Environmental lead and children's intelligence: a systematic review of the epidemiological evidence

OBJECTIVE: To quantify the magnitude of the relation between full scale IQ in children aged 5 or more and their body burden of lead. DESIGN: A systematic review of 26 epidemiological studies since 1979: prospective studies of birth cohorts, cross sectional studies of blood lead, and cross sectional studies of tooth lead. SETTING: General populations of children > or = 5 years. MAIN OUTCOME MEASURES: For each study, the regression coefficient of IQ on lead, after adjustment for confounders when possible, was used to derive the estimated change in IQ for a specific doubling of either blood or tooth lead. RESULTS: The five prospective studies with over 1100 children showed no association of cord blood lead or antenatal maternal blood lead with subsequent IQ. Blood lead at around age 2 had a small and significant inverse association with IQ, somewhat greater than that for mean blood lead over the preschool years. The 14 cross sectional studies of blood lead with 3499 children showed a significant inverse association overall, but showed more variation in their results and their ability to allow for confounders. The seven cross sectional studies of tooth lead with 2095 children were more consistent in finding an inverse association, although the estimated magnitude was somewhat smaller. Overall synthesis of this evidence, including a meta-analysis, indicates that a typical doubling of body lead burden (from 10 to 20 micrograms/dl (0.48 to 0.97 mumol/l) blood lead or from 5 to 10 micrograms/g tooth lead) is associated with a mean deficit in full scale IQ of around 1-2 IQ points. CONCLUSION: While low level lead exposure may cause a small IQ deficit, other explanations need considering: are the published studies representative; is there inadequate allowance for confounders; are there selection biases in recruiting and following children; and do children of lower IQ adopt behaviour which makes them more prone to lead uptake (reverse causality)? Even if moderate increases in body lead burden adversely affect IQ, a threshold below which there is negligible influence cannot currently be determined. Because of these uncertainties, the degree of public health priority that should be devoted to detecting and reducing moderate increases in children's blood lead, compared with other important social detriments that impede children's development, needs careful consideration.