Social and environmental determinants of neuropsychological development from birth to preadolescence

Introduction Neuropsychological development is a genetically guided process which is continuously modified by socio-environmental factors. This thesis aimed to study the main socio-environmental determinants of neuropsychological development in different time-periods, such in the first two years of life, during preschool, and during preadolescence. This thesis also aimed to summarize the work done in environmental epidemiology on neuropsychological development in a novel conceptual framework. Methods This thesis is based on the data of the INMA (Infancia y Medio Ambiente) Project. The main objective of this project is to evaluate the impact of environmental exposures in children’s health in 7 population-based birth cohorts in different regions of Spain. The neuropsychological development of approximate total of 2,650 children was assessed at different time-periods following the same protocols. Results (1) Maternal cognitive capacities were positively related with child cognitive development early in life in more disadvantaged occupational social classes. (2) The levels of child cortisol were not related to child neuropsychological development during the second year of life. (3) Higher levels of long-chain polyunsaturated fatty acids in colostrum due to prolonged periods of breastfeeding improved early neuropsychological development of children, in particular in those children exposed to maternal smoking during pregnancy. (4) Prenatal exposure to PCBs (specially for PCB congener 153) impacted negatively on psychomotor development during the second year in life and on general neuropsychological development at the age of 4 years. (5) Postnatal exposure to organochlorine compounds was associated with a delay in reaction time (speed processing) during the preadolescent period. (6) The conceptual framework proposed will improve the quality of research in this area. Conclusions Social and cultural determinants such as maternal intelligence, educational level or occupational social class, are configuring the proximal environment in which a child develops and determine their neuropsychological development. Current levels of some organochlorine compounds, particularly polychlorinated byphenils, measured in blood samples (from umbilical cord, mothers, or children) are impairing on neuropsychological development in the general population.Introducció El desenvolupament neuropsicològic infantil és un procés guiat genèticament, el qual és contínuament influenciat per factors socials i ambientals. L’objectiu d’aquesta tesis fou l’estudi dels principals determinants socio-ambientals del desenvolupament neuropsicològic infantil en diferents períodes de temps. Aquesta tesis també té l’objectiu de resumir en un marc conceptual els diferents components del desenvolupament neuropsicològic pel seu ús en estudis d’epidemiologia ambiental. Mètodes Aquesta tesis està basada en dades del projecte INMA (Infancia y Medio Ambiente). El principal objectiu d’aquest projecte és avaluar l’impacte de les exposicions ambientals en la salud infantil en 7 cohorts de població establides en diferents regions d’Espanya. Dintre del marc d’aquest projecte, s’ha avaluat el desenvolupament neuropsicològic d’aproximadament 2.650 nens en diferents moments, seguint els mateixos protocols. Resultats (1) Les capacitats cognitives maternes estan positivament relacionades amb el desenvolupament cognitiu infantil en les primeres etapes, en les classes socials ocupacionals menys afavorides. (2) Els nivells de cortisol infantil no mostren associació amb el desenvolupament