Risks to child health from methylmercury exposure in immigrant populations
Article Outline
Abbreviations: EPA, Eicosapentaenoic acid , US EPA, United States Environmental Protection Agency , PBCs, Polychlorinated biphenyls
Innis et al in this issue of The Journal present important information about methylmercury exposure among various immigrant and ethnic groups in Vancouver, British Columbia, including sources of exposure and blood concentrations in preschool-age children.1 The study makes it clear that one subgroup—the ethnic Chinese—is at elevated risk for exposure at levels above the United States Environmental Protection Agency (US EPA)’s reference dose and for adverse neurodevelopmental effects, which are already evident in one of the limited number of outcomes assessed in the study. Perhaps even more important than the specific findings from this well-designed and executed study, however, is that it demonstrates an important role that research on immigrant groups can play in assessment of the risks associated with environmental exposures that occur at lower levels in the general population.
See related article, p 759
The assessment of risks to child health from exposure to environmental contaminants is difficult. In most instances, exposures are at low doses and the effects on health and development are subtle, making it necessary to screen very large numbers of persons to assemble the at-risk cohorts needed to detect and evaluate adverse effects. Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that were used from the mid-1930s until 1975 as heat transfer chemicals and lubricants in electrical transformers and capacitors. In the early 1980s we undertook a study on the effects of prenatal PCB exposure in western Michigan families who had consumed relatively large quantities of PCB-contaminated Lake Michigan fish.2 In assembling the cohort at risk, it was necessary to screen >8000 families to locate 242 recently delivered fish-consuming mothers. In this cohort we detected an adverse effect of prenatal PCB exposure, assessed in umbilical cord blood and maternal blood and milk, on full-scale and verbal IQ at 11 years of age. The magnitude of the effect was small (β = −0.17, after adjustment for potential confounders), however, raising questions about its clinical significance or importance for the child’s day-to-day function.
Despite this apparent subtlety, we suspected that within this cohort there might well be subgroups or persons who were more severely affected by this exposure than was apparent from an examination of average performance for the cohort as a whole. When a study of prenatal PCB exposure in The Netherlands reported stronger effects among non-breast-fed than breast-fed children,3 we examined the potential of breast-feeding as an effect modifier and found that the magnitude of the effect on IQ among the non-breast-fed-children in our cohort was markedly higher (β = − 0.32).4 The greater vulnerability in the non-breast-fed children may be due to the higher quality intellectual stimulation that characterized the breast-feeding families in this cohort. Alternatively, nutrients in breast milk, such as the omega-3 fatty acids, particularly docosahexaenoic acid, have been shown to be associated with more optimal cognitive function and may have played a role in modulating the adverse effects of this exposure.
As noted by Innis et al,1 the blood methylmercury concentrations for preschool-age children reported in the 1999 to 2000 National Health and Nutrition Examination Survey (NHANES) US population survey were low; even the 95th percentile in that survey was well below the US EPA’s reference dose of 28.9 ηmol/L. In light of these data, it would difficult to identify a subgroup of children in the general North American population with sufficiently high exposures to study the neurodevelopmental sequelae of prenatal methylmercury exposure and their etiology. For this reason, the National Institute of Environmental Health Sciences has sponsored large-scale, longitudinal cohort studies in two populations at elevated risk for methylmercury exposure due to high rates of fish consumption—residents of the Faroe Islands in the North Sea5 and of the Seychelles Islands in the Indian Ocean.6 For reasons that have been difficult to determine,7 the Faroes study has confirmed findings from laboratory animal and previous cohort studies indicating adverse effects on cognitive function, but the Seychelles study has not.
This study by Innis et al1 demonstrates that North American immigrant populations may provide another valuable resource for examining methylmercury neurotoxicity. In this ethnically diverse Vancouver sample, the authors found that 12 children had blood methylmercury concentrations above the US EPA reference dose and that all 12 were from Chinese families. Although the authors’ extrapolations suggest that these exposures are generally lower than in the Faroes’ cohort, their extrapolations may be conservative because the Faroes cord blood data reflect prenatal exposure. The Vancouver Chinese prenatal exposures would have been higher than estimated by the authors if we assume that the mothers ate more methylmercury-contaminated fish during pregnancy than they fed to their preschool-age children. What is most important is that these children’s exposures are substantially higher than the 95th percentile in the NHANES study. Animal models are valuable in risk assessment for determining neurotoxicity using experimental designs that control most effectively for potential confounding factors and for examining neurostructural and neurochemical substrates that may mediate neurotoxicological effects. However, animal studies have only limited value for determining and characterizing the cognitive, attentional, and affective end points that may be altered in humans or the doses at which adverse effects become evident. To evaluate these issues, it is critical to find human cohorts exposed at sufficiently high doses for the effects to be observed and studied.
