| IQ and Blood Lead from 2 to 7 Years of Age: Are the Effects in Older Children the Residual of High Blood Lead Concentrations in 2-Year-Olds? Aimin Chen,1 Kim N. Dietrich,2 James H. Ware,3 Jerilynn Radcliffe,4 and Walter J. Rogan1 1Epidemiology Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, USA; 2Department of Environmental Health, University of Cincinnati, Cincinnati, Ohio, USA; 3Harvard School of Public Health, Harvard University, Boston, Massachusetts, USA; 4Department of Psychology, Children's Hospital of Philadelphia, and University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA Abstract
Increases in peak blood lead concentrations, which occur at 18-30 months of age in the United States, are thought to result in lower IQ scores at 4-6 years of age, when IQ becomes stable and measurable. Data from a prospective study conducted in Boston suggested that blood lead concentrations at 2 years of age were more predictive of cognitive deficits in older children than were later blood lead concentrations or blood lead concentrations measured concurrently with IQ. Therefore, cross-sectional associations between blood lead and IQ in school-age children have been widely interpreted as the residual effects of higher blood lead concentrations at an earlier age or the tendency of less intelligent children to ingest more leaded dust or paint chips, rather than as a causal relationship in older children. Here we analyze data from a clinical trial in which children were treated for elevated blood lead concentrations (20-44 µg/dL) at about 2 years of age and followed until 7 years of age with serial IQ tests and measurements of blood lead. We found that cross-sectional associations increased in strength as the children became older, whereas the relation between baseline blood lead and IQ attenuated. Peak blood lead level thus does not fully account for the observed association in older children between their lower blood lead concentrations and IQ. The effect of concurrent blood level on IQ may therefore be greater than currently believed. Key words: chelation, developmental testing, IQ, lead poisoning, longitudinal studies. Environ Health Perspect 113:597-601 (2005) . doi:10.1289/ehp.7625 available via http://dx.doi.org/ [Online 2 February 2005]
Address correspondence to W.J. Rogan, Epidemiology Branch, NIEHS, MD A3-05, P.O. Box 12233, Research Triangle Park, NC 27709 USA. Telephone: (919) 541-4578. Fax: (919) 541-2511. E-mail: rogan@niehs.nih.gov The Treatment of Lead-exposed Children trial was supported by the National Institutes of Environmental Health Sciences in cooperation with the National Institutes of Health Office of Minority Health, and by the Centers for Disease Control and Prevention. Succimer and placebo capsules were gifts from McNeil Labs, Fort Washington, PA, USA. The authors declare they have no competing financial interests. Received 29 September 2004 ; accepted 2 February 2005.
The full version of this article is available for free in HTML or PDF formats. |
