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Lead: Devastating Impact Even at Low Levels

When lead goes up, even a little, intelligence scores go down.

The relation between intelligence and blood lead levels greater than 10 µg/dL is well defined: IQ declines by 2 to 5 points with every increase of 10 to 30 µg/dL. The effect of lower lead concentrations is less certain. Investigators followed 172 children from age 6 months to 5 years, measuring IQ at ages 3 and 5 years and blood lead concentration at ages 6, 12, 18, 24, 36, and 48 months.

After accounting for 9 possible confounders, including maternal IQ and home environment, each increase of 10 µg/dL in lifetime average blood lead level was significantly associated with a 4.6-point decrease from the expected IQ scores in healthy children (P=0.004) in linear analysis. When the analysis was restricted to the 101 children whose maximum blood lead level never reached 10 µg/dL, the association between higher lead concentration and lower IQ was even stronger: In a nonlinear model, IQ declined by 7.4 points with increases in the lifetime average blood lead concentrations of up to 10 µg/dL. Beyond these lower concentrations, there were further, more gradual declines (an additional 2.5-point decrease for concentrations of 10 µg/dL to 30 µg/dL). These results were unchanged whether lead exposure was categorized as peak, concurrent with testing, or average in infancy.

Comment: As an editorialist notes, the CDC has repeatedly lowered its definition of elevated blood lead levels -- it now stands at 10 µg/dL. It is gratifying that the median concentration in U.S. children has fallen almost 4-fold, from 15 µg/dL to 2 µg/dL since gasoline was deleaded. However, almost 500,000 children have lead levels above 10 µg/dL, and many more have levels above 2 µg/dL. What is the primary care physician to do? It is critical that infants receive enough iron, which reduces the gastrointestinal absorption of lead, and it remains important to measure blood lead levels in high-risk children, such as those living in houses built before 1960.

— Howard Bauchner, MD

Published in Journal Watch Pediatrics and Adolescent Medicine May 9, 2003

Citation(s):

Canfield RL et al. Intellectual impairment in children with blood lead concentrations below 10 µg per deciliter. N Engl J Med 2003 Apr 17; 348:1517-26.

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Rogan WJ and Ware JH. Exposure to lead in children--how low is low enough? N Engl J Med 2003 Apr 17; 348:1515-6.

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• Medline abstract (Free)