups remained closer to early-life patterns, and they demonstrated less improvement in the cognitive performance tasks.

“The sizes of the isolated associations are small from a statistical point of view,” Kardan said. “However, given the importance of healthy cognitive development and its downstream consequences on multiple domains of youth mental health, resilience, and academic success, the results hold practical significance that may outsize the statistical effect sizes.”

While these findings are highly informative, the study does contain a few potential limitations. Kardan emphasized that readers should not confuse a slower rate of development with lower overall intelligence. “Lower-than-expected growth in neurocognitive measures is different from general lower cognitive ability (the former is about trajectory while the latter is about overall mean),” Kardan said. “Our finding in this study is mainly about trajectory differences in cognition not overall mean cognitive performance.”

Another important caveat involves the timing of the air quality measurements. “Our method provides some confidence in the specificity of the associations,” Kardan noted. “However, only the brain and cognitive measures were longitudinal and the air pollution measures were not longitudinal in the study, so we can’t infer causality here.”

The air pollution data was based entirely on the outdoor air quality of the participants’ residential neighborhoods at the beginning of the study. This means the measurements might not capture the exact amount of pollution each child actually breathed in while attending school or spending time indoors. The data also does not account for families moving to different neighborhoods or cities during the two years of the study.

Future studies might examine the specific chemical components that make up fine particulate matter in different geographical regions. By looking at specific chemicals, scientists could provide a detailed understanding of which elements pose the highest risk to a developing nervous system. “We plan to expand the analyses to more waves of the ABCD Study to include ages after 13 years old,” Kardan said.

The study, “Neighborhood air pollution is associated with attenuated neurocognitive maturation over early adolescence“, was authored by Omid Kardan, Chacriya Sereeyothin, Kathryn E. Schertz, Mike Angstadt, Alexander S. Weigard, Marc G. Berman, Mary M. Heitzeg, and Monica D. Rosenberg.