The Combined Influence of Air Pollution and Home Learning Environment on Early Cognitive Skills in Children

The Role of Environmental Chemicals in the Etiology of Learning Difficulties: A Novel Theoretical Framework

Children from economically disadvantaged communities have a disproportionate risk of exposure to chemicals, social stress, and learning difficulties. Although animal models and epidemiologic studies link exposures and neurodevelopment, little focus has been paid to academic outcomes in environmental health studies. Similarly, in the educational literature, environmental chemical exposures are overlooked as potential etiologic factors in learning difficulties. We propose a theoretical framework for the etiology of learning difficulties that focuses on these understudied exogenous factors. We discuss findings from animal models and longitudinal, prospective birth cohort studies that support this theoretical framework. Studies reviewed point to the effects of prenatal exposure to polycyclic aromatic hydrocarbons on reading comprehension and math skills via effects on inhibitory control processes. Long term, this work will help close the achievement gap in the United States by identifying behavioral and neural pathways from prenatal exposures to learning difficulties in children from economically disadvantaged families.

The Creation of a Multidomain Neighborhood Environmental Vulnerability Index Across New York City

Compared to previous studies commonly using a single summary score, we aimed to construct a multidomain neighborhood environmental vulnerability index (NEVI) to characterize the magnitude and variability of area-level factors with the potential to modify the association between environmental pollutants and health effects. Using the Toxicological Prioritization Index framework and data from the 2015-2019 U.S. Census American Community Survey and the 2020 CDC PLACES Project, we quantified census tract-level vulnerability overall and in 4 primary domains (demographic, economic, residential, and health status), 24 subdomains, and 54 distinct area-level features for New York City (NYC). Overall and domain-specific indices were calculated by summing standardized feature values within the subdomains and then aggregating and weighting based on the number of features within each subdomain within equally-weighted primary domains. In citywide comparisons, NEVI was correlated with multiple existing indices, including the Neighborhood Deprivation Index (r = 0.91) and Social Vulnerability Index (r = 0.87) but provided additional information on features contributing to vulnerability. Vulnerability varied spatially across NYC, and hierarchical cluster analysis using subdomain scores revealed six patterns of vulnerability across domains: 1) low in all, 2) primarily low except residential, 3) medium in all, 4) high demographic, economic, and residential 5) high economic, residential, and health status, and 6) high demographic, economic and health status. Created using methods that offer flexibility for theory-based construction, NEVI provided detailed vulnerability metrics across domains that can inform targeted research and public health interventions aimed at reducing the health impacts from environmental exposures across urban centers.

Disparities in Toxic Chemical Exposures and Associated Neurodevelopmental Outcomes: A Scoping Review and Systematic Evidence Map of the Epidemiological Literature

BACKGROUND

Children are routinely exposed to chemicals known or suspected of harming brain development. Targeting Environmental Neuro-Development Risks (Project TENDR), an alliance of > 50 leading scientists, health professionals, and advocates, is working to protect children from these toxic chemicals and pollutants, especially the disproportionate exposures experienced by children from families with low incomes and families of color.

OBJECTIVE

This scoping review was initiated to map existing literature on disparities in neurodevelopmental outcomes for U.S. children from population groups who have been historically economically/socially marginalized and exposed to seven exemplar neurotoxicants: combustion-related air pollution (AP), lead (Pb), mercury (Hg), organophosphate pesticides (OPs), phthalates (Phth), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs).

METHODS

Systematic literature searches for the seven exemplar chemicals, informed by the Population, Exposure, Comparator, Outcome (PECO) framework, were conducted through 18 November 2022, using PubMed, CINAHL Plus (EBSCO), GreenFILE (EBSCO), and Web of Science sources. We examined these studies regarding authors' conceptualization and operationalization of race, ethnicity, and other indicators of sociodemographic and socioeconomic disadvantage; whether studies presented data on exposure and outcome disparities and the patterns of those disparities; and the evidence of effect modification by or interaction with race and ethnicity.

RESULTS

Two hundred twelve individual studies met the search criteria and were reviewed, resulting in 218 studies or investigations being included in this review. AP and Pb were the most commonly studied exposures. The most frequently identified neurodevelopmental outcomes were cognitive and behavioral/psychological. Approximately a third (74 studies) reported investigations of interactions or effect modification with 69% (51 of 74 studies) reporting the presence of interactions or effect modification. However, less than half of the studies presented data on disparities in the outcome or the exposure, and fewer conducted formal tests of heterogeneity. Ninety-two percent of the 165 articles that examined race and ethnicity did not provide an explanation of their constructs for these variables, creating an incomplete picture.