In this study, an adverse effect on preschool attentional focusing was observed in relation to methylmercury exposure that we can assume would not have been detectable in a randomly selected general population North American sample, unless the sample were exceptionally large. Given that methylmercury is maximally neurotoxic when exposure occurs in utero, it is impressive that this effect was seen even though only contemporaneous blood levels were available. This study also confirmed the tight association between methylmercury exposure and fish consumption, as indicated by blood eicosapentaenoic acid (EPA) concentration, even in the children exposed below the US EPA reference dose. The individual variability in methylmercury exposure among the children in the highest tercile for fish consumption is also impressive and provides confirmatory evidence that, where relatively noncontaminated fish, such as local Vancouver salmon, are eaten, methylmercury exposure is indeed low. The failure of fish consumption report based on the food frequency questionnaire to relate to blood methylmercury concentrations is disappointing and is presumably attributable to the difficulty of respondents to recall their children’s consumption accurately. The availability of the blood EPA concentration biomarker was clearly critical in clarifying the role of fish consumption in this exposure. The food frequency interview was still of some value, however, because it provided important information about the species of fish eaten and, in particular, that the Chinese tended to eat more imported fish.
The immediate public health implications of these findings are clear. Given the increased risk and relative isolation of immigrant communities, it is important to disseminate public health advisories in the immigrants’ language through channels that they are likely to access, such as ethnic language radio stations, churches, and community groups. In light of the demonstrated nutritional benefits of fish consumption, it is critically important to design public health messages to encourage consumption of locally caught Vancouver salmon and make clear that only certain species obtained from specific locations need to be avoided. Although it is sometimes argued that this type of relatively complex public health message would be difficult for a less-educated consumer to understand, failure to distinguish between high-risk versus beneficial fish would do a major disservice to the health of the targeted community.
Our knowledge about the long-term effects of low dose methylmercury exposure is limited, and the contradictory data emerging from the Faroes and Seychelles studies make it imperative to design and conduct additional studies to clarify the picture. In light of the data from laboratory animal studies suggesting a possible delay of several years or even decades before all the neural damage from prenatal methylmercury exposure is manifest,8 additional studies are needed to investigate long-term sequelae in humans directly. There are also suggestions from the research literature of dietary factors, including selenium and omega-3 fatty acids, that might influence methylmercury absorption and neurotoxicity, which warrant investigation in humans.
Thus, perhaps the greatest contribution of this study is to highlight the potential of targeting heavily exposed immigrant groups in studies designed to assess the risks associated with environmental exposures. A prospective longitudinal study of Vancouver Chinese immigrants could provide valuable information to address the outstanding issues relating to the long-term developmental neurotoxicity of prenatal methylmercury exposure noted above. Other immigrant groups with unusually high exposures have also been identified, including Hmong and Laotian residents in the upper Midwest, who consume large quantities of locally caught environmentally contaminated fish, and migrant farm workers exposed to high doses of pesticides. Although the logistics of recruiting participants from poorly educated, non-English speaking groups, who may also be transient, is challenging, the payoff from the effort invested is likely to be considerable in terms of the valuable information to be gained regarding risks from low-level environmental exposures.
References
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- Prenatal methylmerucry exposure from ocean fish consumption in the Seychelles child development study . Lancet . 2003;361:1686–1692
- . Contending with contradictory data in a risk assessment context (the case of methylmercury) . NeuroToxicology . 2001;22:667–675
- . Methods and rationale for derivation of a reference dose for methyl mercury by the US Environmental Protection Agency . Risk Anal . 2003;23:107–115
Supported by the National Institute of Environmental Health Sciences (R01-ES07902) and the Joseph Young, Sr., Fund from the State of Michigan.
PII: S0022-3476(06)00266-6
doi:10.1016/j.jpeds.2006.03.047
© 2006 Elsevier Inc. All rights reserved.
Refers to article:
- Increased levels of mercury associated with high fish intakes among children from Vancouver, Canada