neuropsicològic durant el segon any de vida. (3) Nivells elevats d’àcids grassos poliinsaturats de cadena llarga en conjunció amb períodes prolongats de lactancia materna afavoreixen el desenvolupament neuropsicològic a les primeres etapes, especialment en aquells nens de mares fumadores durant l’embaràs. (4) L’exposició prenatal a PCBs (espcialment, al congener 153) impacta negativament en el desenvolupament psicomotor infantil durant el segon any de vida i en el desenvolupament neuropsicològic general als 4 anys d’edat. (5) L’exposició postnatal a compostos organocloroats està associada a una capacitat de reacció (velocitat de processament) més alentida en etapes preadolescents. (6) L’ús del marc conceptual propossat tesis afavorirà la qualitat de l’investigació en aquesta àrea. Conclussions Els determinants socials, com l’intel•ligència materna, el nivell educatiu o la classe social basada en l’ocupació, configuren l’entorn més proper del nen i determinen el seu desenvolupament neuropsicològic. Els nivells actuals de certs compostos organoclorats, especialment els bifenils policlorinats, mesurats en mostres de sang (de cordó umbilical, mares o nens) mostren efectes negatius sobre el desenvolupament neuropsicològic en la població general.Introducción El desarrollo neuropsicológico infantil es un proceso guiado genéticamente, el cual está contínuamente influenciado por factores sociales y ambientales. El objetivo de esta tesis fue estudiar los principales determinantes socio-ambientales del desarrollo neuropsicológico infantil en diferentes periodos de tiempo. Esta tesis también tenía como objetivo resumir en un marco conceptual el trabajo hecho en epidemiología ambiental en el estudio del desarrollo neuropsicológico infantil. Métodos Esta tesis está basada en datos del proyecto INMA (Infancia y Medio Ambiente). El principal objetivo de esto proyecto es evaluar el impacto de las exposiciones ambientales en la salud infantil en 7 cohortes de población establecidas en diferentes regiones de España. El desarrollo neuropsicológico de aproximadamente 2.650 niños ha sido evaluado en diferentes momentos siguiendo los mismos protocolos. Resultados (1) Las capacidades cognitivas maternas están positivamente relacionadas con el desarrollo cognitivo infantil en edades tempranas en las clases sociales ocupacionales menos aventajadas. (2) Los niveles de cortisol en el niño no se asocian con el desarrollo neuropsicológico durante el segundo año de vida. (3) Niveles altos de ácidos grasos poliinsaturados de cadena larga debido a periodos largos de lactancia materna mejoraron el desarrollo neuropsicológico a edades tempranas, especialmente en esos niños cuyas madres fumaron durante el embarazo. (4) La exposición prenatal a PCBs (espcialmente para el congener 153) impacta negativamente en el desarrollo psicomotor durante el segundo año de vida y en desarrollo neuropsicológico general a la edad de 4 años. (5) La exposición postnatal a compuestos organoclorados está asociado con una peor capacidad de tiempo de reacción (velocidad de procesamiento) durante la preadolescencia. (6) El marco conceptual propuesto mejorará la calidad de la investigación en esta área. Conclusiones Los determinantes sociales tales como inteligencia maternal, nivel educativo o clase social basada en la ocupación, configuran el entorno más cercano en el cual el niño se desarrolla y determinan su desarrollo neuropsicológico. Los niveles actuales de ciertos compuestos organoclorados, especialmente los bifeniles policlorinados, medidos en sangre (de cordón umbilical, madre, o niño) tienen efectos negativos sobre el desarrollo neuropsicológico de la población general