DISCUSSION

As a whole, the studies we reviewed indicated a complex story about how racial and ethnic minority and low-income children may be disproportionately harmed by exposures to neurotoxicants, and this has implications for targeting interventions, policy change, and other necessary investments to eliminate these health disparities. We provide recommendations on improving environmental epidemiological studies on environmental health disparities. To achieve environmental justice and health equity, we recommend concomitant strategies to eradicate both neurotoxic chemical exposures and systems that perpetuate social inequities. https://doi.org/10.1289/EHP11750.

Cumulative Neurotoxicological Air Pollution Exposure Is Associated with Lower Reading Improvement and Diminished Benefits of Literacy Interventions for Urban Elementary Students of Color

The cognitive and behavioral deficits associated with air pollution exposure may have far-reaching negative effects on children's scholastic achievement. Moreover, air pollution may be conditioning the success of educational investments that support students who face greatest levels of societal adversity. This study examined the direct main effects of cumulative neurotoxicological exposure on annual reading improvement. We also tested the statistical interaction (i.e., moderation) of neurotoxicological exposure and academic intervention sessions on annual reading improvement for a large sample of ethnic minority (95%) elementary school children (n = 6080, k-6th grade) enrolled in a standard literacy enrichment program. These children were all behind grade level in reading and attended predominantly low-income schools (n = 85) in urban settings across the state of California. Multi-level modeling assessments accounted for random effects associated with school and neighborhood environments, and incorporated extensive individual, school, and community level covariates. Findings show individual elementary students of color to progress less in reading when exposed to greater accumulations of neurotoxin air pollution in their home and school environments, with the average deficit equivalent to 1.5 weeks of learning delay per year. Findings also show neurotoxicological exposure to diminish the efficacy of literacy intervention sessions received on reading improvement throughout the school year. Results suggest that pollution abatement can be a salient strategy to help bridge the child educational achievement gap. In addition to several methodological strengths, this study is one of the first to show that ambient pollution can undermine program efficacy of a literacy enrichment program.

Prenatal socioenvironmental exposures and autism spectrum disorder: A web of confusion

Although evidence of heritability for autism spectrum disorder (ASD) is strong, studies of twin pairs suggest that at least some portion of the etiology is attributable to environmental factors, either directly or through interaction with genes. Given the multitude of environmental and psychosocial exposures that have been reported to increase atypical neurodevelopment in offspring, in this article, we summarize what prenatal air pollutant, chemical, and occupational exposures and psychosocial stressors have been reportedly associated with ASD and co-occurring neurodevelopmental disorders. We highlight consistencies in reported associations and recommend areas for research to close gaps in our understanding of environmental risk for ASD. Because this issue is of particular importance in historically marginalized communities and low- and middle-income countries, we also discuss the importance of environmental justice issues and exposure disparities in research, and we advocate for prioritizing policies to reduce disparities and improve service provision in vulnerable populations.

The relationship between home environment and early childhood development of left-behind children under 3 years in rural China

BACKGROUND

Home environment provides stimulation and learning opportunities required for children's early development. However, few studies have focused on the effects of home environment on left-behind children's development in rural China. The study aimed to investigate the relationship between home environment and early childhood development of left-behind children under 3 years old in rural China.

METHODS

Information about sociodemographic characteristics, caregivers' mental health, home environment and children's development was collected. Infant/Toddler Home Observation for Measurement of the Environment Inventory (IT-HOME) was used to assess home environment. Child's development was evaluated with the Chinese version of the Ages & Stages Questionnaire - third edition.

RESULTS

Elevated caregivers' depressive symptoms were associated with a higher risk of total suspected development delay. An increase of one score in the IT-HOME resulted in a 7% decrease in total suspected development delay. Higher scores in the dimensions of involvement and variety were associated with a lower risk of suspected development delay adjusting for covariates.

CONCLUSIONS

Our results have implications for home-based intervention aiming at promoting nurturing care as well as caregivers' mental health, which is required for early development of young left-behind children in rural areas of China.

