Multiple studies have demonstrated higher rates of inattention, distractibility, and impulsivity in lead-exposed children than would be expected given the prevalence of attention-deficit/hyperactivity disorder (ADHD) in the general population. The incidence of these symptoms goes up in a dose-dependent fashion and appears to have no threshold (so they occur at even the lowest measurable blood lead levels). In a 2006 study of nearly 5,000 children between ages 4-15 years, those with blood lead levels greater than 2 mcg/dL (still below the level the CDC deems toxic) were four times more likely to be carrying a diagnosis of ADHD and be on stimulant medication than their peers with blood lead levels less than 0.8mcg/dL.
Cognition
Closely related to impulse control and attention, the cognitive domains of intelligence and executive function are clearly damaged by lead exposure. Poor performance on tasks requiring focus, cognitive flexibility, and inhibition of automatic responses was directly associated with higher blood lead levels in a group of preschoolers with levels between 0 and 13 mcg/dL.5
Dr. Michael S. Jellinek
IQ has been found to be so consistently diminished by increasing blood lead levels that it is used as an overall index of neurodevelopmental morbidity of lead exposure, leading to the CDC’s adoption of a lower standard definition of toxic lead levels. Even very low blood lead levels are associated with decrements in IQ: children with blood lead levels less than 7.5 mcg/dL lost an average of 3 IQ points for every 1 mcg/dL increase in blood lead levels.6 In a study of 57,000 elementary school students in 2009, Miranda et al. found that those who had a blood lead level of 4 mcg/dL at 3 years old were significantly more likely to be diagnosed with a learning disability in elementary school. Another study of 48,000 children who had a blood lead level of 5 mcg/dL were 30% more likely to fail third grade reading and math tests than their peers without measurable lead levels.
Speech and language
More recent studies have demonstrated that children with higher bone lead concentrations had poorer performance on several language-processing measures, suggesting that childhood lead exposure damages language processing and function as the young people grow. These deficits in language processing can make social development and self-regulation much more challenging in adolescence, and make school and work settings much more challenging. These findings also have implications for the utility of psychotherapy, a language-based treatment, for the other behavioral problems of lead exposure.
Motor skills
Several recent studies have assessed both fine and gross motor skills in lead-exposed children. Findings have demonstrated that balance, coordination, gross motor and fine motor skills all appear to be compromised in a dose-dependent fashion by childhood lead exposure. These findings suggest that not only are children at greater risk for accident and injury through childhood and into adulthood, a risk already increased by their compromised attention and impulse control. But they also are likely to be physically clumsy, compromising an opportunity to cultivate strengths or experience mastery when cognitive tasks may prove frustrating for them.
With deficits in such fundamental cognitive, motor, and behavioral processes, exposed children are clearly vulnerable to more than ADHD, conduct disorder, and learning disabilities. These struggles may lead to secondary vulnerabilities to anxiety or mood symptoms or substance abuse as these children grow into teenagers who face frustration at every turn. In addition to treatment for their deficits in attention and executive function, these children will ideally receive specialized supports in school and at home, to be able to master cognitive tasks, manage new social circumstances and make friends, discover their interests and talents, and generally stay on their best developmental trajectories. Lastly, the specific consequences of lead exposure will vary for any individual child, so parents will have to deal with the uncertainty of their child’s behavior and development over many years. Clearly, the children of Flint face a long road that has been substantially impacted by their lead exposure. The only good that can come from the exposure in Flint is to heighten efforts to ensure that it never happens again.
1. Environ Health Perspect. 2008 Jul;116(7):956-62.
2. Environ Res. 2000 May;83(1):1-22.
3. Environ Res. 2007 Jul;104(3):315-36.
4. Arch Pediatr Adolesc Med. 2001 May;155(5):579-82.
5. Dev Neuropsychol. 2004;26(1):513-40.
6. Environ Health Perspect. 2005 Jul;113(7):894-9.
Dr. Swick is an attending psychiatrist in the division of child psychiatry at Massachusetts General Hospital, Boston, and director of the Parenting at a Challenging Time (PACT) Program at the Vernon Cancer Center at Newton (Mass.) Wellesley Hospital. Dr. Jellinek is professor of psychiatry and of pediatrics at Harvard Medical School, Boston.