Air Pollution Exposure during Pregnancy and Childhood Autistic Traits in Four European Population-Based Cohort Studies: The ESCAPE Project.

BACKGROUND: Prenatal exposure to air pollutants has been suggested as a possible etiologic factor for the occurrence of autism spectrum disorder. OBJECTIVES: We aimed to assess whether prenatal air pollution exposure is associated with childhood autistic traits in the general population. METHODS: Ours was a collaborative study of four European population-based birth/child cohorts-CATSS (Sweden), Generation R (the Netherlands), GASPII (Italy), and INMA (Spain). Nitrogen oxides (NO2, NOx) and particulate matter (PM) with diameters of ≤ 2.5 μm (PM2.5), ≤ 10 μm (PM10), and between 2.5 and 10 μm (PMcoarse), and PM2.5 absorbance were estimated for birth addresses by land-use regression models based on monitoring campaigns performed between 2008 and 2011. Levels were extrapolated back in time to exact pregnancy periods. We quantitatively assessed autistic traits when the child was between 4 and 10 years of age. Children were classified with autistic traits within the borderline/clinical range and within the clinical range using validated cut-offs. Adjusted cohort-specific effect estimates were combined using random-effects meta-analysis. RESULTS: A total of 8,079 children were included. Prenatal air pollution exposure was not associated with autistic traits within the borderline/clinical range (odds ratio = 0.94; 95% CI: 0.81, 1.10 per each 10-μg/m3 increase in NO2 pregnancy levels). Similar results were observed in the different cohorts, for the other pollutants, and in assessments of children with autistic traits within the clinical range or children with autistic traits as a quantitative score. CONCLUSIONS: Prenatal exposure to NO2 and PM was not associated with autistic traits in children from 4 to 10 years of age in four European population-based birth/child cohort studies.Funding was provided as follows: ESCAPE Project— European Community’s Seventh Framework Program (FP7/2007-2011-GA#211250). CATSS, Sweden— Swedish Research Council for Health, Working Life and Welfare (FORTE), Swedish Research Council (VR) Formas, in partner hip with FORTE and VINNOVA (cross-disciplinary research program concerning children’s and young people’s mental health); VR through the Swedish Initiative for Research on Microdata in the Social And Medical Sciences (SIMSAM) framework grant 340-2013-5867; HKH Kronprinsessan Lovisas förening för barnasjukvård; and the Strategic Research Program in Epidemiology at Karolinska Institutet. Generation R, the Netherlands—The Generation R Study is conducted by the Erasmus University Medical Center in close collaboration with the School of Law and Faculty of Social Sciences of the Erasmus University Rotterdam; the Municipal Health Service Rotterdam area, Rotterdam; the Rotterdam Homecare foundation, Rotterdam; and the Stichting Trombosedienst & Artsenlaboratorium Rijnmond (STAR-MDC), Rotterdam. The general design of the Generation R Study is made possible by financial support from the Erasmus University Medical Center, Rotterdam; the Erasmus University Rotterdam; the Netherlands Organization for Health Research and Development (ZonMw); the Netherlands Organization for Scientific Research (NWO); and the Ministry of Health, Welfare and Sport. The Netherlands Organisation for Applied Scientific Research (TNO) received funding from the Netherlands Ministry of Infrastructure and the Environment to support exposure assessment. GASPII, Italy—grant from the Italian Ministry of Health (ex art.12, 2001). INMA, Spain— grants from Instituto de Salud Carlos III (Red INMA G03/176 and CB06/02/0041 FIS-FEDER 03/1615, 04/1509, 04/1112, 04/1931, 05/1079, 05/1052, 06/1213, 07/0314, 09/02647, 11/01007, 11/02591, CP11/00178, FIS-PI041436, FIS-PI081151, FIS-PI06/0867, FIS-PS09/00090), PI13/1944, PI13_02032, PI14/0891, PI14/1687, MS13/00054, UE (FP7-ENV-2011 cod 282957, and HEALTH.2010.2.4.5-1); Generalitat de Catalunya-CIRIT 1999SGR 00241; La Fundació La Marató de TV3 (090430); Conselleria de Sanitat Generalitat Valenciana; Department of Health of the Basque Government (2005111093 and 2009111069); and Provincial Government of Gipuzkoa (DFG06/004 and DFG08/001). V.W.V.J. received an additional grant from the Netherlands Organization for Health Research and Development (ZonMw 90700303, 916.10159). A.G.’s work was supported by a research grant from the European Community’s 7th Framework Programme (FP7/2008–2013-GA#212652). A full roster of the INMA project investigators can be found online (http://www. proyectoinma.org/presentacion-inma/listado-investigadores/ en_listado-investigadores.html)

