December 12th, 2017
Lead is a confirmed neurotoxin, but questions remain about lead-associated intellectual deficits at blood lead levels < 10 micrograms per deciliter (µg/dL) and whether lower exposures are, for a given change in exposure, associated with greater deficits. The objective of this study is to examine the association of intelligence test scores and blood lead concentration, especially for children who had maximal measured blood lead levels < 10 µg/dL. We examined data collected from 1,333 children who participated in 7 international population-based longitudinal cohort studies, followed from birth or infancy until 5-10 years of age. The full scale IQ score was the primary outcome measure. The geometric mean blood lead concentration of the children peaked at 17.8 µg/dL and declined to 9.4 µg/dL by 5 to 7 years of age; 244 (18%) children had a maximal blood lead < 10 µg/dL and 103 (8%) had a maximal blood lead < 7.5 µg/dL.After adjustment for covariates, we found an inverse relationship between blood lead concentration and IQ score. Using a log-linear model, there was a 6.9 IQ point decrement (95% CI=4.2, 9.4) associated with an increase in concurrent blood lead levels from 2.4 µg/dL to 30 µg/dL. The estimated IQ point decrements associated with an increase in blood lead from 2.4 µg/dL to 10 µg/dL, 10 µg/dL to 20 µg/dL and 20 µg/dL to 30 µg/dL were 3.9 (95% CI=2.4, 5.3), 1.9 (95% CI=1.2, 2.6) and 1.1 (95% CI=0.7, 1.5), respectively. For a given increase in blood lead, the lead-associated intellectual decrement for children with a maximal blood lead level < 7.5 µg/dL was significantly greater than that observed for those with a maximal blood lead level > 7.5 µg/dL (p= 0.015). We conclude that environmental lead exposure in children who have maximal blood lead levels < 7.5 µg/dL is associated with intellectual deficits.