School-age lead exposures most harmful to IQ

Testing for lead only in infants and toddlers may be a mistake, a new study suggests. Pediatricians routinely test very young children because this is the age when blood concentrations of the neurotoxic heavy metal tend to be highest. But older children can face significant lead exposures, and lead’s ability to lower IQ, the new study shows, is much greater for exposures in early school-age children than in toddlers.

The study, which will appear in an upcoming Environmental Health Perspectives, also finds that the later childhood exposures correlate more strongly than earlier ones with an exaggerated risk of incurring future criminal arrests for violent behavior.

The new data “get at a key concept in environmental health: that there may be some windows of vulnerability — stages of development — that are more vulnerable than others,” notes environmental epidemiologist Howard Hu of the University of Michigan in Ann Arbor. If school-age brains are more susceptible to lead toxicity than younger ones, “that’s important to know, from a public health perspective,” he says. Looking for lead in older children would be a first step in identifying families that need counseling on reducing sources of lead in and around the home.

Richard Hornung and his colleagues at Cincinnati Children’s Hospital Medical Center analyzed data on lead levels and IQ from 462 children. About half of the data were collected from kids in Cincinnati during the early 1980s, the rest from kids in Rochester, N.Y., during the mid-1990s.

Many studies have linked elevated lead exposures in 2-year-olds with a diminished IQ at school age — even when peaks never exceeded the federal action level, 10 micrograms of lead per deciliter of blood. However, two studies previously hinted that where blood-lead values were available for school-age children, these tracked with IQ better than those from toddlerhood. Hornung’s team has now probed that possibility in two groups of intensively monitored inner-city children.

The researchers compared IQ at age 6 — about the youngest that intelligence can be reliably assessed — with ratios of lead levels. The team looked at the ratio of lead at each age through age 6 to age 2 (for example ages 3:2, 4:2, etc.). The researchers also compared IQ with the average lead value through age 6 and with a youngster’s peak lead value. Far and away, lead ratios at ages 5 and 6 were most predictive of IQ losses, the Cincinnati-based scientists report.

IQ averaged 7 points lower when the age-6 blood-lead value was 50 percent greater than the value at age 2, as compared with when the age-6 value was 50 percent less than at age 2.

Indeed, for children having identical average lifetime blood-lead values, higher ratios at early ages showed a much smaller impact on IQ than elevated ratios at ages 5 and 6, Hornung says.

The Cincinnati group “used a relatively innovative statistical method,” says epidemiologist Aimin Chen of Creighton University in Omaha, Neb. In 2005, while working at the National Institute of Environmental Health Sciences, he reported a similar observation: Blood-lead values at age 7 were more predictive than those at age 2 of IQ decrements. But this study wasn’t able to point to where the potency of lead seemed to pick up, which the new study suggests was age 5.

The Cincinnati team’s new study strengthens this link and suggests that “we can’t just screen 2-years-olds.” The data are now strong enough, Chen says, that “we should expand community screening to school-age kids.”

Adds Hu, “If there are major differences in vulnerability to lead, we need to understand why.”

Kim Cecil, also of Cincinnati Children’s Hospital, thinks she’s homing in on part of the answer by analyzing magnetic resonance images of now-adult participants of the Cincinnati cohort studied by Hornung’s team.

Last year, she and her colleagues linked lead exposures through age 6 in the Cincinnati cohort to reduced tissue volumes in areas of the adult brain — especially regions of the frontal lobe related to judgment, impulsivity and mood. The researchers used magnetic resonance imaging to compare the relative size of brain regions in 157 participants against records of their childhood lead levels.

In a new analysis that she’ll be publishing soon, Cecil finds that here, too, the size of affected brain areas — the reduced number of cells in them — tracked best with lead exposures at ages 5 and 6.

Hornung’s team also surveyed data on criminal records for the now-adult Cincinnati participants. “For every 5 µg/dl increase in a child’s average blood-lead level, we saw approximately a 25 percent increase in the number of violent activities associated with criminal arrests,” notes Kim Dietrich, a coauthor of the study. In the new EHP paper, this team shows that the likelihood those participants had acquired a juvenile or adult arrest for violent behavior such as assault, rape and homicide correlated best with childhood elevations in lead at ages 5 and 6.