Lead exposure, IQ, and behavior in urban 5- to 7-year-olds: does lead affect behavior only by lowering IQ?

BACKGROUND Lead exposure in childhood lowers IQ scores, but its effect on children's behavior is less clear. Because IQ, per se, affects behavior, measuring the direct effect of lead requires measuring and then adjusting for IQ. In addition, either peak blood lead concentration, usually at 2 years old, or the lower blood lead level measured at school age may be the most relevant. Few studies have all of this information. OBJECTIVES The purpose of this work was to differentiate the direct effect of lead on behavior and the indirect effect through IQ and to examine the strength of the association for peak and concurrent blood lead concentration. METHODS Data come from a clinical trial of the chelating drug succimer to prevent cognitive impairment in 780 urban 12- to 33-month-olds with blood lead concentrations of 20 to 44 microg/dL. The children were followed from ages 2 to 7 years. The trial data were analyzed as a prospective observational study. RESULTS Blood lead concentration at 2 years old was not associated with Conners' Parent Rating Scale-Revised scores at 5 years of age or Behavioral Assessment Systems for Children scores at 7 years of age. Blood lead level at 7 years of age had direct effects on the Behavioral Assessment Systems for Children behavioral symptoms index, externalizing, and school problems at age 7. CONCLUSIONS Concurrent blood lead concentration was associated with externalizing and school problems scales at 7 years of age, and the effect was not entirely mediated through the effect of lead on IQ.