Air pollution impacts brain development in children

Exposure to certain pollutants, such as fine particles (PM2.5) and nitrogen oxides (NOx), during pregnancy and childhood has been linked to alterations in the microstructure of the brain’s white matter. 

Some of these effects persist into adolescence, according to a new study led by the Barcelona Institute for Global Health (ISGlobal). 

White matter is an essential brain structure responsible for connecting different brain regions. The findings highlight the urgent need to address air pollution as a public health concern, particularly for pregnant women and children.

Air pollution and brain development

There is mounting evidence suggesting that air pollution negatively impacts neurodevelopment in children. 

While earlier research using imaging techniques has explored how air pollutants affect white matter, these studies were limited to snapshots at a single timepoint. They also failed to track participants over the course of childhood.

“Following participants throughout childhood and including two neuroimaging assessments for each child would shed new light on whether the effects of air pollution on white matter persist, attenuate, or worsen,” explained study senior author Mònica Guxens. This is precisely what her team set out to investigate.

Childhood exposure to various pollutants 

The study examined over 4,000 children who had been followed from birth as part of the Generation R Study in Rotterdam, the Netherlands. 

The research team estimated the children’s exposure to 14 different air pollutants during pregnancy and childhood based on where their families lived. 

For 1,314 of these children, the researchers were able to analyze data from two brain scans – one at around age 10 and the other at age 14 – to assess changes in the microstructure of white matter.

Changes in white matter development 

The analysis revealed that exposure to pollutants like fine particles (PM2.5) and nitrogen oxides (NOx) was associated with differences in white matter development. 

Specifically, higher exposure to PM2.5 during pregnancy, and to PM2.5, PM10, PM2.5-10, and NOx during childhood, correlated with lower levels of fractional anisotropy (FA) – a measure of how water molecules diffuse within the brain. 

In more developed brains, water flows more in one direction, leading to higher fractional anisotropy values. The study found that this impact persisted into adolescence, with evidence of long-term effects on brain development. 

Each increase in air pollution exposure corresponded to a developmental delay in fractional anisotropy equivalent to over five months.

Are small particles reaching the brain?

“We think that the lower fractional anisotropy is likely the result of changes in myelin, the protective sheath that forms around nerves, rather than changes in the structure or packaging of the nerve fibers,” said lead author Michelle Kusters, a researcher at ISGlobal. 

The exact mechanism by which air pollutants affect myelin remains unclear, but it may involve small particles reaching the brain or inflammatory responses triggered by particles entering the lungs. 

This process could lead to neuroinflammation, oxidative stress, and, ultimately, neuronal death, as seen in animal studies.

Additional impacts on white matter

The experts also discovered that some pollutants influenced another measure of white matter, called mean diffusivity (MD), which typically decreases as the brain matures. 

Higher exposure to pollutants like silicon in fine particles (PM2.5) during pregnancy was associated with initially higher mean diffusivity, which then decreased more rapidly as the children aged. This suggests that certain effects of air pollution may lessen over time.

“Even if the size of the effects were small, this can have a meaningful impact on a population scale,” Guxens said.

Stricter pollution guidelines are needed

The researchers also found that these effects occurred at levels of PM2.5 and PM10 above the World Health Organization’s recommended limits but below those currently allowed by the European Union. 

“Our study provides support to the need for more stringent European guidelines on air pollution, which are expected to be approved soon by the European Parliament,” Guxens added.

Lasting changes in white matter

In a previous study, Guxens and her team demonstrated that white matter microstructure could also be affected by early exposure to heat and cold, particularly among children living in lower-income neighborhoods.

“Even though we found an attenuation of the initial association between air pollution and higher MD in early adolescence, residential outdoor air pollution exposure during pregnancy and childhood was also associated with lasting changes in white matter microstructure across adolescence,” wrote the study authors.  

Each increase in exposure to air pollution corresponded to more than a 5-month delay in the development of fractional anisotropy, noted the researchers.

The study is published in the journal Environmental Research. 

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