Relationships between maternal body mass index and child cognitive outcomes at 3 years of age are buffered by specific early environments in a prospective Canadian birth cohort

Fetal and child development are shaped by early life exposures, including maternal health states, nutrition and educational and home environments. We aimed to determine if suboptimal pre-pregnancy maternal body mass index (BMI; underweight, overweight, obese) would associate with poorer cognitive outcomes in children, and whether early life nutritional, educational and home environments modify these relationships. Self-reported data were obtained from mother-infant dyads from the pan-Canadian prospective Maternal-Infant Research on Environmental Chemicals cohort. Relationships between potential risk factors (pre-pregnancy maternal BMI, breastfeeding practices and Home Observation Measurement of the Environment [HOME] score) and child cognitive development at age three (Weschler's Preschool and Primary Scale of Intelligence, Third Edition scale and its subcategories) were each evaluated using analysis of variance, multivariable regression models and moderating analyses. Amongst the 528 mother-child dyads, increasing maternal pre-pregnancy BMI was negatively associated with scores for child full-scale IQ (β [95% CI]; -2.01 [-3.43, -0.59], p = 0.006), verbal composite (-1.93 [-3.33, -0.53], p = 0.007), and information scale (-0.41 [-0.70, -0.14], p = 0.003) scores. Higher maternal education level or HOME score attenuated the negative association between maternal pre-pregnancy BMI and child cognitive outcome by 30%-41% and 7%-22%, respectively, and accounted for approximately 5%-10% greater variation in male children's cognitive scores compared to females. Maternal education and higher quality home environment buffer the negative effect of elevated maternal pre-pregnancy BMI on child cognitive outcomes. Findings suggest that relationships between maternal, social and environmental factors must be considered to reveal pathways that shape risk for, and resiliency against, suboptimal cognitive outcomes in early life.

Parents' Perceived Social Support and Children's Approaches to Learning in Rural China: A Moderated Mediation Model

This study aimed to investigate the relationship between parents' perceived social support (PSS) and young children's approaches to learning (ATLs) in rural China, as well as the mediating effect of the home learning environment (HLE) and the moderating effect of parental negative emotions (PNE). Using the cluster random sampling method, 2714 kindergarteners (M_age = 52.99 months, SD = 10.28; 52.00% boys; 27.43% only child) were recruited from rural areas of eight provinces in China. With questionnaires, parents reported their PSS and PNE, HLE, and the children's ATLs. The results indicated that: (1) parents' PSS positively predicted children's ATLs; (2) the HLE partially mediated the relationship between parents' PSS and children's ATLs; and (3) PNE moderated the relationship between parents' PSS and children's ATLs. Thus, the results supported a moderated mediation model on the relationship between parents' PSS and young children's ATLs, with the HLE as a mediator and the parents' PSS as a moderator. These findings offer new avenues for intervening and supporting the development of young children's ATLs.

Air Pollution and Cognitive Impairment across the Life Course in Humans: A Systematic Review with Specific Focus on Income Level of Study Area

Cognitive function is a crucial determinant of human capital. The Lancet Commission (2020) has recognized air pollution as a risk factor for dementia. However, the scientific evidence on the impact of air pollution on cognitive outcomes across the life course and across different income settings, with varying levels of air pollution, needs further exploration. A systematic review was conducted, using Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) Guidelines to assess the association between air pollution and cognitive outcomes across the life course with a plan to analyze findings as per the income status of the study population. The PubMed search included keywords related to cognition and to pollution (in their titles) to identify studies on human participants published in English until 10 July 2020. The search yielded 84 relevant studies that described associations between exposure to air pollutants and an increased risk of lower cognitive function among children and adolescents, cognitive impairment and decline among adults, and dementia among older adults with supportive evidence of neuroimaging and inflammatory biomarkers. No study from low- and middle-income countries (LMICs)was identified despite high levels of air pollutants and high rates of dementia. To conclude, air pollution may impair cognitive function across the life-course, but a paucity of studies from reLMICs is a major lacuna in research.

Prenatal exposure to air pollution is associated with childhood inhibitory control and adolescent academic achievement

BACKGROUND

Prenatal air pollution exposure is associated with reductions in self-regulation and academic achievement. Self-regulation has been separately linked with academic achievement. Understudied, however, are the contributions of pollution exposure to inhibitory control, a facet of self-regulation, and whether pollution-related inhibitory control deficits are associated with impairment in academic achievement.