Air Pollution Exposure during Pregnancy and Childhood Autistic Traits in Four European Population-Based Cohort Studies: The ESCAPE Project.

BACKGROUND: Prenatal exposure to air pollutants has been suggested as a possible etiologic factor for the occurrence of autism spectrum disorder. OBJECTIVES: We aimed to assess whether prenatal air pollution exposure is associated with childhood autistic traits in the general population. METHODS: Ours was a collaborative study of four European population-based birth/child cohorts-CATSS (Sweden), Generation R (the Netherlands), GASPII (Italy), and INMA (Spain). Nitrogen oxides (NO2, NOx) and particulate matter (PM) with diameters of ≤ 2.5 μm (PM2.5), ≤ 10 μm (PM10), and between 2.5 and 10 μm (PMcoarse), and PM2.5 absorbance were estimated for birth addresses by land-use regression models based on monitoring campaigns performed between 2008 and 2011. Levels were extrapolated back in time to exact pregnancy periods. We quantitatively assessed autistic traits when the child was between 4 and 10 years of age. Children were classified with autistic traits within the borderline/clinical range and within the clinical range using validated cut-offs. Adjusted cohort-specific effect estimates were combined using random-effects meta-analysis. RESULTS: A total of 8,079 children were included. Prenatal air pollution exposure was not associated with autistic traits within the borderline/clinical range (odds ratio = 0.94; 95% CI: 0.81, 1.10 per each 10-μg/m3 increase in NO2 pregnancy levels). Similar results were observed in the different cohorts, for the other pollutants, and in assessments of children with autistic traits within the clinical range or children with autistic traits as a quantitative score. CONCLUSIONS: Prenatal exposure to NO2 and PM was not associated with autistic traits in children from 4 to 10 years of age in four European population-based birth/child cohort studies.Funding was provided as follows: ESCAPE Project— European Community’s Seventh Framework Program (FP7/2007-2011-GA#211250). CATSS, Sweden— Swedish Research Council for Health, Working Life and Welfare (FORTE), Swedish Research Council (VR) Formas, in partner hip with FORTE and VINNOVA (cross-disciplinary research program concerning children’s and young people’s mental health); VR through the Swedish Initiative for Research on Microdata in the Social And Medical Sciences (SIMSAM) framework grant 340-2013-5867; HKH Kronprinsessan Lovisas förening för barnasjukvård; and the Strategic Research Program in Epidemiology at Karolinska Institutet. Generation R, the Netherlands—The Generation R Study is conducted by the Erasmus University Medical Center in close collaboration with the School of Law and Faculty of Social Sciences of the Erasmus University Rotterdam; the Municipal Health Service Rotterdam area, Rotterdam; the Rotterdam Homecare foundation, Rotterdam; and the Stichting Trombosedienst & Artsenlaboratorium Rijnmond (STAR-MDC), Rotterdam. The general design of the Generation R Study is made possible by financial support from the Erasmus University Medical Center, Rotterdam; the Erasmus University Rotterdam; the Netherlands Organization for Health Research and Development (ZonMw); the Netherlands Organization for Scientific Research (NWO); and the Ministry of Health, Welfare and Sport. The Netherlands Organisation for Applied Scientific Research (TNO) received funding from the Netherlands Ministry of Infrastructure and the Environment to support exposure assessment. GASPII, Italy—grant from the Italian Ministry of Health (ex art.12, 2001). INMA, Spain— grants from Instituto de Salud Carlos III (Red INMA G03/176 and CB06/02/0041 FIS-FEDER 03/1615, 04/1509, 04/1112, 04/1931, 05/1079, 05/1052, 06/1213, 07/0314, 09/02647, 11/01007, 11/02591, CP11/00178, FIS-PI041436, FIS-PI081151, FIS-PI06/0867, FIS-PS09/00090), PI13/1944, PI13_02032, PI14/0891, PI14/1687, MS13/00054, UE (FP7-ENV-2011 cod 282957, and HEALTH.2010.2.4.5-1); Generalitat de Catalunya-CIRIT 1999SGR 00241; La Fundació La Marató de TV3 (090430); Conselleria de Sanitat Generalitat Valenciana; Department of Health of the Basque Government (2005111093 and 2009111069); and Provincial Government of Gipuzkoa (DFG06/004 and DFG08/001). V.W.V.J. received an additional grant from the Netherlands Organization for Health Research and Development (ZonMw 90700303, 916.10159). A.G.’s work was supported by a research grant from the European Community’s 7th Framework Programme (FP7/2008–2013-GA#212652). A full roster of the INMA project investigators can be found online (http://www. proyectoinma.org/presentacion-inma/listado-investigadores/ en_listado-investigadores.html)

Interaction between airborne copper exposure and ATP7B polymorphisms on inattentiveness in scholar children

Recent research indicates that airborne copper exposure in scholar children negatively affects brain functioning. These effects are likely to be influenced by the efficiency of copper metabolism, which is partly regulated by the ATPase copper transporting beta (ATP7B) gene. We investigated whether indoor and outdoor airborne copper exposure is differentially associated with child inattentiveness depending on genetic variation within the ATP7B gene in 1645 scholar children from the BREATHE project. Outdoor (courtyard) and indoor (classroom) air pollution levels were measured during class hours in each school. Inattentiveness was assessed through a follow-up with four measurements via the Attentional Network Test (4475 observations). Linear mixed models considering repeated measures were conducted to assess genetic and exposure main and interaction effects. Two interactions were detected indicating that ATP7B-rs1061472 (P for interaction 0.016) and ATP7B-rs1801243 (P for interaction 0.003) polymorphisms modified the association between indoor copper exposure and inattentiveness. Stratified analysis by genotypes revealed that both outdoor and indoor copper exposure increased inattentiveness in rs1061472-CC and rs1801243-CC carriers. These findings suggest that the genetic background promotes the association between airborne copper exposure at school with inattentiveness in children.The research leading to these results has received funding from the European Research Council under the ERC Grant Agreement number 268479- the BREATHE project. S. Alemany thanks the ‘Instituto de Salud Carlos III’ for her postdoctoral grant (CD14/00214). N. Vilor-Tejedor is funded by a pre-doctoral grant from the Agència de Gestió d’Ajuts Universitaris i de Recerca (2015 FI_B 00636) Generalitat de Catalunya

Airborne copper exposure in school environments associated with poorer motor performance and altered basal ganglia.

