Blood Lead Concentrations <10 Micrograms Per Deciliter and Child IQ at Age 6 Years

Abstract

ISEE-432 Objective: To examine the association between blood lead concentrations assessed throughout early childhood and children's intelligence test scores at age 6 years. Material and Methods: This prospective cohort study followed-up 242 infants and their families from 6 months to 6 years of age, measuring blood lead concentrations at 6, 12, 18, and 24 months, and yearly thereafter. Full-scale performance and verbal IQ at age 6 years was assessed using the Wechsler Preschool and Primary Scale of Intelligence – Revised. At the 6-year follow-up, 194 children (80%) completed IQ testing; complete data were available for 174. Results: Lifetime average blood lead concentrations, based on up to 8 blood samples collected over the 6 month to 6 year age period, averaged 7.2 ìg/dL (median, 6.2 ìg/dL). After adjustment for maternal IQ, HOME scale scores, and other potential confounding factors, lifetime average blood lead concentration was inversely associated with full-scale (P for trend = 0.006) and performance IQ scores (P for trend = 0.002). Compared with children who had lifetime average blood lead concentrations <5 ìg/dL, children with lifetime average concentrations between 5 and 9.9 ìg/dL scored 4.9 points lower on measures of full-scale IQ (91.3 vs. 86.4, P = 0.03). However, in comparison with children with blood lead concentrations between 5 and 9.9 ìg/dL, children with blood lead concentrations ≥10 ìg/dL scored only 2.3 points lower on full-scale IQ, and this difference was not statistically significant (86.4 vs. 84.1, P = 0.34). Verbal IQ scores were marginally associated with lifetime average blood lead concentration, but the dose-response relation was less consistent than it was for full-scale and performance IQ. Conclusions: Children's intellectual functioning at 6 years may be impaired by blood lead concentrations in the range of 5 to 10 ìg/dL, below the current CDC definition of an elevated blood lead level.