ENVIRONMENTAL LEAD AND CHILDRENS INTELLIGENCE - A SYSTEMATIC REVIEW OF THE EPIDEMIOLOGIC 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: prospecti ve studies of birth cohorts, cross sectional studies of blood lead, an d cross sectional studies of tooth lead. Setting-General populations o f children greater than or equal to 5 years. Main outcome measures-For each study, the regression coefficient of IQ on lead, after adjustmen t for confounders when possible, was used to derive the estimated chan ge in IQ for a specific doubling of either blood or tooth lead. Result s-The five prospective studies with over 1100 children showed no assoc iation of cord blood lead or antenatal maternal blood lead with subseq uent IQ. Blood lead at around age 2 had a small and significant invers e 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 synthes is of this evidence, including a meta-analysis, indicates that a typic al doubling of body lead burden (from 10 to 20 mu g/dl (0.48 to 0.97 m u mol/l) blood lead or from 5 to 10 mu g/g tooth lead) is associated w ith a mean deficit in full scale IQ of around 1-2 IQ points. Conclusio n-While low level lead exposure may cause a small IQ deficit, other ex planations need considering: are the published studies representative; is there inadequate allowance for confounders; are there selection bi ases in recruiting and following children; and do children of lower IQ adopt behaviour which makes them more prone to lead uptake (reverse c ausality)? Even if moderate increases in body lead burden adversely af fect 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 reduci ng moderate increases in children's blood lead, compared with other im portant social detriments that impede children's development, needs ca reful consideration.