revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 245
2. HEALTH EFFECTS
In addressing the potential influence of concurrent PCB exposure on the Faroe results, the Confounders and
Variables (Epidemiology) panel indicated that with respect to four of the prenatal outcomes (related
primarily to verbal and memory performance), when PCBs were included in the model, only one of these
outcomes is specifically related to mercury exposure. Concerning this matter, the panel wrote that “... the
most likely explanation is that both (mercury and PCBs)... affect these three outcomes, but their relative
contributions cannot be determined given their concurrence in this population.” The Neurobehavioral
Endpoints Panel also looked at this issue, and noted that “PCB exposure might act as an effect modifier,
increasing the susceptibility to MeHg”; however, this panel further indicated that it did not believe that the
effects seen in the Faroe Islands were due to uncontrolled confounding by PCBs. A third panel that
addressed the issue of concurrent PCB exposures, the Statistics/Design Panel, noted that only 3 of 208 PCB
congeners were measured in the Faroe study, and stated that it “seems likely that mercury was measured
more accurately than the biologically relevant PCB exposure. Consequently even if the neurological effects
seen in this study were caused entirely by PCBs, it is possible that mercury would still be more highly
correlated with these effects than PCBs.” The Statistics/Design Panel also said that “the best method to deal
with this problem would be to study a population where exposure to PCBs is not an issue.” This statement
points directly to the Seychelles study as the study most appropriate for MRL derivation.
Another issue raised at Raleigh workshop concerned the taking of hair samples for determining pre-natal
exposure. In the Seychelles, hair samples were collected 6 months post-partum, and segments
corresponding to pregnancy were selected for analysis. In the case of the Faroese, hair samples were taken
at the scalp. Regarding that, the Confounders and Variables (Epidemiology) panel stated that “Given the
time it takes the Hg to be excreted into the hair, we can assume that samples collected at parturition do not
cover the last 6 weeks of gestation, during which critically important neuronal proliferation and
differentiation is taking place.”
Regarding the Seychelles and Faroe studies, the Neurobehavioral Endpoints Panel found “no specific
neurobehavioral signature injury from MeHg” in the data from either study (Seychelles or Faroe). The
same panel also noted that episodic exposure in the Faroese (1–2 fish meals/week and