Protocol: Neurodevelopmental Disorder Symptoms In Children And Adolescents Exposed To Ambient Air Pollution: A Scoping Review Protocol

This scoping review will examine epidemiologic studies assessing associations between ambient air pollution exposure and neurodevelopmental disorder symptoms for three childhood neurodevelopmental disorders: ADHD, ASD, and OCD. It will also characterize the modifying roles of socioeconomic and neighbourhood characteristics.

This protocol has been registered with Open Science Foundation: https://doi.org/10.17605/OSF.IO/KDN7U.

Background

Neurodevelopmental disorders in children and adolescents

There is established evidence for the interaction between childhood neurodevelopmental disorders (NDDs) and genetic, social, and environmental factors (1–5). Attention Deficit Hyperactivity Disorder (ADHD; prevalence 5-11%), Autism Spectrum Disorder (ASD; 1-3%), and Obsessive-Compulsive Disorder (OCD; 0.5-2%) are among the most common NDDs, affecting children and adolescents globally (6). Childhood NDDs are often co-occurrent with other NDDs (homotypic comorbidity) and with additional psychiatric pathologies (heterotypic comorbidity) such as anxiety and mood disorders (6).

Measuring the mental health burden in children and young people requires looking beyond clinical samples to dimensional symptom scores from population-based cohorts (7). Symptom rating scales implemented in community-based samples capture children who meet threshold criteria for an NDD but have not received a formal diagnosis given various healthcare disparities related to low income, race/ethnicity, etc. (8)

Note on OCD classification: In DSM-5, OCD is classified under “Obsessive-Compulsive and Related Disorders” rather than the Neurodevelopmental Disorders chapter. Its inclusion in this review is justified by its childhood onset, high co-occurrence with ADHD and ASD, and its established use as a neurodevelopmental outcome in the environmental epidemiology literature (5).

Environmental exposures and neurodevelopmental outcomes

Air pollution is a significant and increasing global risk factor for children and youth (9). Several ambient environmental exposures are associated with ADHD, ASD, and OCD incidence, acting as either risk or protective factors depending on exposure level and timing (10–14). Prenatal and early-life PM2.5 exposure is reported in multiple cohort studies to raise ASD risk and impair cognitive development (11,15). NO2, a marker of traffic-related pollution, is linked to psychomotor delays and attention difficulties in children (13). Children with greater greenspace access score lower on NDD symptom measures including OCD, plausibly because greenspace reduces pollutant exposure, supports physical activity, and lowers physiological stress (14,16,17).

Most of this evidence concerns clinical diagnosis rather than dimensional symptom severity, with some exceptions (18,19). Prior systematic reviews and meta-analyses have predominantly treated ASD or ADHD diagnosis as a binary outcome, rather than examining scale scores that capture sub-threshold and severity variation (10). Most prior reviews have also been confined to a single NDD and a single exposure type, which limits understanding of multi-exposure effects and cross-diagnostic patterns. Clarifying how environmental exposures shape NDD trait severity, rather than diagnosis incidence, could directly inform community intervention priorities and strengthen the case for urban environmental standards protective of child neurodevelopment.

Rationale and justification for a scoping review

This review examines studies assessing associations between environmental exposures and NDD trait scores, and characterizes the modifying roles of social, neighbourhood, and genetic factors (20). Given the heterogeneity of NDD outcome measures, exposure types, and study designs in this area, a scoping review is more appropriate than a systematic review or meta-analysis. It maps evidence across multiple exposures and NDD outcomes without requiring pooling of effect estimates from incompatible measurement scales.

Prior meta-analyses have typically been restricted to a single exposure-outcome pair: Shang et al. (2020) for NO2 and psychomotor development; Lin et al. (2022) for particulate matter and NDDs; Alter et al. (2024) for PM2.5 and IQ (2–4). The present review takes a broader mapping approach to characterize the breadth of measurement approaches across multiple exposures (environmental and social) and NDD types, and to identify evidence gaps and research priorities.

The format is also appropriate because the use of psychometric outcomes in environmental NDD research is still emerging. A scoping review lays the groundwork for subsequent systematic reviews with more precisely defined questions.

Prior reviews

Current systematic and scoping reviews in this area examine studies that measure only the incidence rate or clinical diagnosis of NDDs in relation to environmental exposures (2–4,10–12,21–23). From an initial search of systematic and scoping reviews in December 2025, a search of PROSPERO, MEDLINE, the Cochrane Database of Systematic Reviews, and JBI Evidence Synthesis was conducted. No current or in-progress scoping reviews or systematic reviews were identified that specifically address trait scores and NDD symptoms for childhood NDDs in relation to multiple environmental and social exposures.

Gao et. al. (2025) conducted an umbrella review of environmental pollutant exposure and neurodevelopmental outcomes, including collection of assessment criteria or tools used (10). However, this review didn’t provide a discussion of included studies or the relationship between NDD symptoms and environmental exposures.

Numerous reviews have examined individual neurodevelopmental disorders or specific environmental exposures, but most focus on clinical incidence for a single NDD and do not address symptom severity or multi-morbidity. Prior reviews also rarely examine multiple environmental and social exposures and simultaneously. This review addresses both gaps.

Primary research question(s) 

The objective of this review is to map epidemiologic studies that assess the associations between ambient air pollution and neighbourhood environmental exposures (ambient air pollution and greenspace) and neurodevelopmental disorders (ADHD, ASD, and OCD) symptoms in children and adolescents, and to characterize the modifying roles of neighbourhood-level socioeconomic characteristics.

The following questions operationalize the review objective and guide the inclusion criteria and data extraction:

1. What validated ADHD, ASD, and OCD-related behavioural and/or cognitive measures have been used to (Concept) in children and adolescents aged 0-18 years (Population) exposed to ambient air pollution or greenspace, across any geographic setting (Context)?
2. What types of environmental exposures (PM2.5, PM10, NO2, O3, SO2, greenspace, and other ambient exposures) and what exposure measurement methods (ground monitoring, satellite-derived estimates, land-use regression, NDVI) have been employed in included studies?
3. 3) What socioeconomic and neighbourhood-level characteristics have been examined as potential modifiers of the exposure-NDD symptom score relationship?

Secondary research question(s) 

1. What exposure windows (prenatal, early childhood, school-age, cumulative) have been examined, and how have studies operationalized these windows?
2. What evidence gaps exist regarding multi-exposure designs, under-studied NDD types, and under-represented populations or geographic settings?