Attention deficity hyperactivity disorder (ADHD) occurs in 3-7% of children, causing impairment due to extreme hyperactivity, impulsivity and/or inattention. The syndrome frequently co-occurs with other disruptive behaviours, particularly conduct and oppositional defiant disorders. Earlier studies have associated levels of lead in children's bloodstreams with ADHD but these were at levels much higher than are found today. A U.S. study of 115 children between the ages of 8 and 17 found that, even at relatively low concentrations, blood lead was associated with problems with hyperactivity and impulsivity and a lower IQ although not with inattention or disorganisation.
Nigg, Janet T. ... [et al] - Low blood levels associated with clinically diagnosed attention-deficity hyperactivity disorder and mediated by weak cognitive control. Biological Psychiatry February 2008, 63(3), 325-331