Intellectual Impairment in Children With Blood Lead Concentrations Below 10 μg per Deciliter

Abstract

Blood lead levels in children have declined, and the level of concern specified by the Centers for Disease Control and Prevention now is 10 μg/dL. It is not clear, however, whether cognitive deficits consequent to lead exposure are a problem at blood lead levels less than 10 μg/dL. This study estimated blood lead concentrations in 172 children at ages 6, 12, 18, 24, 36, 48, and 60 months. At ages 3 and 5 years, the children were tested with the Stanford-Binet Intelligence Scale, an instrument that evaluates vocabulary, spatial pattern analysis, quantitative ability, and memory. Intelligence quotient (IQ) was related to blood lead levels after adjusting for maternal IQ, quality of the home environment, and other potentially confounding factors. Mean blood lead levels were lowest at age 6 months (3.4 μg/dL) and maximal at age 2 years (9.7 μg/dL). By age 5, the mean level was 6 μg/dL. The lifetime average blood lead level was 7.7 μg/dL at age 3 years and 7.4 μg/dL at age 5. The proportions of children with peak lead levels below 10 μg/dL were 57.0% and 55.8% at ages 3 and 5 years, respectively. At both these ages, the mean IQ (composite score) was approximately 90. After adjusting for numerous covariates, IQ correlated inversely and significantly with blood lead concentration. An increase in the lifetime average blood lead level of 1 μg/dL correlated with a change in IQ of -0.46. The estimated overall difference in IQ for each 1-μg/dL increase in lifetime average lead level was -1.37 points. Other significant predictors of IQ included maternal IQ, material income, and the child's birth weight. These findings suggest that substantially more children in the United States undergo adverse cognitive change from environmental exposure to lead than was previously thought. Primary prevention is essential in view of the lack of effective treatment for children with moderate blood lead elevations.