Lead in Children's Blood May Contribute to ADHD's Hyperactivity Component

Thanks to government regulation, lead poisoning in children is no longer common in the United States. However, American youngsters are still widely exposed to low levels of lead in water, soil, and other venues.

But even these lower levels of lead in turn may contribute to attention-deficit/hyperactivity disorder (ADHD) in some American children, a new study suggests.

The study was headed by Joel Nigg, Ph.D., a professor of clinical psychology at Michigan State University. Results are in press with Biological Psychiatry.

Studies done in the 1980s have already linked the amount of lead in children's blood with ADHD. But the levels—10-20 ug/dl—were higher than the levels usually found in American children today, which average 1-2 ug/dl. So Nigg and his colleagues wanted to see whether these lower blood levels could also be associated with ADHD.

The researchers recruited subjects for the study from schools, clinics, and advertisements. To obtain the broadest and most representative sample possible, the investigators advertised for both healthy children and children suspected of having or diagnosed with ADHD. A total of 845 families expressed interest in participating in the study.

The children in these families were then screened extensively with various instruments—say, the Child Behavior Checklist and the Kiddie Schedule for Affective Disorders and Schizophrenia—to determine whether they had ADHD or other psychiatric disorders.

One hundred-and-fifty children aged 8 to 17 were finally selected to participate in the study. A third had ADHD, combined type; a third had ADHD, predominantly inattentive type; and the remainder did not have ADHD and could thus serve as controls. Moreover, the levels of lead in the blood of all the subjects closely matched the American average of 1-2 ug/dl, with a maximum level of 3.4 ug/dl.

Blood lead levels were significantly higher in the children with ADHD, combined type, than in the control children, but this was not the case in children who had ADHD, predominantly inattentive type. So Nigg and his group believe that low lead levels in the blood might play a role in the hyperactivity component of ADHD, but not in the inattention component of it.

How lead might contribute to hyperactivity is not clear. It does not seem to be mediated by lead's negative impact on intelligence, because while the researchers found that blood lead levels in their subjects were correlated with IQ, they also found that IQ could not explain the link between lead and hyperactivity. Low-level lead exposure is known to disrupt mid-brain dopamine circuitry, and this circuitry, however, is involved in ADHD.

Although the researchers suspect that low blood levels of lead may play a role in a number of cases of ADHD-related hyperactivity, they do not believe that this is so in all cases, because some of their subjects with mildly elevated blood levels did not have ADHD-related hyperactivity, while some of their subjects with ADHD-related hyperactivity did not have elevated blood levels of lead.

The study's results have public health policy implications, Nigg told Psychiatric News, emphasizing that “prevention of lead exposure may be important at even lower levels than we thought.”

The study was financed by Michigan State University and the Centers for Disease Control and Prevention.

An abstract of “Low Blood Lead Levels Associated With Clinically Diagnosed Attention-Deficit/Hyperactivity Disorder and Mediated by Weak Cognitive Control” is posted at<www.journals.elsevierhealth.com//periodicals/bps> under “Articles in Press.” ▪