Questions surrounding how exposure to outdoor air pollution may alter brain developmental trajectories are gaining traction, in part due to the urgent climate crisis impacting the global community. To this end, the field of environmental neuroscience is growing rapidly, but there is still a lack of studies investigating the long-term impacts of air pollution on child and adolescent brain health, and what it could mean for mental health in youth.

Regulatory bodies across the globe track ambient air pollution and develop air quality standards or recommendations aimed at protecting public health overall. Specifically, the World Health Organization (WHO) and the United States Environmental Protection Agency (U.S. EPA) track particulate matter (PM) of different sizes. The smaller the particulate matter size, the more harmful it is, as smaller particles can travel greater distances and penetrate deeper into lung tissue, potentially crossing over to the bloodstream and, eventually, the brain. PM with a diameter of <2.5 micrometers (PM_2.5) is thought to be especially harmful to human health. Other pollutants, like gaseous toxins such as nitrogen dioxide (NO₂) and ground-level ozone (O₃), also pose important health risks. Children may be particularly vulnerable – they have higher respiratory rates and underdeveloped immune systems, higher rates of change to brain structure and function, and generally spend more time outside, compared to adults. Additionally, sex assigned at birth, pubertal status, and hormonal fluctuations during adolescence all may play a role in how the brain reacts to a toxicant like air pollution.

The study: Childhood exposure to three criteria ambient pollutants and their effects on the longitudinal changes in structural connectivity

White matter is comprised of a fatty layer of tissue (called myelin) that coats an axon and allows for rapid transfer of information from one brain region to the next. This type of brain tissue continues to mature across childhood and adolescence, with notable differences between the sexes. Reductions in the structural integrity of myelin may result in disruptions to speed of information transfer throughout the brain, making it incredibly important for normal sensorimotor, cognitive, behavioral, and emotional function. In a recently published paper in Communications Medicine, our team investigated the longitudinal associations between outdoor air pollution exposure during childhood (ages 9-10 years old) and changes to white matter microstructural integrity over a two-year follow-up period using data from the large U.S.-based cohort, the Adolescent Brain Cognitive Development (ABCD) Study. 

We assessed 8,182 children ages 9-13 years, who had been scanned 1-2 times in a magnetic resonance imaging (MRI) scanner. Of these, 45% had two MRI scans taken about two years apart. We estimated annual average air pollution concentrations by linking three criteria pollutants (PM_2.5, NO₂, and O₃) to geocoded locations based on the child’s residential address in 2016, coinciding with the year the ABCD Study began enrollment. We measured white matter microstructure using restriction spectrum imaging, a novel diffusion MRI technique that allows for the compartmentalization of intracellular spaces, extracellular spaces, and free water to probe the biophysical structure of brain tissue. The two imaging parameters that were of most importance to this study were restricted (aka, intracellular) directional and isotropic diffusion. Intracellular directional diffusion may represent movement of water within an axon - the higher the directional diffusion, the more intact the axonal fiber bundles connecting various brain regions. Intracellular isotropic diffusion may represent water movement within glial cells. Glial cells include astrocytes, oligodendrocytes, and microglia, with the latter supporting the brain in various functions, including (but not limited to) surveilling for debris and acting as the brain’s resident “immune system.” Higher isotropic diffusion could mean there are more cells, or perhaps changes to cell shape or size. Both intra-axonal and intra-glial diffusion have been found to increase with normal development during adolescence.

Air pollution is related to altered white matter development in youth

We found that higher PM_2.5 exposure was related to higher diffusion within the axon in both female and male youth at age 9, with more widespread effects noted in female as compared to male youth. However, PM_2.5 did not affect white matter development over time.

NO₂ had near-global effects on white matter development over time in female youth – indeed, female youth with higher exposure had diminished increases in intra-glial diffusion with age. In male youth, intra-glial diffusion was higher at age 9 only in commissural tracts (i.e., tracts that span both hemispheres of the brain, like the corpus callosum), with no effects on change in white matter microstructure over time.

O₃ had widespread effects on white matter in both sexes at age 9, such that higher exposure related to lower intra-axonal diffusion, but higher intra-glial diffusion. Additionally, O₃ had effects on white matter microstructural development over time in both sexes but were more prevalent in male youth. Specifically, O₃ accelerated change in intra-axonal diffusion in males within the superior corticostriate, a tract implicated in several important motor, sensory, cognitive, and emotional functions.  However, higher O₃ exposure was related to diminished increases in intra-glial diffusion with age in tracts throughout the brain in males, with a similar, yet more focal effect (e.g., affecting only the corticospinal tract, involved in motor function) in female youth.

Implications for future policy

Air pollution has been linked to various neurodevelopmental disorders as well as mood or psychiatric disorders with onset later in life. Exposure to air pollution during these important developmental periods could contribute to individual differences in brain development which may increase an individual’s risk of developing neurocognitive and mental health problems down the line. Importantly, the exposure concentrations of the criteria pollutants in this study were low, falling below current EPA standards but still exceeding the latest WHO guidelines released in September 2021. The results from this study suggest that even at low levels, changes in adolescent brain developmental trajectories are linked to air pollution exposure. Given this, U.S. regulatory bodies should consider more stringent air quality policies in the interest of brain health for all youth.