Although lead has long been recognized as toxic and lead paint has been banned in the United States for 40 years, it continues to wreak developmental havoc on children. Most recently, the water crisis in Flint, Michigan, brought this decades-old problem to the fore yet again. Now, a new study from Duke University is the latest to confirm that lead exposure not only damages cognitive function but also leads to lower socioeconomic status.
The researchers examined data from the Dunedin Multidisciplinary Health and Development Study. That study, a lifelong periodic examination of more than 1,000 people from Dunedin, New Zealand, which began in the early 1970s, has provided a plethora of data during the past 40 years. About half the participants were tested for lead exposure when they were 11 years old. Aaron Reuben, a graduate student in clinical psychology at Duke, wondered whether blood lead levels during childhood correlated with any aspects of their adulthood. Because the Dunedin study has so carefully documented the lives of its enrollees, the level of lead in an 11-year-old’s bloodstream can be scrutinized as a potential predictor of educational attainment, career, finances and health.
Reuben and his team extracted Dunedin data on childhood blood levels, cognitive function and socioeconomic status. Cognitive function was assessed through IQ tests at ages 7, 9 and 38. Socioeconomic status was determined by the occupation of the parents when the participants were children and at age 38 by their own job.
Among the 565 children who’d been tested for lead at age 11, about half had a concentration deemed concerning at the time of testing, in the early 1980s. Today, half that level is considered too high, according to the Centers for Disease Control and Prevention.
Dunedin study adults with poorer cognitive performance had higher concentrations of lead in their blood as children, according to this new report, published recently in JAMA. “There’s a pretty linear gradient,” says Reuben. “Those with the most exposure had the worst outcomes.” Reuben emphasizes that the effect was mild: For every 5 micrograms of lead in the blood, between one and two IQ points were lost. And because this study is observational—it was not specifically designed to examine the influence of childhood lead exposure on adult intelligence—the toxin cannot be established as the conclusive cause of diminished intelligence. But, says Reuben, after eliminating all other potential factors that could affect cognition, “lead exposure was associated with lower IQs down the line.”
The same gradient held true for socioeconomic status. Children who had a “concerning” blood lead concentration reached a socioeconomic level that was, on average, 4.51 points lower than their less-leaded peers by age 38. The socioeconomic status of children with high lead concentrations was also, on average, lower than that of their parents. “Lead exerted a downward trend on their socioeconomic outcomes,” says Reuben, “regardless of where they started out in life.”
But the researchers didn’t stop there. They wanted to know if the weakened socioeconomic status of the high-lead children was connected to diminished cognition. The answer was yes. Variation in economic outcomes following lead exposure, says Reuben, “could be accounted for by what was happening in their brains.” In other words, the lead made it harder for the children to succeed financially as adults because their thinking was impaired by the exposure. The downward trend is slight, says Reuben, “but it’s there.”
This study is not the first to elucidate the link. Kim Dietrich, an environmental epidemiologist at the University of Cincinnati, chronicled an association between childhood lead exposure and violent crime in a long-term study of nearly 300 people. Dietrich notes that the Dunedin study is missing data on lead exposure during the earliest years of life, when, for example, normal oral behavior led to toddlers sticking lead-based paint chips in their mouths. But if that absence has any impact on the study results, says Dietrich, it would be to underestimate the effect of lead on cognitive performance.
Although contaminated water in Flint led to recent outcries over lead, Dietrich’s concern is strongly tied to inner cities with a large stock of older housing in disrepair. “That is where the problem mostly resides,” he says. As these houses degrade, lead from the paint enters interior dust and exterior soil, both of which end up in children’s mouths and subsequently their bloodstreams. Baltimore, Philadelphia, Detroit, Atlanta, St. Louis and Newark, New Jersey, are all at risk for this problem, says Dietrich, who has done imaging studies showing a loss in adult cortical gray matter—part of the brain critical to cognitive and behavioral functioning—tied to childhood lead exposure.
The presence of lead has largely declined since its neurotoxic effects were first established. But, says Dietrich, lead concentrations in children and adults “remain a problem.”
Reuben and his colleagues are not taking on the role of advocates, but he does hope the study raises continued awareness about the dangers of lead. “Our findings are a call for intervention,” he says. “If you do nothing, there are real long-term consequences.”
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