Effect of low-level body burdens of lead on the mental development of children: limitations of meta-analysis in a review of longitudinal data

Abstract

The effect of low-level body burdens of lead on the intelligence of children, as measured by intelligence quotient (IQ), was assessed. We reviewed 35 reports from five longitudinal studies conducted in the United States and Australia. In each of these studies, infants were followed for 58 mo or less. The study populations consisted of low- and middle-socioeconomic-class infants who had low-level exposure to environmental lead. Blood-lead levels were measured in a standard fashion at various times, beginning in the prenatal period, and intelligence was first measured at 6 mo of age and was followed by subsequent assessments. Studies were assessed for quality by a review panel blinded to the identity of the investigators and their affiliations. Efforts were made to pool the data with meta-analytic techniques, but efforts were unsuccessful because the methods used to analyze and report data were inconsistent. Inconsistencies were as follows: (a) there were few instances in which IQ and blood-lead levels were measured at comparable times in different studies; (b) incompatibilities existed among the studies, including differences in independent variables, data transformations, and statistical parameters reported; (c) results conflicted when measurement intervals were comparable (i.e., heterogeneity); (d) patterns of regression and correlation coefficients were inconsistent; and (e) data were insufficient to interconvert the parameters reported. Consequently, definitive conclusions regarding the effect of low-level body burdens of lead on IQ could not be determined from the longitudinal data. Examination of the weight of the evidence from this and other studies, however, suggests an adverse relationship of lead on the intelligence of children.