Aromatic Polyclinical Hydrocarburates (APHs), found in exhaust fumes, cigarette smoke, burning fossil fuels, smoke from burning food, and other pollutants such as industrial fumes and burning rubber and chemicals are associated with a higher incidence of Attention-Deficit Hyperactivity Disorder (ADHD), according to the study published in Environment International.
The main culprit is traffic pollution, reveals the team at Barcelona's Global Health Institute (ISGlobal), whose BREATHE Project studied 242 male and female children aged between eight and 12 from 39 schools in the metropolitan area.
Air contamination levels were tested, and the participant pupils' brain structure examined via MRI scans.
Results showed that high exposure to APHs, particularly Benzopyrene, correlated with a reduction in the caudate nucleus, a component of the basal ganglia.
The caudate nuclei are located near the central part of the brain, straddling the thalamus – which controls basic functions such as feeling hungry, thirsty, hot or cold – and play a major role in motor processes, or movement and organ function.
These nuclei have also been associated with non-motor functions such as procedural learning – or skills that are exercised automatically once acquired, such as walking or writing – associative learning, and inhibitory control, or the 'moral brake' on instinctive behaviour which falls within the brain's 'reward system'.
Alterations in either caudate nucleus are believed to play a part in the onset of Alzheimer's, Parkinson's and Huntington's Diseases, as well as in OCD, Type I bipolar (manic) depression, schizophrenia and ADHD.
Reduction in the basal ganglia, specifically in the caudate nucleus, is associated with alterations in the neurotransmitter dopamine – excessive dopamine is believed to be behind schizophrenia, psychosis and hyperactivity.
The ISGlobal has found that a reduction of just 2% in the caudate nuclei is enough to cause ADHD in children.
And this reduction only needs an increase in air pollution of 70 picograms (70pg) – a picogram being one-trillionth of a gram – of Benzopyrene per cubic metre of air.
In simpler terms, says ISGlobal researcher Marion Mortamais, levels of traffic pollution considered 'safe' in accordance with EU guidelines and even those which are barely-perceptible to the layperson are enough to affect brain structure leading to psychological disorders, especially in children – whilst greater levels of pollutants may be needed to actively contribute to schizophrenia, Alzheimer's and Parkinson's, for example, very low quantities can still cause ADHD.
Whether the studies extrapolate to ADD, or Attention-Deficit Disorder without the hyperactivity element, is not known, but given that ADD often responds to dopamine-enhancers, it is likely this would not be the case.
The upshot, says Dr Mortamais, is that air pollution, especially from industry and traffic, 'needs to be reduced urgently'.
“These results add to the abundant scientific evidence that highlights the extreme dangers of atmospheric contamination, particularly from traffic, and suggest it would be convenient for EU directives covering maximum annual pollution readings to be re-evaluated,” says Dr Jordi Sunyer, dean of Barcelona's Pompeu Fabra University and head of the child health programme at ISGlobal.
The long- and short-term perils of air pollution are no great secret – as well as being associated with headaches, respiratory problems, tiredness and nausea, traffic and industry pollutants kill six million people worldwide every year.
Directly-caused fatal conditions include heart and lung problems, strokes and cancer.
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