Fish Consumption Advice for Alaskans
The Faroe Islands The other large-scale study took place in the Faroe Islands, where methylmercury exposure occurs primarily through consumption of pilot whale meat (1–2 meals a week) containing an average total mercury concentration of 3.3 ppm (1.6 ppm methylmercury).57 Of 1,023 consecutive births, the median umbilical cord blood mercury concentration was 24.2 ppb; 25.1% (n=250) had blood-mercury concentrations that exceeded 40 ppb. The median maternal hair mercury concentration was 4.5 ppm, with 12.7% (n=130) of women having concentrations exceeding 10 ppm. 58
Evaluation of 583 subjects during infancy (age < 12 months) demonstrated that infants with higher hair mercury concentrations had more rapid achievement of developmental milestones than other infants. Increased frequency of breast-feeding was associated with better test performance and higher hair mercury concentrations. 59
Possible in utero neurologic effects were evaluated at 7 years of age. Neurologic and developmental tests included the Neurobehavioral Evaluation System (NES) Finger Tapping Test, the NES Hand-Eye Coordination Test, NES Continuous Performance Test, the Tactual Performance Test, the Boston Naming Test for language skills, the Wechsler Intelligence Scale for Children-Revised (WISC-R), WISC-R Digit Spans, WISC-R Block Designs, WISC-R Similarities, Bender Gestalt Test for visuospatial skills, California Verbal Learning Test for memory, and the Nonverbal Analogue Profile of Mood States.
Analyses of 917 children at 7 years of age found no clinical or neurophysiological mercury-related abnormalities. However, subtle decreases in neuropsychological test performance were associated with prenatal mercury exposure at maternal hair levels below 10 ppm, “although test scores obtained by most of the highly exposed children were mainly within the range seen in the rest of the children....”60 Interestingly, the Faroese children had excellent visual contrast sensitivity that may be attributed to the ample supply of dietary omega-3 fatty acids. At age 14 years, an association with prenatal methylmercury exposure and delays in the response of the brain to sound was reported; however, hearing thresholds were not affected by methylmercury exposure. 61
Pilot whales also contain relatively high concentrations of PCBs and organochlorine pesticides. In 2001 Grandjean, et al, reported neurobehavioral deficits associated with PCBs in this cohort.62 PCBs were quantified by multiplying the sum concentration of 3 congeners by 2 to derive the total. This is a relatively crude method with which to quantify PCBs; more rigorous methods quantify many more congeners (typically 40 or more; 209 are possible) and sum them for a more accurate total. Such analyses allow consideration of structure-activity relationships of individual congeners, and increase power to detect significant associations with outcome variables.63 Four of the neuropsychological outcomes measured showed possible decrements associated with wet-weight PCB concentration, but not lipid-adjusted PCB concentrations. Adjustment for methylmercury reduced the association to a nonsignificant level. The strongest PCB effect was noted in those within the highest tertile of methylmercury exposure. Interestingly, the most sensitive parameter to the PCB exposure was the Boston Naming test, the endpoint selected by EPA to derive its reference dose for methylmercury. EPA concluded that “…methylmercury neurotoxicity may be a greater hazard than that associated with PCBs, but PCBs could possibly augment the neurobehavioral deficits at increased levels of mercury exposure.” Previous statistical analysis by this group indicated methylmercury-associated neurobehavioral deficits were unlikely to be affected by PCB exposure.64 A consideration of the potential neurobehavioral effects of PCBs and methylmercury suggests a need for further study to determine whether the effects noted in the Faroe Islands study result from methylmercury exposure alone.