METHODS

Participants were recruited from a prospective birth cohort. Measures of prenatal airborne polycyclic aromatic hydrocarbons (PAH) during the third trimester of pregnancy, inhibitory control (NEPSY Inhibition) at mean age = 10.4 years, and Woodcock-Johnson Tests of Achievement-III at mean age = 13.7 were available for N = 200 participants. Multiple linear regression examined sex-dependent and sex independent associations among prenatal PAH, childhood inhibitory control, and academic achievement during adolescence, and whether childhood inhibitory control mediated associations between prenatal PAH and academic achievement during adolescence, controlling for ethnicity, maternal country of birth, language of prenatal interview, maternal marital status, maternal years of education, material hardship, quality of home caregiving environment, and early life stress.

RESULTS

Across all participants, higher prenatal PAH was significantly associated with worse spelling skills (WJ-III Spelling, β = -0.16, 95%Confidence Interval [CI]: 0.30, -0.02, p = .02). Trend level associations between higher prenatal PAH and worse reading comprehension (WJ-III Passage Comprehension, β = -0.13, 95%CI: 0.28, 0.01, p = .07) and math skills (WJ-III Broad Math, β = -0.11, 95%CI: 0.25, 0.03, p = .11) were detected. Across all participants, higher PAH was significantly associated with worse inhibitory control (β = -0.15, 95%CI: 0.29,-0.01 p = .03). Better inhibitory control was significantly associated with better reading comprehension (WJ-III Passage Comprehension, β = 0.22, 95%CI: 0.09, 0.36, p < .002) and math skills (WJ-III Broad Math Index, β = 0.32, 95%CI: 0.19, 0.45, p < .001), and trend level associations with better spelling skills (WJ-III Spelling, β = 0.12, 95%CI: 0.02, 0.26, p = .10). Inhibitory control significantly mediated PAH-related achievement effects for Passage Comprehension (β = -0.61, 95%CI: 1.49, -0.01) and Broad Math Index (β = -1.09, 95%CI: 2.36, -0.03).

CONCLUSIONS

Higher prenatal PAH exposure and lower childhood inhibitory control were associated with worse spelling, passage comprehension, and math in adolescence. Notably, childhood inhibitory control mediated PAH exposure-related effects on achievement in adolescents. Identifying these potential exposure-related phenotypes of learning problems may promote interventions that target inhibitory control deficits rather than content specific deficits.

Inequalities in Exposure to Ambient Air Neurotoxicants and Disparities in Markers of Neurodevelopment in Children by Maternal Nativity Status

Exposure levels to environmental pollutants vary significantly among different populations. These inequities in exposure to hazardous air pollutants (HAP) among different populations can contribute to disparities in neurodevelopmental outcomes. The aim of this study was to determine if exposure to HAP varies by maternal nativity status, a demographic marker often overlooked in the study of health disparities. We also assessed if those inequalities in exposure levels are associated with neurodevelopmental measures in young children. To do this, we obtained data from the Early Childhood Longitudinal Study-Birth cohort (ECLS-B), a nationally representative sample of children born in the U.S. in the year 2001 (n = 4750). Bayley’s Short Form-Research Edition (BSF-R) was used to measure cognitive development at 2 years of age. Using residential location at nine months of age, participants were assigned exposures to ten HAPs identified as potentially neurotoxic. Linear regression models were used to assess the joint effect of maternal nativity status and HAP exposure on neurodevelopment. Results showed inequities in exposure levels to ten different HAPs among the populations, as approximately 32% of children of foreign-born mothers were exposed to high levels of HAPs, compared to 21% of children born to U.S.-born mothers. Adjusting for socioeconomic factors, both isophorone exposure (a marker of industrial pollution) (−0.04, 95% CI, −0.12, 0.04) and maternal nativity status (−0.17, 95% CI, −0.27, −0.06) were independently associated with lower standardized BSF-R mental scores in children. Interaction between nativity status and isophorone was not statistically significant, but the change in mental scores associated with isophorone exposure was greater in children of foreign-born mothers compared to children of U.S.-born mothers (−0.12, vs. −0.03, p = 0.2). In conclusion, exposure to HAPs within the highest quartile was more commonly found among children of foreign-born mothers as compared to children of US-born mothers, indicating inequities in pollutant exposure by nativity status within urban populations. Exposures associated with nativity status may negatively contribute to children’s neurodevelopment.

