(principal investigator in the SCDS, personal commun., January 20, 2000) indicated to the committee that the most heavily contaminated fish consumed in the Seychelles islands — swordfish, shark, and tuna — tend to be among the most expensive fish, so that, if anything, exposure levels might be higher among mothers with higher socioeconomic status. There is, therefore, no reason to expect a confounding of exposure with lower socioeconomic status, and low socioeconomic status is not likely to explain the association of Hg exposure with adverse development outcomes in the Seychelles pilot study.

ASSESSMENT OF PRENATAL Hg EXPOSURE: CORD BLOOD VERSUS MATERNAL HAIR AND TIMING OF EXPOSURE

The principal measure of prenatal exposure in the Faroe study was Hg in cord blood; in the Seychelles, it was Hg in maternal hair. The Faroe investigators also analyzed maternal-hair samples, but no cord-blood specimens were obtained in the Seychelles. In a recently published analysis, the Faroe investigators compared the relation of the cord-blood and maternal-hair Hg measures with their 7-year end points (Grandjean et al. 1999). As shown in Table 6-1, cord-blood Hg concentration was significantly associated with a slightly larger number of end points than maternal-hair Hg concentration, and in most cases the associations were slightly stronger. For various pharmacokinetic and neurodevelopmental reasons, cord-blood measurements might be more sensitive indicators of the neurodevelopmental effects of MeHg. However, given that hair Hg concentrations in the Faroe Islands study were only a slightly weaker predictor of Hg effects than cord blood, it would be reasonable to expect that, if children were affected in the main Seychelles study, some indication of an association between child performance and maternal-hair Hg concentration would be apparent in that study. With the possible exception of the Bender Gestalt scores for boys, there is no indication of even a trend in the predicted direction in the data published to date from the main SCDS (e.g., see Figures 5-7 and 5-8).

It should be noted that the maternal-hair samples obtained in the Faroe and Seychelles studies did not necessarily reflect exactly the same period of pregnancy. In part, this is because the Seychelles study ob-



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