Environmental Exposure to Lead and Children's Intelligence at the Age of Seven Years

Abstract

Exposure to lead in early childhood is thought to result in delayed neuropsychological development. As yet there is little longitudinal evidence to establish whether these effects persist into later childhood. We measured IQ scores in 494 seven-year-old children from the lead-smelting community of Port Pirie, Australia, in whom developmental deficits associated with elevated blood lead concentrations had already been reported at the ages of two and four years. Exposure to lead was estimated from the lead concentrations in maternal blood samples drawn antenatally and at delivery and from blood samples drawn from the children at birth (umbilical-cord blood), at the ages of 6 and 15 months and 2 years, and annually thereafter. Data relating to known covariates of child development were collected systematically for each child throughout the first seven years of life. We found inverse relations between IQ at the age of seven years and both antenatal and postnatal blood lead concentrations. After adjustment by multiple regression for sex, parents' level of education, maternal age at delivery, parents' smoking status, socioeconomic status, quality of the home environment, maternal IQ, birth weight, birth order, feeding method (breast, bottle, or both), duration of breast-feeding, and whether the child's natural parents were living together, the relation with lead exposure was still evident for postnatal blood samples, particularly within the age range of 15 months to 4 years. For an increase in blood lead concentration from 10 μg per deciliter (0.48 μmol per liter) to 30 μg per deciliter (1.45 μmol per liter), expressed as the average of the concentrations at 15 months and 2, 3, and 4 years, the estimated reduction in the IQ of the children was in the range of 4.4 points (95 percent confidence interval, 2.2 to 6.6) to 5.3 points (95 percent confidence interval, 2.8 to 7.8). This reduction represents an approximate deficit in IQ of 4 to 5 percent. Low-level exposure to lead during early childhood is inversely associated with neuropsychological development through the first seven years of life. (N Engl J Med 1992;327:1279–84.), EXPOSURE to low levels of lead in childhood may result in impaired neuropsychological development and classroom performance.1 The extent of this relation, however, after adjustment for the confounding effects of socioeconomic and environmental covariates, has been debated.234567891011121314151617 Taken together, the epidemiologic studies indicate a moderate inverse relation between the body burden of lead (measured as blood or tooth lead concentrations) and the neuropsychological or cognitive performance of children.1819202122 Whether the effects disappear once lead exposure ceases or whether early exposure to lead has effects on neuropsychological development that persist into later life is uncertain. Longitudinal studies are clearly the best… © 1992, Massachusetts Medical Society. All rights reserved.