Lead and PCBs as risk factors for attention defcit/hyperactivity disorder

Objectives: Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed neurobehavioral disorder of childhood, yet its etiology is not well understood. In this review we present evidence that environmental chemicals, particularly polychlorinated biphenyls (PCBs) and lead, are associated with defcits in many neurobehavioral functions that are also impaired in ADHD. Datas Ources: Human and animal studies of developmental PCB or lead exposures that assessed specifc functional domains shown to be impaired in ADHD children were identifed via searches of PubMed using "lead" or "PCB exposure" in combination with key words, including "attention," "working memory," "response inhibition," "executive function," "cognitive function," "behavior," and "ADHD." Datasynthesis: Children and laboratory animals exposed to lead or PCBs show defcits in many aspects of attention and executive function that have been shown to be impaired in children diagnosed with ADHD, including tests of working memory, response inhibition, vigilance, and alertness. Studies conducted to date suggest that lead may reduce both attention and response inhibition, whereas PCBs may impair response inhibition to a greater degree than attention. Low-level lead exposure has been associated with a clinical diagnosis of ADHD in several recent studies. Similar studies of PCBs have not been conducted. Conclusions: We speculate that exposures to environmental contaminants, including lead and PCBs, may increase the prevalence of ADHD.