Test: Environmental Pollution as Risk Factors for Autism Spectrum Disorders: A Systematic Review and Meta-Analysis of Cohort Studies. Photo source: Alena Lom/Shutterstock.com​​​​​​​

In a recent review article published in BMC Public HealthScientists have discussed the link between environmental pollution and the occurrence of autism spectrum disorders (ASD).

The researchers concluded that specific pollutants, including nitrogen dioxide, copper, and some phthalates, have a significant association with ASD, emphasizing the need to identify risk factors in order to develop effective prevention strategies.

Background

ASD, a neurodevelopmental disorder that affects behavior, communication, and social interactions, is becoming increasingly common around the world.

A systematic review estimates that ASD affects 1 in 100 children worldwide, and the Centers for Disease Control and Prevention (CDC) reports that 1 in 36 children in the U.S. will be diagnosed with ASD.

Environmental pollutants can disrupt cellular metabolism, cause oxidative stress, and trigger brain inflammation, which may contribute to ASD. Pollutants can also cause genetic damage and epigenetic changes that affect brain development.

The timing of exposure to these contaminants is critical because early exposure during the prenatal and early postnatal periods can significantly impact neurological development.

However, reviews of these links have limitations, often focusing on air pollution and using limited search criteria, potentially missing important evidence. Further research into these associations could lead to a better understanding of how pollutants affect ASD risk.

About the study

This systematic review and meta-analysis examined the association between ASD and various environmental pollutants. After screening 5,780 individual studies, the researchers included 27 articles in the systematic review and used 22 in their meta-analysis.

The included studies covered almost 1.3 million people, mostly children, with smaller numbers of adolescents. The studies together examined the effects of 129 different pollutants, including air pollutants and toxic substances.

Individual pollutants considered risk factors

Several pollutants have been frequently studied, including nitrogen dioxide, particulate matter (PM 2.5 and PM 10), ozone, and various phthalates and polychlorinated biphenyls (PCBs). Studies have shown significant associations between some pollutants and increased risk of ASD.

Contaminants such as monobutyl phthalate, nitrogen dioxide, mono-3-carboxypropyl phthalate, copper, and one PCB were clearly associated with ASD. For example, exposure to nitrogen dioxide was associated with a 20% increased risk of ASD, although this finding had large heterogeneity between studies.

In contrast, copper exposure showed a smaller but significant association with ASD, with little variability between studies.

Pollution Categories

The meta-analysis also revealed a significant association between ASD and several pollutants grouped by category. Carbon monoxide, metals such as iron and molybdenum, and nitrogen oxides were positively associated with ASD.

In contrast, carbamates and organophosphates (a type of pesticide) were negatively associated with ASD, indicating that higher exposure to these compounds was associated with a lower likelihood of ASD, although these results were highly variable.

Sensitivity and immunity checks

Sensitivity analyses to check the robustness of the results confirmed significant associations between nitrogen dioxide, copper and some phthalates and ASD.

These results were consistent even when controlling for different methods of diagnosing or screening ASD. However, one PCB could not be included in these analyses due to insufficient studies.

Conclusions

This systematic review, including meta-analysis, examined how environmental pollutants and ASD are associated in adolescents and children. The results showed that exposure to some pollutants, such as copper, nitrogen dioxide, certain phthalates, and PCBs, increases the risk of ASD.

Further analyses also found an association between exposure to metals, nitrogen oxides and carbon monoxide and a higher risk of developing ASD.

The study’s results are consistent with those of previous studies on contaminants such as PCBs and carbon monoxide, but showed some differences for nitrogen dioxide and some phthalates, likely due to differences in study designs and timing of exposure to contaminants.

Although these findings suggest that environmental pollutants may be associated with an increased risk of ASD, the certainty of the evidence was rated as low or very low. This was primarily due to inconsistencies between studies, such as differences in how pollutants were measured and different study designs.

The researchers also noted the potential risk of publication bias, which means studies that show significant results may be more likely to be published than those that do not produce significant findings.

In summary, this study highlights a potential association between exposure to certain environmental contaminants and the risk of developing ASD, particularly contaminants such as nitrogen dioxide, copper, and some phthalates and PCBs. Strengths of the study include the comprehensive search strategy and focus on high-quality studies, but limitations were noted, such as variability in exposure time and detection methods.

The researchers emphasize the need to use standardized methods in future studies to better understand how environmental pollutants contribute to ASD and to develop effective prevention strategies. Further studies with more consistent study designs and measurements are needed to strengthen these conclusions and improve the certainty of the evidence.