Environmental Chemical Contributions to ADHD and the Externalising Disorders of Childhood – A Review of Epidemiological Evidence

Abstract

externalising disorders of childhood, Attention Deficit hyperactivity Disorder (ADhD), conduct Disorder and Oppositional Defiant Disorder are leading causes of childhood morbidity. Aetiology is poorly understood but includes both genetic and environmental factors. here we review the observational evidence for an association between environmental chemical exposures and increased externalising behaviour in childhood. With respect to AD hD, we additionally consider whether there is evidence that exposures are associated with impairments in information processing skills characteristic of AD hD. We conclude that large, well designed, prospective studies combining behavioural and cognitive outcomes now provide strong evidence for an association between environmental lead exposure and the externalising disorders of childhood, as well as evidence for an association between prenatal exposure to environmental methylmercury and difficulties associated with ADhD. Existing epidemiological research on polychlorinated biphenyls is however inconclusive and we discuss evidence that the association between environmental tobacco smoke and externalising disorders may not be causal in nature. Finally modern environmental chemicals are reviewed where early studies have observed an association between increased exposure and externalising disorders, externalising behaviours, or information processing difficulties associated with AD hD, but where further replication is required. Implications for future research are discussed.