These days, doctors warn parents that there's no safe level of lead exposure. This toxin can damage the developing nervous system in young children, and blood lead levels as low as 5 micrograms per deciliter may lower intelligence quotient (IQ), according to the World Health Organization.
Participants in the current study had average blood lead levels more than twice that high when they were 11 years old in the early 1980s: 10.99 micrograms/dl.
Every 5 microgram/dl increase in blood lead levels early in life was associated with a 1.61-point lower IQ by the time these children reached age 38, as well as reductions in perceptual reasoning and working memory, researchers report in JAMA.
"This suggests at the very least that individuals don't fully recover from lead-related cognitive injuries received in childhood," said lead study author Aaron Reuben of Duke University in Durham, North Carolina.
"It also suggests that lead exerts a downward pull on an individual's cognitive abilities over time regardless of where they start out in life," Reuben said by email.
For the study, researchers examined data on cognitive function, IQ and socioeconomic status for 565 adults in Dunedin, New Zealand when they were 38 years old, as well as results from blood tests for lead done in childhood.
Childhood blood lead levels ranged from 4 to 31 micrograms/dl.
There were no meaningful differences in lead exposure based on socioeconomic status, and elevated blood lead levels were found in children from poor and affluent families alike.
Participants with childhood blood lead levels above 10 micrograms/dl had average adult IQ test scores 4.25 points lower than their peers with lower blood lead levels.
After accounting for factors that can influence adult IQ and earnings such as childhood IQ and socioeconomic status as well as mothers' IQ, researchers still found that higher lead levels in childhood were tied to what's known as downward social mobility, or adult kids earning less or having less prestigious jobs than their parents.
"The normal trend for this generation is for sons and daughters to achieve better occupations than their parents," said senior study author Terrie Moffitt, also of Duke University.
"But among those with elevated lead levels the trend was opposite," Moffitt said by email. "Much of this could be attributed to the IQ effect."
Some previous research has linked each 1-point decrease in IQ scores to $200 to $600 less in annual income, Moffitt said. The average 4.25-point lower IQ scores tied to high lead exposure in the study could translate into a net worth reduction of several thousands dollars, Moffitt said.
While lead exposure has long been linked to poor academic achievement, this study offers fresh evidence of how high blood lead levels in childhood could lead to lower socioeconomic status in adulthood, said David Bellinger, a researcher at the Harvard T.H. Chan School of Public Health in Boston and author of an accompanying editorial.
"Effects on IQ are just the tip of the iceberg," Bellinger said by email. "The adverse effects extend far beyond, to include impaired attention, including ADHD, impairments of executive function, and different forms of social pathologies - impairments that are likely to be more important in determining an individual's success in life than a modest reduction in IQ."
In some cities in the states of New York, Ohio and Pennsylvania, at least one in seven kids have unsafe levels of lead in their blood, a study released last June in the Journal of Pediatrics found.
A Reuters investigation of blood testing data in California also found dozens of communities had rates of unsafe childhood lead exposure that surpass those of Flint, Michigan (http://reut.rs/2o40nKc). In one zip code in Fresno, California, 13.6 percent of blood tests on kids under age 6 came back high for lead. That compares to 5 percent across the city of Flint during the recent water contamination crisis there.
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