The global increase in air pollution constitutes a worrying environmental risk factor for neurodevelopmental disorders, and more particularly autism spectrum disorders (ASD) which today affect between 1 and 1.5% of the population.
A review of recent epidemiological studies
In an article published in Brain Medicinetwo authors analyzed recent epidemiological studies that revealed convincing associations between exposure to specific air pollutants and an increased risk of ASD. Among these pollutants we find fine particles (PM), including PM10 and PM2.5, the latter being particularly dangerous. Nitrogen oxides (NO, NO2) are among the common pollutants originating mainly from emissions from thermal vehicles and fossil and industrial fuels.
Risk factors also include ozone (O3), a highly reactive oxygen gas produced at ground level by a photochemical reaction between two classes of atmospheric pollutants: volatile organic compounds and nitrogen oxides. Finally, more and more evidence points to sulfur dioxide (SO2), an air pollutant produced in particular by thermal vehicles and the combustion of fossil fuels in power plants.
While the overall prevalence of ASD partly reflects the expansion of diagnostic criteria and improved screening, a growing body of data points to the role of interactions between genes and environment in the etiology of these disorders.
Potential mechanisms of action
Airborne pollutants can trigger multiple pathogenic mechanisms, including neuroinflammation, oxidative and nitrosative stress, epigenetic modifications, disruptions of the glutamatergic/GABAergic neurotransmitter system, endocrine disorders, and metabolic dysregulation.
The timing of exposure appears to be crucial, with greatest vulnerability during prenatal development and early childhood, when critical neurodevelopmental processes, such as neuronal migration, synaptogenesis and myelination, occur.
To date, no study has looked at biomarkers specifically reflecting the impact of air pollutants. According to the authors, such biomarkers could enable early prevention of ASD by identifying individuals at high risk during the prenatal period or pre-symptomatic stages.
This would allow early interventions in cases of neurodevelopmental abnormality or to crowd out environmental triggers such as exposure to air pollution. Possible biomarkers discussed in the article are elevation of 3-nitrotyrosine, changes in GABA and glutamate in the blood, high expression of PCR during pregnancy and the presence of micronuclei.
What avenues for research?
Different confounding factors should be taken into account when studying the link between air pollutants and ASD. These factors can include microbiome, nutrition, financial aspect, education level, social aspects and workplace. Lifestyle factors, such as active and passive smoking during pregnancy, should also be considered.
It also seems essential to take into account the place of residence and socio-economic status, the poorest neighborhoods being more likely to suffer more pollution, to present a greater vulnerability to these factors as well as a risk of ASD increased.
For the authors of this article, research priorities should focus on how pollutants affect brain development in genetically predisposed individuals, particularly during pregnancy and early childhood. Better tools are needed to identify individuals at highest risk and to develop practical prevention measures.
Given the increasing level of global pollution, this knowledge could help shape public health policies to protect future generations from environmental factors that may contribute to ASD.
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