INTRODUCTION: Children are more vulnerable to the effects of environmental elements. A variety of air pollutants are among the identified factors causing neural damage at toxic concentrations. It is not obvious, however, to what extent the tolerated high levels of air pollutants are able to alter brain development. We have specifically investigated the neurotoxic effects of airborne copper exposure in school environments. METHODS: Speed and consistency of motor response were assessed in 2836 children aged from 8 to 12 years. Anatomical MRI, diffusion tensor imaging, and functional MRI were used to directly test the brain repercussions in a subgroup of 263 children. RESULTS: Higher copper exposure was associated with poorer motor performance and altered structure of the basal ganglia. Specifically, the architecture of the caudate nucleus region was less complete in terms of both tissue composition and neural track water diffusion. Functional MRI consistently showed a reciprocal connectivity reduction between the caudate nucleus and the frontal cortex./nCONCLUSIONS: The results establish an association between environmental copper exposure in children and alterations of basal ganglia structure and function.This work was supported by the European Research Council under the ERC (grant number 268479)—the BREATHE project. The Agency of University and Research Funding Management of the Catalonia Government participated in the context of Research Group SGR2014-1673

Traffic pollution exposure is associated with altered brain connectivity in school children.

Children are more vulnerable to the effects of environmental elements due to their active developmental processes. Exposure to urban air pollution has been associated with poorer cognitive performance, which is thought to be a result of direct interference with brain maturation. We aimed to assess the extent of such potential effects of urban pollution on child brain maturation using general indicators of vehicle exhaust measured in the school environment and a comprehensive imaging evaluation. A group of 263 children, aged 8 to 12years, underwent MRI to quantify regional brain volumes, tissue composition, myelination, cortical thickness, neural tract architecture, membrane metabolites, functional connectivity in major neural networks and activation/deactivation dynamics during a sensory task. A combined measurement of elemental carbon and NO2 was used as a putative marker of vehicle exhaust. Air pollution exposure was associated with brain changes of a functional nature, with no evident effect on brain anatomy, structure or membrane metabolites. Specifically, a higher content of pollutants was associated with lower functional integration and segregation in key brain networks relevant to both inner mental processes (the default mode network) and stimulus-driven mental operations. Age and performance (motor response speed) both showed the opposite effect to that of pollution, thus indicating that higher exposure is associated with slower brain maturation. In conclusion, urban air pollution appears to adversely affect brain maturation in a critical age with changes specifically concerning the functional domain.This work was supported by the European Research Council under the ERC [grant number 268479]—the BREATHE project. The Agency of University and Research Funding Management of the Catalonia Government participated in the context of Research Group SGR2014-1673

Neurodevelopmental deceleration by urban fine particles from different emission sources: a longitudinal observational study.

BACKGROUND: A few studies have reported associations between traffic-related air pollution exposure at schools and cognitive development. The role of PM components or sources other than traffic on cognitive development has been little explored. OBJECTIVES: We aimed to explore the role of PM sources in school air on cognitive development. METHODS: A cohort of 2,618 schoolchildren (average age, 8.5 years) belonging to 39 schools in Barcelona (Spain) was followed up for a year. Children completed computerized tests assessing working memory, superior working memory, and inattentiveness during four visits. Particulate matter ≤ 2.5 μm (PM2.5) was measured during two 1-week campaigns in each school, both outdoors and in the classroom. Source apportionment resulted in nine sources: mineral, organic/textile/chalk, traffic, secondary sulfate and organics, secondary nitrate, road dust, metallurgy, sea spray, and heavy oil combustion. Differences in cognitive growth trajectories were assessed with mixed models with age-by-source interaction terms. RESULTS: An interquartile range increase in indoor traffic-related PM2.5 was associated with reductions in cognitive growth equivalent to 22% (95% CI: 2%, 42%) of the annual change in working memory, 30% (95% CI: 6%, 54%) of the annual change in superior working memory, and 11% (95% CI: 0%, 22%) of the annual change in the inattentiveness scale. None of the other PM2.5 sources was associated with adverse effects on cognitive development. CONCLUSIONS: Traffic was the only source of fine particles associated with a reduction in cognitive development. Reducing air pollution from traffic at primary schools may result in beneficial effects on cognition.This work was supported by the European Research Council under the ERC [grant number 268479] – the BREATHE project. Fulvio Amato is beneficiary of the Juan de la Cierva postdoctoral Grant (JCI-2012-13473) from the Spanish Ministry of Economy and Competitiveness. We are acknowledged with all the families participating into the study for their altruism and particularly to the schools Antoni Brusi, Baloo, Betània – Patmos, Centre d’estudis Montseny, Col.legi Shalom, Costa i Llobera, El sagrer, Els Llorers, Escola Pia de Sarrià, Escola Pia Balmes, Escola concertada Ramon Llull, Escola Lourdes, Escola Tècnica Professional del Clot, Ferran i Clua, Francesc Macià, Frederic Mistral, Infant Jesús, Joan Maragall, Jovellanos, La Llacuna del Poblenou, Lloret, Menéndez Pidal, Nuestra Señora del Rosario, Miralletes, Ramon Llull, Rius i Taulet, Pau Vila, Pere Vila, Pi d'en Xandri, Projecte, Prosperitat, Sant Ramon Nonat - Sagrat Cor, Santa Anna, Sant Gregori, Sagrat Cor Diputació, Tres Pins, Tomàs Moro, Torrent d'en Melis, Virolai. We also acknowledge the ESCAPE project for the design and supervision modelling of air pollution. Furthermore, Xavier Mayoral did the technical development of the n-back test; Cecilia Persavento, Judit Gonzalez, Laura Bouso and Pere Figueras contributed to the field work

