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Background: The most prominent non-occupational source of exposure to methylmercury is the consumption of fish. In this study we examine a fish consuming population to determine the extent of temporal exposure and investigate the extent to which single time estimates of methylmercury exposure based on blood-Hg concentration can provide reliable estimates of longer-term average exposure., Methods: Blood-mercury levels were obtained from a portion of the Arsenic Mercury Intake Biometric Study (AMIBS) cohort. Specifically, 56 Japanese women residing in the Puget Sound area of Washington State, US were sampled on three occasions across a one-year period., Results: An average of 135 days separated samples, with mean blood-mercury levels for the visits being 5.1, 6.6 and 5.0 μg/l and geometric means being 2.7, 4.5 and 3.1 μg/l. The blood-mercury levels in this group exceed national averages with geometric means for two of the visits being between the 90th and 95th percentiles of nationally observed levels and the lowest geometric mean being between the 75th and 90th percentile. Group means were not significantly different across sampling periods suggesting that exposure of combined subjects remained relatively constant. Comparing intra-individual results over time did not reveal a strong correlation among visits (r = 0.19, 0.50, 0.63 between 1st and 2nd, 2nd and 3rd, and 1st and 3rd sample results, respectively). In comparing blood-mercury levels across two sampling interval combinations (1st and 2nd, 2nd and 3rd, and 1st and 3rd visits, respectively), 58% (n = 34), 53% (n = 31) and 29% (n = 17) of the individuals had at least a 100% difference in blood-Hg levels., Conclusions: Point estimates of blood-mercury, when compared with three sample averages, may not reflect temporal variability and individual exposures estimated on the basis of single blood samples should be treated with caution as indicators of long-term exposure. Reliance on single blood samples can make predicting ongoing methylmercury exposure highly speculative due to the large intra-individual variability.

Efforts to provide for public health protection from environmental contaminants in fish have resulted in various advisories or recommendations with regard to fish consumption from local, state, and federal agencies. These advisories are based on measured levels of contaminants in fish that are combined with values for body weight and portion size to produce an estimate of an "acceptable" consumption frequency (e.g., eat no more than once per month). Because values for body weight and portion size are generally generic default values, they do not necessarily relate to a specific population or to any individual in that population. Thus, the use of default values may result in underprotection or overprotection in any given case. Given the benefits of fish consumption and the risks from overexposure to various toxicants, vigilance is required by custodians of public health to ensure that populations are protected while being cautious not to over- or underprotect them. In this analysis, we examine the "acceptable" consumption limits derived for fish species/groups consumed by three specific populations and determine the extent of public health protection afforded by these limits. To accomplish this, the "acceptable" consumption frequencies are derived based, in part, on default assumptions and are compared to intakes calculated from empirically derived species-specific individual consumption and demographic data. Sensitivity analyses and population-specific probabilistic assessments of exposure are conducted to identify those values and/or assumptions which might significantly influence the resulting fish consumption advisories. Three populations were chosen for study based on their ability to represent populations of greatest concern: those most sensitive and/or those most exposed. We conclude from this investigation that consumption pattern data, contaminant data and body weight data together can be used to make fish consumption advisories more focused and, therefore, less likely to be under- or overprotective.