Secondary Analysis of Existing Data Sets for Developmental Behavioral Pediatrics

ABSTRACT

Secondary analysis of existing large, national data sets is a powerful method to address many of the complex, key research questions in developmental behavioral pediatrics (DBP). Major advantages include decreasing the time needed to complete a study and reducing expenses associated with research by eliminating the need to collect primary data. It can also increase the generalizability of research and, with some data sets, provide national estimates that may form the basis for developing policy. However, few resources are available to direct researchers who seek to develop expertise in this area. This study aims to guide investigators with limited experience in this area who wish to improve their skills in performing secondary analysis of existing large data sets. This study provides direction on the steps to perform secondary analysis of existing data sets. It describes where and how data sets can be identified to answer questions of interest to DBP. Finally, it offers an overview of a number of data sets relevant to DBP.

Placental programming, perinatal inflammation, and neurodevelopment impairment among those born extremely preterm

Individuals born extremely preterm are at significant risk for impaired neurodevelopment. After discharge from the neonatal intensive care, associations between the child's well-being and factors in the home and social environment become increasingly apparent. Mothers' prenatal health and socioeconomic status are associated with neurodevelopmental outcomes, and emotional and behavioral problems. Research on early life risk factors and on mechanisms underlying inter-individual differences in neurodevelopment later in life can inform the design of personalized approaches to prevention. Here, we review early life predictors of inter-individual differences in later life neurodevelopment among those born extremely preterm. Among biological mechanisms that mediate relationships between early life predictors and later neurodevelopmental outcomes, we highlight evidence for disrupted placental processes and regulated at least in part via epigenetic mechanisms, as well as perinatal inflammation. In relation to these mechanisms, we focus on four prenatal antecedents of impaired neurodevelopment, namely, (1) fetal growth restriction, (2) maternal obesity, (3) placental microorganisms, and (4) socioeconomic adversity. In the future, this knowledge may inform efforts to detect and prevent adverse outcomes in infants born extremely preterm. IMPACT: This review highlights early life risk factors and mechanisms underlying inter-individual differences in neurodevelopment later in life. The review emphasizes research on early life risk factors (fetal growth restriction, maternal obesity, placental microorganisms, and socioeconomic adversity) and on mechanisms (disrupted placental processes and perinatal inflammation) underlying inter-individual differences in neurodevelopment later in life. The findings highlighted here may inform efforts to detect and prevent adverse outcomes in infants born extremely preterm.

The Relationship Between Air Pollution and Cognitive Functions in Children and Adolescents: A Systematic Review

Air pollution has a negative impact on one's health and on the central nervous system. We decided to assess studies that evaluated the relationship between air pollution and cognitive functions in children and adolescents by reviewing studies that had been published between January 2009 and May 2019. We searched three major databases for original works (26 studies) and for studies using brain imaging methods based on MRI (six studies). Adverse effects of air pollutants on selected cognitive or psychomotor functions were found in all of the studies. Exposure to nitrogen dioxide, for example, was linked to impaired working memory, general cognitive functions, and psychomotor functions; particulate matter 2.5 was linked to difficulties in working memory, short-term memory, attention, processing speed, and fine motor function; black carbon was linked to poor verbal intelligence, nonverbal intelligence, and working memory; airborne copper was linked to impaired attentiveness and fine motor skills; isophorone was linked to lower mathematical skills; and polycyclic aromatic hydrocarbons in fetal life were linked to lower intelligence scores. The studies using MRI showed that high concentrations of air pollutants were linked to changes in the brain's white matter or lower functional integration and segregation in children's brain networks. In view of the global increase in air pollution, there is a need for further research to elucidate the relationship between air pollution and cognitive and motor development in children. According to some studies, neuroinflammation, the e4 allele of the apolipoprotein E gene, and gutathione-S-transferase gene polymorphism processes may play a role.

Prepregnancy obesity is associated with cognitive outcomes in boys in a low-income, multiethnic birth cohort

Background

Maternal obesity and high gestational weight gain (GWG) disproportionally affect low-income populations and may be associated with child neurodevelopment in a sex-specific manner. We examined sex-specific associations between prepregnancy BMI, GWG, and child neurodevelopment at age 7.

