A toxic air pollutant widespread in urban environments has been shown to shrink the brains of young children.
The discovery, revealed in brain scan images, may explain links between exposure to polycyclic aromatic hydrocarbons (PAHs) and behaviour and learning problems, scientists believe.
Previous studies have suggested that the chemicals can impair the development of unborn babies’ brains, leading to reduced IQ and childhood anxiety and depression.
The new research associated the effects seen in the scans with slower mental processing and behavioural problems, including attention deficit hyperactivity disorder (ADHD) and aggression.
If confirmed, our findings have important public health implications
US lead scientist Bradley Peterson, director of the Institute for the Developing Mind at the Children’s Hospital of Los Angeles, said: “Our findings raise important concerns about the effects of air pollutants on brain development in children, and the consequences of those brain effects on cognition and behaviour.
“If confirmed, our findings have important public health implications, given the ubiquity of PAH in air pollutants in the population.”
PAHs are neurotoxins found in motor vehicle exhaust fumes, oil and coal burning emissions, and tobacco smoke.
Other sources include hazardous waste sites, charred foods, wildfires and agricultural burning.
Peterson’s team studied 40 urban children with Latin and African American mothers drawn from minority communities in New York City.
The children’s progress was followed up to the ages of seven to nine, and their parents completed questionnaires designed to assess PAH exposure.
Previously the researchers had associated higher exposure to PAHs before birth with multiple neurodevelopmental disturbances in the same group of children.
They included developmental delay by age three, reduced verbal IQ at age five, and symptoms of anxiety and depression at age seven.
Magnetic resonance imaging (MRI) scans of the children’s brains showed clear signs of damage to the left hand side of the brain linked to increased PAH exposure before birth.
Reduced amounts of white matter, the nerve fibres that make up the brain’s ‘wiring’, were seen over almost the whole of the left brain surface.
This was associated with slower processing during intelligence testing, and behavioural effects including ADHD symptoms and conduct disorder problems such as aggression.
Higher PAH exposure at age five was also linked to additional white matter disturbances in the dorsal pre-frontal region of the brain, which is important for concentration, reasoning, judgment and problem-solving.
Peterson said: “This is the largest MRI study to date of how early life exposure to air pollutants, specifically PAH, affect the developing mind.
“Our findings suggest that PAHs are contributors to ADHD and other behavioural problems due to the pollutants’ disruptive effects on early brain development.”
The scientists, whose results are reported in the journal Jama Psychiatry, stress that the research was limited by the small number of participants and the fact that the children all came from backgrounds with high levels of poverty and low educational attainment.
Impoverished urban minority populations are known to be disproportionately exposed to air pollutants.
A much larger study is now being undertaken to confirm and extend the findings.