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Toxicological Effects of Methylmercury
heavier doses per occasion, has a more adverse impact on neuronal development than the more gradual exposure in the Seychelles. However, it is difficult to compare the 12 fish meals per week reported in the Seychelles with the three fish “dinners” per week in the Faroe Islands, because the types of fish eaten and their Hg concentrations are different. Moreover, the exposure-associated differences in neurobehavior found in the New Zealand cohort and the Seychelles pilot study where no whale meat was eaten suggest that bolus doses are not necessary to generate cognitive deficits at those levels of exposure.
The importance of high episodic (“spiking”) exposures is unclear. However, as discussed in Chapter 4, the degree of spiking in the Faroe study is likely to be in the low-to-moderate range (i.e., less than a doubling in hair Hg concentrations, assuming an individual at the Faroe Islands median exposure level consumes three consecutive 4-ounce whale meals). Spiking might also occur in the Seychelles given the availability of fish species with characteristically moderate-to-high concentrations of Hg (e.g. tuna), although the absence of dietary data does not allow this issue to be examined further.
STUDY DIFFERENCES IN CONTROL FOR CONFOUNDERS
A potential confounder is a variable related to both the exposure of interest (e.g., MeHg) and to the outcome of interest (e.g., neurobehavior). If the relation between exposure and outcome is no longer significant after controlling statistically for a potential confounder, it is inferred that the relation between exposure and outcome is spurious and due to confounding by the control variable being examined. Because random assignment to predetermined exposure levels cannot be used to control for confounding in human exposure studies, it is important to assess whether a broad range of control variables confound any associations observed between exposure and outcome. Table 6-2 lists the control variables examined in the Faroe and Seychelles studies. Both studies evaluated most of the variables that are known to be at least moderately related to childhood cognitive outcome, including maternal cognitive competence (e.g. Ravens test), child age, gender, maternal alcohol consumption and smoking during pregnancy, and parental income. A few variables that are sometimes modestly related to those outcomes