Improbable blood lead concentration–IQ relationships

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To the Editor:

The recent article by Solon et al¹ describes a dramatic reduction in IQ associated with increases in blood lead concentrations for a population of children in the Philippines. The authors report a decrease in cognitive function of 3.32 points in young children and 2.47 points in older children, for each 1-μg/dL increase in blood lead concentrations. Given the range of blood lead concentrations and cognitive function scores described in this study, it is highly improbable that the reported relationships are accurate.

The mean and standard deviation of population blood lead concentrations reported by Solon et al¹ suggest that approximately 90% of the data fall between blood lead concentrations of 1 and 20 μg/dL. A 3.32 (2.47)-point decline in cognitive function per 1-μg/dL blood lead concentration corresponds to a cognitive function deficit of 63 (47) points (5th to 95th percentile). This estimate is considerably higher than those reported elsewhere; for example, Lanphear et al² report an IQ deficit of 7.1 points between 5th and 95th percentile blood lead concentrations. Furthermore, the results are inconsistent with research suggesting that lead typically accounts for less than 5% of the variance in IQ.³ In contrast, the estimates by Solon et al¹ provide for cognitive declines that are greater than the estimated range of cognitive data in their study, estimated here as 54 (24) points between the 5th and 95th percentiles on the basis of means and standard deviations reported by Solon et al.¹

Other authors² have regressed measures of cognitive function against the logarithm of blood lead or have performed regressions on a small subset of blood lead data. Both approaches sometimes yield numerical coefficients similar in magnitude to those reported by Solon et al.¹ However, Solon et al¹ do not indicate that their reported slope applies over less than the full range of blood lead concentrations or that the slope coefficients arise from a nonlinear fit of the blood lead concentrations. The relationships presented by Solon et al¹ are highly improbable, and I conclude that there is an error in either the analysis or the presentation of blood lead concentration–IQ relationships in the study by Solon et al.¹

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References 

1. Solon O, Riddell TJ, Quimbo SA, Butrick E, Aylward GP, Bacate ML, et al. Associations between cognitive function, blood lead concentration, and nutrition among children in the Central Philippines. J Pediatr. 2008;152:237–243
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2. Lanphear BP, Hornung R, Khoury J, Yolton K, Baghurst P, Bellinger DC, et al. Low-level environmental lead exposure and children's intellectual function: An international pooled analysis. Environ Health Persp. 2005;113:894–899
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3. Koller K, Brown T, Spurgeon A, Levy L. Recent developments in low-level lead exposure and intellectual impairment in children. Environ Health Persp. 2004;112:987–994
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