Early Blood Lead Levels Linked to Risk of ADHD, Especially in Boys

Xiaobin Wang, MD, MPH, ScD

Having elevated blood lead levels in early childhood raises the risk of developing attention-deficit/hyperactivity disorder (ADHD) in children, with boys being especially vulnerable, according to new data.

In an investigation seeking to determine any prospective associations between early lead exposure and ADHD, a research team consisting of Xiaobin Wang, MD, MPH, ScD, the director of the Center on the Early Life Origins of Disease in the Department of Population, Family and Reproductive Health at Johns Hopkins University’s Bloomberg School of Public Health, and colleagues analyzed data from 1479 mother-infant pairs in the Boston Birth Cohort. In total, 299 pairs included a child with ADHD, and 1180 consisted of neurotypical children.

The team utilized electronic medical records to obtain the children’s first blood lead measurements and physician diagnoses of ADHD. Additionally, graphic plots and multiple logistic regression were used to examine the dose-response links between exposure and the condition, as well as the potential effect modifiers, with adjustments made for the pertinent covariables.

“Even though lead exposure has decreased significantly over the past decades since the removal of lead from paints and gasoline and the adoption of other environmental safety measures, exposure to low lead levels continues to be widespread in urban low-income populations,” Wang and colleagues told MD Magazine. “We found that 8.9% of the children in the Boston Birth Cohort had elevated lead levels—5-10 ug/dL—in early childhood.”

“While this low level of lead was in the past, considered to be safe, there is growing evidence that there is no safe level for lead,” Wang added. That rang true with the findings, as that 8.9% was associated with a 66% increased risk of ADHD (odds ratio [OR], 1.66; 95% CI, 1.08 to 2.56) in the overall sample. For boys, the association was stronger (OR, 2.49; 95% CI, 1.46 to 4.26) than with girls (P value for sex-lead interaction = .017).

“We found a striking gender difference: among boys, the lead-ADHD association was significantly stronger—a 149% increased risk,” Wang said. “In girls, the association was largely attenuated, suggesting that boys are particularly vulnerable to the adverse effect of lead exposure.”

Although, the odds ratio of ADHD was associated with elevated lead levels among boys was cut essentially in half if their mothers had adequate high-density lipoprotein levels compared with low high-density lipoprotein. That effect was mirrored if their mothers had low stress compared with high stress during pregnancy, as well.

“First, we would like to stress that lead exposure is harmful for all children, but elevated lead in boys is even more harmful in terms of ADHD risk,” Wang said. “Our study identified some maternal factors during pregnancy that can help reduce the risk of ADHD associated with lead exposure in boys.”

These findings, Wang and colleagues noted, should shed a new light on the gender differences in ADHD, as well as point to openings for—and the importance of—earlier risk assessment and primary prevention of the condition.

The study, “A Prospective Birth Cohort Study on Early Childhood Lead Levels and Attention Deficit Hyperactivity Disorder: New Insight on Sex Differences,” was published in The Journal of Pediatrics.