The possible role of air pollution in the link between ADHD and obesity

A critical review of developmental exposure to particulate matter, autism spectrum disorder, and attention deficit hyperactivity disorder

Autism spectrum disorder (ASD) and attention deficit (hyperactivity) disorder (ADD/ADHD) are key focuses of current health research due to their increasing prevalence. The objective of this systematic literature search and critical review was to evaluate whether the human epidemiologic data indicate a pattern of association between ASD or ADD/ADHD and developmental exposure to particulate matter (PM), with a focus on exposures encountered before the age of three. A MEDLINE and EMBASE search was conducted; following preliminary and full-text screening, 14 relevant articles were identified for review. Three of the 14 studies were prospective cohort studies evaluating exposure to PM₁₀; 11 studies had a case-control design. There was no consistent association between developmental PM exposure and ASD across the three of the cohort studies. Seven of the case-control studies examined the relationship between PM_2.5 and/or PM₁₀ and ASD; four examined the relationship between developmental diesel PM exposure and ASD. Overall, there was low external consistency in results among studies of PM_2.5/PM₁₀ and ASD, with some reporting high internal consistency without significant associations, others showing associations with high internal consistency for specific exposure windows only (e.g., third trimester), and still others showing high consistency for moderate to strong associations between PM and ASD. The majority of studies reporting significant results had low effect sizes in conjunction with small sample sizes. The four studies of diesel PM and ASD also had low external consistency of results. Only one study evaluated associations with ADD/ADHD, and it found no significant associations with PM₁₀. The inconsistent findings across studies of developmental exposure to PM and ASD may be attributed to differences in the study populations, exposure assessments, outcome assessments, or chance. Further research is needed to understand the underlying biological mechanisms that lead to ASD and ADD/ADHD and how PM might be involved in those mechanisms, if at all. High-quality epidemiologic studies are also needed to conclusively determine whether developmental PM exposure is a causal factor for ASD or ADD/ADHD, with focus on a well-developed exposure assessment.

Exposure to ambient PM10 and nitrogen dioxide and ADHD risk: A reply to Min & Min (2017)

Min and Min (2017) conducted an epidemiological investigation that revealed further support of a link between exposure to air pollution and risk for attention-deficit hyperactivity disorder (ADHD) in childhood. We have previously reported that exposure to the agricultural and combustion pollutant, nitrous oxide (N₂O), may be a primary environmental trigger in the onset of neurodevelopmental disorders, like ADHD and autism spectrum disorders. In order to validate our prior work pointing to an association between farm use of nitrogen fertilizers and a severe ADHD phenotype, we have utilized a different statistical approach (i.e., Poisson regression methodology) including two-way fixed effects. The results reported in this correspondence indicate that for a one-log unit increase in the farm use of nitrogen fertilizers, hospitalization risk for ADHD and conduct disorders increases by a factor of 1.16 (p<0.017), which was a statistically significant increase in risk after multiple pollutant comparison correction. Exposure to PM10 and NOx in this analysis was not associated with an increased risk of hospitalization for ADHD and conduct disorder. We are able to validate our prior conclusions and, therefore, suggest that future analyses dedicated to improving the literature on the association between air pollution and risk of ADHD take into account environmental emissions of N₂O.

Does environmental exposure to the greenhouse gas, N2O, contribute to etiological factors in neurodevelopmental disorders? A mini-review of the evidence

BACKGROUND

Neurodevelopmental disorders are increasing in prevalence worldwide. Previous work suggests that exposure to the environmental air pollutant and greenhouse gas - nitrous oxide (N₂O) - may be an etiological factor in neurodevelopmental disorders through the targeting of several neural correlates.

METHODOLOGY

While a number of recent systematic reviews have addressed the role of general anesthesia in the surgical setting and neurodevelopmental outcomes, a narrative mini-review was conducted to first define and characterize the relevant variables (i.e., N₂O, attention-deficit hyperactivity disorder [ADHD] and autism spectrum disorders [ASD]) and their potential interactions into a coherent, hypothesis-generating work. The narrative mini-review merges basic principles in environmental science, anesthesiology, and psychiatry to more fully develop the novel hypotheses that neurodevelopmental impairment found in conditions like ADHD and ASD may be due to exposure to the increasing air pollutant, N₂O.

RESULTS

The results of the present mini-review indicate that exposure to N₂O, even at non-toxic doses, may modulate central neurotransmission and target many neural substrates directly implicated in neurodevelopmental disorders, including the glutamatergic, opioidergic, cholinergic, and dopaminergic systems. Epidemiological studies also indicate that early and repeated exposure to general anesthesia, including N₂O, may contribute to later adverse neurodevelopmental outcomes in children.

CONCLUSIONS

The current evidence and subsequent hypotheses suggest that a renewed interest be taken in the toxicological assessment of environmental N₂O exposure using validated biomarkers and psychiatric endpoints. Given the relevance of N₂O as a greenhouse gas, societies may also wish to engage in a more robust monitoring and reporting of N₂O levels in the environment for climactic benefit as well.

Gluten Intolerance and Neurodevelopmental Disorders: Is Nitric Oxide the Common Biomarker Linking These Conditions?

Cruchet et al. attempt to tease out the myths and facts surrounding the growing popularity of certain dietary approaches in the management of neurodevelopmental disorders, like attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASDs). The authors identify a particular exclusionary-type approach that seeks to eliminate dietary gluten. Although the relationship between celiac disease (CD) and ADHD/ASD is not well established, a repeated clinical feature noted in CD is the elevated levels of nitric oxide in serum and urine. Elevated oxidative stress has also been observed in neurodevelopmental conditions, and the author of this correspondence has been the first to propose that chronic, environmental exposure to the air pollutant, nitrous oxide may contribute to these oxidative stress profiles through neural cholinergic perturbation. Therefore, the purpose of this correspondence is to highlight this biochemical connection between these conditions so as to identify the clinical populations who may realize the greatest benefit of these dietary approaches, while minimizing any potential risk of nutrient deficiencies.