Methylmercury is present throughout the world in seafood and freshwater fish. Environmental methylmercury derives almost solely from methylation of inorganic mercury. The major natural source of atmospheric mercury is degassing from the earth's crust. Global anthropogenic releases of mercury to the atmosphere are estimated to be about similar in magnitude to natural emissions, and local releases may cause serious contamination and be more readily bioavailable than more distant ones, e.g., releases from deep-sea volcanic sources. Methylated mercury is absorbed by fish and shellfish from food and sediments, and possibly also as dissolved species directly from water. Thus, the concentration of methylmercury in finfish tends to be proportional to the mercury concentration in water and also accumulates with time. As a result of biomagnification in food chains, the highest concentrations are found in predatory fish with longer life spans, including freshwater trout, pike, bass, and, in marine species, particularly tuna and swordfish, as well as shark, seals and cetaceans. Mercury also accumulates in a variety of molluscs, crustaceans, and echinoderms, including species that contribute to the human diet. Accumulation of methylmercury in such species results in increased exposures in many fishing communities.
Persistent organic pollutants
Industrial compounds that are only slowly degraded in the environment may accumulate in food chains and result in food-mediated human exposures. Among members of this group, the polychlorinated biphenyls (PCBs) and the pesticide metabolite p,p'-dichlorodiphenyldichloroethylene (DDE) are common contaminants of lipid-containing tissues and reach high concentrations, especially in pilot whale blubber. Several PCB congeners and DDE have elimination half-lives in humans that exceed five years and therefor accumulate with age. Increased PCB exposure is associated with delayed neurobehavioral development and possible endocrine disruption – both outcomes being examined in the Faroes cohorts.
The perfluorinated compounds are also highly persistent and accumulate in marine food chains, but they are not lipophilic. The therefore occur in fish and pilot whale meat, and traditional seafood diets in the Faroes are known to be associated with increased concentrations of PFCs in serum. Of special interest, animal experiments suggest that serum concentrations similar to those that are common in humans are associated with immune system dysfunction in mice. Thus, immunotoxicity is being studied in the Faroese cohorts.
- Traditional food includes pilot whale meat (MeHg) and blubber (POPs)
- Individual reliance on traditional foods varies, depends on seasonal availability
- Traditional diet only weakly associated with confounders
- Otherwise homogeneous, western culture
- Methylmercury and lipophilic POPs originate from different food items (whale meat / blubber)
- Highest exposure about 1,000-fold higher than lowest