Dose–Response Relationship of Prenatal Mercury Exposure and IQ: An Integrative Analysis of Epidemiologic Data

Abstract

BackgroundPrenatal exposure to mercury has been associated with adverse childhood neurologic outcomes in epidemiologic studies. Dose–response information for this relationship is useful for estimating benefits of reduced mercury exposure.ObjectivesWe estimated a dose–response relationship between maternal mercury body burden and subsequent childhood decrements in intelligence quotient (IQ), using a Bayesian hierarchical model to integrate data from three epidemiologic studies.MethodsInputs to the model consist of dose–response coefficients from studies conducted in the Faroe Islands, New Zealand, and the Seychelles Islands. IQ coefficients were available from previous work for the latter two studies, and a coefficient for the Faroe Islands study was estimated from three IQ subtests. Other tests of cognition/achievement were included in the hierarchical model to obtain more accurate estimates of study-to-study and end point–to–end point variability.ResultsWe find a central estimate of −0.18 IQ points (95% confidence interval, −0.378 to −0.009) for each parts per million increase of maternal hair mercury, similar to the estimates for both the Faroe Islands and Seychelles studies, and lower in magnitude than the estimate for the New Zealand study. Sensitivity analyses produce similar results, with the IQ coefficient central estimate ranging from −0.13 to −0.25.ConclusionsIQ is a useful end point for estimating neurodevelopmental effects, but may not fully represent cognitive deficits associated with mercury exposure, and does not represent deficits related to attention and motor skills. Nevertheless, the integrated IQ coefficient provides a more robust description of the dose–response relationship for prenatal mercury exposure and cognitive functioning than results of any single study.

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“…Griffiths et al (2007), mainly with respect to the economic analysis but also because of the restriction to the Faroe cohort. However, the corrected dose-response slope estimated by Trasande et al is close to that derived by Axelrad et al (2007) on basis of the three most relevant cohorts (Faroe Islands, Seychelles, New Zealand).…”

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“…The scores for the IQ tests in the Pop study have standard deviations of 16 (Pop et al 1999). We multiplied the results by 15/16 to be on the same scale as the other results, as suggested in Axelrad et al (2007). We converted from units pmol/L FT 4 to pg/mL FT 4 using the molecular weight of thyroxine (776.87).…”

“…The IQ/cognitive test results from the various studies had different standard deviations. We scaled the slopes to a common standard deviation of 15 (Axelrad et al, 2007). For results before and after scaling, see Table 4.…”

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“…These DRFs are then revised in Trasande et al 2006. The revised function of DRF for Hg by Axelrad et al (2007) is based on an integrative analysis of the New Zealand, the Seychelles, and the Faroe Islands studies as well as the estimates of Trasende et al (2005). This function is also used in the Spadaro and Rabl (2008) study.…”