Lead Exposure in Childhood Linked to Lower IQ in Midlife

March 29, 2017

Exposure to lead in childhood is associated with lower cognitive function and socioeconomic status in midlife, a new study suggests.

The study, which prospectively followed 565 children from New Zealand for four decades, found that those with higher lead exposure in childhood had significantly lower IQ scores and socioeconomic status at age 38 years than those with lower lead levels.

Greater childhood lead exposure was also associated with greater declines in IQ from childhood to adulthood and greater declines relative to parents in occupational socioeconomic status.

The results are published in the March 28 issue of JAMA.

"This is the first study where children with routine lead exposure have been followed into midlife," lead author, Aaron Reuben, MEM, Duke University, Durham, North Carolina, commented to Medscape Medical News. "We have now shown that lead exposure has long-term effects that don't go away. We can still detect the consequences 30 years later."   

He noted that the population included in this study was a normal cohort of children born in 1972–1973 shown to be representative of that age group of the general population of the western world.

"Most people who are now middle aged would have been exposed to a similar level of lead as we have seen in this study, so our results will probably apply to a large percentage of the population in whom we can expect a downward trend in IQ."  

The current analysis is part of the Dunedin Multidisciplinary Health and Development Study, a prospective cohort study based on a population representative 1972–1973 birth cohort from New Zealand, and followed so far to age 38 years.

Of 1037 original participants, 1007 were alive at age 38 years, of whom 565 (56%) had undergone lead testing at age 11 years, with mean levels of 10.99 μg/dL.

Results showed that after adjustment for maternal IQ, childhood IQ, and childhood socioeconomic status, each 5-μg/dL higher level of blood lead in childhood was associated with a 1.61-point lower IQ score at age 38 years, a 2.07-point lower score in perceptual reasoning, and a 1.26-point lower score in working memory.

Associations of childhood blood lead level with deficits in verbal comprehension and processing speed were not statistically significant.

After adjustment for confounders, each 5-μg/dL higher level of blood lead in childhood was associated with a 1.79-unit lower score in socioeconomic status.

An association between greater blood lead levels and a decline in IQ and socioeconomic status from childhood to adulthood was observed, with 40% of the association with downward mobility mediated by cognitive decline from childhood.

Another analysis showed that those who had higher lead exposure as a child (>10 μg/dL) had a 4.25-point lower IQ score at age 38 than those with lead levels below 10 μg/dL in childhood.

Dr Reuben noted that the effect of a 4-point drop in IQ would differ greatly in different individuals. "It depends on where you start — if you are at the genius level then you probably wouldn't notice a 4-point drop. But if you start and an average level or below then this would have a big effect.  

"That is why we looked at socioeconomic status and we also saw a reduction in that so that even a mild drop in IQ seemed to have a knock-on effect on socioeconomic status," he added. "Importantly, we found this occurred regardless of where people started out — high or low in socioeconomic terms."

The researchers also showed that those with high lead levels at age 11 years had a decline from their own childhood IQ. Dr Reuben explained that IQ scores normally stayed the same or possibly increased a little from childhood into midlife. "But in high lead–exposed individuals we found that IQ scores actually dropped."

Dr Reuben noted that the childhood lead levels in this study (average, 11 μg/dL) were normal for a developed country at that time but would now be much lower.  

"To show how well things have improved, the average in the Western world is now under 3 μg/dL.  We now think of 5 as being high, but it doesn't mean that under 5 is safe — actually it is thought that any level of lead is harmful."

"At the start of this study lead exposure was similar across all socioeconomic groups, mainly because of the high levels in gasoline, and the use of lead paint and pipes.  In the 70s and 80s it was everywhere, but since the mid-90s we have dramatically lowered the lead use. Now in most parts of the world the lead exposure burden is highly focused in low-income communities, and related to industrial sites or older residential homes."

Curtailing Upward Mobility?

He says these latest results point to lead levels as being one factor that could be curtailing upward mobility. "Our results reinforce the need to make extra efforts to reduce lead levels in these low-income communities."

And Dr Reuben suggests that in areas that have recently been exposed to lead pollution, extra efforts to compensate for its deleterious effect on IQ should be considered. "These could include programs to improve education and nutrition in children, as we know these things can help brain development."

In an accompanying editorial,  David C. Bellinger, PhD, Boston Children's Hospital, Massachusetts, notes that the total IQ loss in young children that is attributable to current exposures to lead still exceeds that attributable to many pediatric conditions, including brain tumors, congenital heart disease, and traumatic brain injury.

"Some exposure to lead is still virtually universal, and, because there is no safe level of lead, any exposure is detrimental. As a result, and in contrast to rare conditions, all children contribute to the total lead-related IQ loss at the population level."

He points out that because effective options for secondary prevention are limited, reducing the health burden related to lead exposure will require vigorous primary prevention measures. "Environments must be screened, in addition to children, so that the most hazardous sites can be effectively abated before rather than after a child's exposure occurs."

Dr Bellinger adds that identifying educational interventions that improve the cognitive outcomes of lead-exposed children is an important research need because no data are available.

He concludes that until these steps are taken, problems are almost certain to occur periodically.

"Children will continue to experience the greatest harm from lead exposure, and disadvantaged children will bear a disproportionate share of the burden," he writes. "The findings of the study by Reuben et al show that these avoidable harms are long lasting and may prevent children from achieving their full potential."

The Dunedin Multidisciplinary Health and Development Research Unit is supported by the New Zealand Health Research Council and the New Zealand Ministry of Business, Innovation, and Employment. This research received grant support from the US National Institute on Aging; the UK Medical Research Council; the Economic Social Research Council; and the Jacobs Foundation. Dr Bellinger reports receipt of compensation as an expert witness for plaintiffs and defendants in civil litigation involving lead poisoning.

JAMA. Published online March 28, 2017. Abstract, Editorial

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