Methods

Data are from a prospective low-income cohort of African American and Dominican women (n = 368; 44.8% male offspring) enrolled during the second half of pregnancy from 1998 to 2006. Neurodevelopment was measured using the Wechsler Intelligence Scale for Children (WISC-IV) at approximately child age 7. Linear regression estimated associations between prepregnancy BMI, GWG, and child outcomes, adjusting for race/ethnicity, marital status, gestational age at delivery, maternal education, maternal IQ and child age.

Results

Overweight affected 23.9% of mothers and obesity affected 22.6%. At age 7, full-scale IQ was higher among girls (99.7 ± 11.6) compared to boys (96.9 ± 13.3). Among boys, but not girls, prepregnancy overweight and obesity were associated with lower full-scale IQ scores [overweight β: − 7.1, 95% CI: (− 12.1, − 2.0); obesity β: − 5.7, 95% CI: (− 10.7, − 0.7)]. GWG was not associated with full-scale IQ in either sex.

Conclusions

Prepregnancy overweight and obesity were associated with lower IQ among boys, but not girls, at 7 years. These findings are important considering overweight and obesity prevalence and the long-term implications of early cognitive development.

Active Commute in Relation to Cognition and Academic Achievement in Children and Adolescents: A Systematic Review and Future Recommendations

Active commuting to school (ACS) is an important source of physical activity among children. Recent research has focused on ACS and its benefits on cognition and academic achievement (AA), factors important for success in school. This review aims to synthesize literature on the relationship between ACS and cognition or AA among children and adolescents. Peer-reviewed articles in PubMed, Web of Science, PsycINFO and Cochrane Library assessing ACS with cognition and/or AA among children, until February 2019, were selected. Twelve studies across nine countries (age range 4–18.5 years) were included. One study used accelerometers, whereas all others used self-report measures of ACS. A wide range of objective assessments of cognitive functioning and AA domains were used. Five among eight studies, and four among six found a positive relationship between ACS and cognitive or AA measure, respectively. Four studies found dose–response relationships, and some studies found sex differences. The quantitative analysis found that ACS was not significantly associated with mathematics score (odds ratio = 1.18; CI = 0.40, 3.48). Findings are discussed in terms of methodological issues, potential confounders, and the strength of the evidence. Future studies should conduct longitudinal studies and use objective measures of ACS to understand this relationship further.

Combined association of BTEX and material hardship on ADHD-suggestive behaviours among a nationally representative sample of US children

BACKGROUND

Previous research shows that environmental and social factors contribute to the development of attention-deficit/hyperactivity disorder (ADHD).

OBJECTIVE

To determine the relationship between early-life exposure to common ambient air pollutants (benzene, toluene, ethylbenzene, and xylene, also known as BTEX), household material hardship (a measure of socio-economic status), and ADHD-suggestive behaviours in kindergarten-age children.

METHODS

Pollutant exposure estimated from the 2002 National Air Toxics Assessment at each child's residential ZIP code at enrolment was linked to the Early Childhood Longitudinal Study Birth Cohort (n = 4650). Material hardship was assigned as a composite score of access to food, health care, and housing. Kindergarten teachers rated children's behaviours and activity in the classroom using a five-point Likert scale. Children with summary scores in the bottom decile were classified as displaying ADHD-suggestive behaviours. Logistic regression models were constructed to estimate the association between both BTEX exposure and material hardship on ADHD-suggestive behaviours.

RESULTS

The odds of displaying ADHD-suggestive behaviours were greater in children with combined high-level exposure to BTEX and in those experiencing material hardship (odds ratio 1.54, 95% confidence interval [CI] 1.12, 2.11, and OR 2.12, 95% CI 1.25, 3.59, respectively), adjusting for covariates. These associations were stronger when restricting the study population to urban areas. There was no evidence of interaction between early life BTEX exposure and material hardship, although the effects of BTEX exposure were slightly greater in magnitude among those with higher material hardship scores.

CONCLUSIONS

Children exposed to air toxics, material hardship, or both early in life are more likely to display signs of ADHD-suggestive behaviours as assessed by their kindergarten teachers. The associations between exposures to air pollution and to socio-economic hardship were observed in all children but were particularly strong in those living in urban areas.