Blood lead concentrations < 10 microg/dL and child intelligence at 6 years of age

Abstract

Background: Few studies provide data directly relevant to the question of whether blood lead concentrations < 10 microg/dL adversely affect children's cognitive function.

Objective: We examined the association between blood lead concentrations assessed throughout early childhood and children's IQ at 6 years of age.

Methods: Children were followed from 6 months to 6 years of age, with determination of blood lead concentrations at 6, 12, 18, and 24 months, and 3, 4, 5, and 6 years of age. At 6 years of age, intelligence was assessed in 194 children using the Wechsler Preschool and Primary Scale of Intelligence-Revised. We used general linear and semiparametic models to estimate and test the association between blood lead concentration and IQ.

Results: After adjustment for maternal IQ, HOME scale scores, and other potential confounding factors, lifetime average blood lead concentration (mean = 7.2 microg/dL; median = 6.2 microg/dL) was inversely associated with Full-Scale IQ (p = 0.006) and Performance IQ scores (p = 0.002). Compared with children who had lifetime average blood lead concentrations < 5 microg/dL, children with lifetime average concentrations between 5 and 9.9 microg/dL scored 4.9 points lower on Full-Scale IQ (91.3 vs. 86.4, p = 0.03). Nonlinear modeling of the peak blood lead concentration revealed an inverse association (p = 0.003) between peak blood lead levels and Full-Scale IQ down to 2.1 microg/dL, the lowest observed peak blood lead concentration in our study.

Conclusions: Evidence from this cohort indicates that children's intellectual functioning at 6 years of age is impaired by blood lead concentrations well below 10 microg/dL, the Centers for Disease Control and Prevention definition of an elevated blood lead level.

Keywords: GAM; HOME; IQ; LOESS; Rochester; WPPSI-R; cohort; electrothermal atomic absorption spectrometry.

Figure 1

Distributions of blood lead concentrations (n = 174). In each box plot, the median value is indicated by the center horizontal line and the 25th and 75th percentiles are indicated by the lower and upper horizontal lines, respectively. The vertical lines represent the 5th and 95th percentiles.

Figure 2

Differences in Full-Scale, Performance, and Verbal IQ associated with increasing lifetime average blood lead concentrations (n = 174). Mean IQ levels are adjusted for child’s sex, birth weight, and transferrin saturation; mother’s race, IQ, and education level; HOME-SF total score, family income, and maternal prenatal smoking. Error bars represent 95% confidence intervals. White bars represent the mean IQ of children with blood lead concentrations < 5 μg/dL (n = 64), the blue bars represent the mean IQ of children with blood lead concentrations 5–9.9 μg/dL (n = 70), and the black bars represent the mean IQ of children with blood lead concentrations ≥ 10 μg/dL (n = 40). Values above the brackets represent the mean difference in IQ for adjacent groups and associated p-values.

Figure 3

Differences in Full-Scale, Performance, and Verbal IQ associated with increasing concurrent blood lead concentrations (n = 174). Mean IQ levels are adjusted for child’s sex, birth weight, and transferrin saturation; mother’s race, IQ, and education level; HOME-SF total score, family income, and maternal prenatal smoking. Error bars represent 95% confidence intervals. White bars represent the mean IQ of children with blood lead concentrations < 5 μg/dL (n = 107), the blue bars represent the mean IQ of children with blood lead concentrations 5–9.9 μg/dL (n = 53), and the black bar represent the mean IQ of children with blood lead concentrations ≥ 10 μg/dL (n = 14). Values above the brackets represent the mean difference in IQ for adjacent groups and associated p-values.

Figure 4

Differences in Full-Scale, Performance, and Verbal IQ associated with increasing infancy average blood lead concentrations (n = 174). Mean IQ levels are adjusted for child’s sex, birth weight, and transferrin saturation; mother’s race, IQ, and education level; HOME-SF total score, family income, and maternal prenatal smoking. Error bars represent 95% confidence intervals. White bars represent the mean IQ of children with blood lead concentrations < 5 μg/dL (n = 62), the blue bars represent the mean IQ of children with blood lead concentrations 5–9.9 μg/dL (n = 79), and the black bars represent the mean IQ of children with blood lead concentrations ≥ 10 μg/dL (n = 33). Values above the brackets represent the mean difference in IQ for adjacent groups and associated p-values.

Figure 5

Differences in Full-Scale, Performance, and Verbal IQ associated with increasing peak blood lead concentrations (n = 174). Mean IQ levels are adjusted for child’s sex, birth weight, and transferrin saturation; mother’s race, IQ, and education level; HOME-SF total score, family income, and maternal prenatal smoking. Error bars represent 95% confidence intervals. The bars represent the mean IQ of children with blood lead concentrations < 5 μg/dL (n = 17; white); 5–9.9 μg/dL (n = 79; blue); 10–14.9 μg/dL (n = 41; black); and ≥ 15 μg/dL (n = 37; gray). Values above the brackets represent the mean difference in IQ for adjacent groups and associated p-values.

Figure 6

Full-Scale IQ as a function of peak blood lead concentration from 6 months to 6 years (n = 169), with 95% confidence intervals. The individual points represent the unadjusted peak blood lead concentrations and Full-Scale WPPSI-R IQ scores.