Study links environmental pollutants to increased risk of autism spectrum disorder

By Priyanjana Pramanik, MSc.Reviewed by Lily Ramsey, LLMSep 5 2024

​​​​​​​Study: Environmental pollutants as risk factors for autism spectrum disorders: a systematic review and meta-analysis of cohort studies. Image Credit: Alena Lom/Shutterstock.com​​​​​​​

In a recent review article published in BMC Public Health, researchers discussed the association between environmental pollutants and the incidence of Autism Spectrum Disorder (ASD).

They concluded that specific pollutants, including nitrogen dioxide, copper, and certain phthalates, are significantly linked to ASD, highlighting the need to identify factors that increase risk to develop effective prevention strategies.

Background

ASD, which is a neurodevelopmental condition that affects behavior, communication, and social interaction, has become more prevalent around the world.

A systematic review estimates that globally, 1 in 100 children are affected, and the Centers for Disease Control and Prevention (CDC) report that 1 out of every 36 children in the U.S. will receive an ASD diagnosis.

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

The timing of exposure to these pollutants is crucial, as early exposure during prenatal and early postnatal periods can significantly impact neurodevelopment.

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

About the study

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

The included studies involved nearly 1.3 million individuals, mostly children, with a smaller number of adolescents. The studies collectively investigated the impact of 129 different pollutants, including air pollutants and toxic substances.

Individual pollutants considered risk factors

Several pollutants were frequently studied, including nitrogen dioxide, particulate matter (PM 2.5 and PM 10), ozone, and various phthalates and polychlorinated biphenyls (PCBs). The studies showed significant associations between certain pollutants and an increased risk of ASD.

Pollutants such as monobutyl phthalate, nitrogen dioxide, mono-3-carboxypropyl phthalate, copper, and one PCB were notably linked to ASD. For instance, nitrogen dioxide exposure was associated with a 20% increase in the risk of ASD, although this finding had high heterogeneity between studies.

Conversely, copper exposure showed a smaller but significant association with ASD, with low variability among the studies.

Categories of pollutants

The meta-analysis also revealed a significant association between ASD and several pollutants when grouped by categories. Carbon monoxide, metals like iron and molybdenum, and nitrogen oxides were positively linked to ASD.

Conversely, carbamates and organophosphates, a type of pesticide, were negatively associated with ASD, indicating that higher exposure to these compounds was linked to a lower likelihood of ASD, though these findings had high variability.

Sensitivity and robustness checks

Sensitivity analyses, which tested the robustness of the results, confirmed the significant associations between nitrogen dioxide, copper, and certain phthalates with ASD.

These results held consistent even when considering different methods for diagnosing or screening for ASD. However, one PCB could not be included in these analyses due to a lack of sufficient studies.

Conclusions

This systematic review, including a meta-analysis, explored how environmental pollutants and ASD are linked in adolescents and children. The findings showed that being exposed to certain pollutants, such as copper, nitrogen dioxide, specific phthalates, and PCBs, increases ASD risk.

Further analyses also linked exposure to metals, nitrogen oxides, and carbon monoxide with a higher risk of ASD development.

The study's results align with previous research on pollutants like PCBs and carbon monoxide but showed some differences regarding nitrogen dioxide and certain phthalates, likely due to variations in study designs and pollutant exposure times.

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

The researchers also noted a potential risk of publication bias, where studies showing significant results might be more likely to be published than those without significant findings.

In summary, this study highlights a potential link between exposure to certain environmental pollutants and ASD development risk, particularly pollutants like nitrogen dioxide, copper, and certain phthalates and PCBs. The study's strengths include a comprehensive search strategy and a focus on high-quality studies, but limitations such as variability in exposure times and detection methods were noted.

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