Sleeping, TV, cognitively stimulating activities, physical activity, and attention-deficit hyperactivity disorder symptom incidence in children: a prospective study

Objective: To analyze associations between time spent sleeping, watching TV, engaging in cognitively stimulating activities, and engaging in physical activity, all at 4 years, and (1) attention-deficit/hyperactivity disorder (ADHD) symptoms and (2) behavior problems, both assessed at 7 years, in ADHD-free children at baseline. Method: In total, 817 participants of the Infancia y Medio Ambiente birth cohort, without ADHD at baseline, were included. At the 4-year follow-up, parents reported the time that their children spent sleeping, watching TV, engaging in cognitively stimulating activities, and engaging in physical activity. At the 7-year follow-up, parents completed the Conners' Parent Rating Scales and the Strengths and Difficulties Questionnaire, which measure ADHD symptoms and behavior problems, respectively. Negative binomial regression models were used to assess associations between the activities at 4 years and ADHD symptoms and behavior problems at 7 years. Results: Children (48% girls) spent a median (p25-p75) of 10 (10-11) hours per day sleeping, 1.5 (0.9-2) hours per day watching TV, 1.4 (0.9-1.9) hours per day engaging in cognitively stimulating activities, and 1.5 (0.4-2.3) hours per day engaging in physical activity. Longer sleep duration (>10 hours per day) was associated with a lower ADHD symptom score (adjusted incidence rate ratio = 0.97, 95% confidence interval, 0.95-1.00). Longer time spent in cognitively stimulating activities (>1 hours per day) was associated with lower scores of both ADHD symptoms (0.96, 0.94-0.98) and behavior problems (0.89, 0.83-0.97). Time spent watching TV and engaging in physical activity were not associated with either outcomes. Conclusion: A shorter sleep duration and less time spent in cognitively stimulating activities were associated with an increased risk of developing ADHD symptoms and behavior problems

Outdoor infiltration and indoor contribution of UFP and BC, OC, secondary inorganic ions and metals in PM2.5 in schools

Infiltration of outdoor-sourced particles into indoor environments in 39 schools in Barcelona was assessed during school hours. Tracers of road traffic emissions (NO2, Equivalent Black Carbon (EBC), Ultrafine Particles (UFP), Sb), secondary inorganic aerosols (SO42−, NO3−, NH4+) and a number of PM2.5 trace elements showed median indoor/outdoor (I/O) ratios ≤ 1, indicating that outdoor sources importantly contributed to indoor concentrations. Conversely, OC and mineral components had I/O ratios>1. Different infiltration factors were found for traffic and secondary components (0.31–0.75 and 0.50–0.92, cold and warm season respectively), with maxima corresponding to EBC and Cd. Higher concentrations of indoor-generated particles were observed when closed windows hindered dispersion (cold season). Building age was not a major determinant of indoor levels. Neither were the window's material, except for NO2 (with an increase of 8 μg m−3 for wood framed windows) and the mineral components (also dependent on the presence of sand in a distance <20 m) that reach the indoor environment via soil adhering to footwear with their dispersion being more barred by Aluminium/PVC framed windows than the wooden ones. Enlarged indoor concentrations of some trace elements suggest the presence of indoor sources that should be further investigated in order to achieve a healthier school indoor environment.The research leading to these results has received funding from the European Community's Seventh Framework Program (ERC Advanced Grant) under grant agreement number 268479 - the BREATHE project. Thanks are due to all the participant schools and colleagues. Additional instrumentation was kindly provided by national projects IMPACT (CGL2011-26574), VAMOS (CLG2010-19464-CLI) and CECAT (CTM2011-14730-E)