of methylmercury in blood in humans is estimated to be 50 days, and the whole-body half-life to be 70–80 days, although the residence time of mercury in the brain appears to be considerably longer (NRC, 2000). Hair is frequently used as an exposure biomarker for methylmercury. Hair is a route of methylmercury excretion, and approximately 80 to 90 percent of the total mercury found in hair is in the methylated form. Hair mercury is a good biomarker in fish-consuming populations. Autopsy studies suggest that maternal hair mercury level correlates reasonably well with the level of mercury in the fetal brain (Cernichiari et al., 1995).
The first nationally representative estimates of blood and hair mercury levels were provided by the National Health and Nutrition Examination Survey (NHANES) of 1999–2000. Among women 16–49 years old, the geometric mean hair mercury level was 0.2 parts per million (ppm), with 75th, 90th, and 95th percentiles of 0.42, 1.11, and 1.73 ppm, respectively (McDowell et al., 2004). The geometric mean blood mercury level was 1.02, with 75th, 90th, and 95th percentiles of 2.07, 4.84, and 7.13 ppm, respectively (Mahaffey et al., 2004). The prevalence of levels in excess of 5.8 µg/L (benchmark dose lower bound [BMDL] adjusted for uncertainty and for population variability) was 5.66 percent. Levels were 50 percent higher among older women (30–49 years) compared to younger women, and levels were highest among women who self-identified as “Other” racial/ethnic category (Asians, Native Americans, Pacific Islanders). Mercury burdens were strongly associated with the amount of self-reported fish consumption (Mahaffey et al., 2004). Among women reporting eating 5–8 fish meals per month, these figures were 2.56, 4.54, 8.80, and 11.60 ppm, respectively. Levels were seven times greater among women who reported eating nine or more fish meals in the previous 30 days, compared to women who reported no consumption. Among these relatively high fish-consumers, the 50th, 75th, 90th, and 95th percentiles for blood mercury were 3.02, 6.68, 12.00, and 13.40 ppm, respectively.
Data on blood and hair mercury levels in adult men in the United States were not collected as part of NHANES until 2003, and no data for this group has been reported. Therefore, estimates must be made based on mercury biomarker data reported as part of large cohort studies. Urine and blood mercury levels of 1127 Vietnam-era pilots were measured for a study of the health effects of exposure to dental amalgam (Kingman et al., 1998). The mean blood mercury level in this group of men was 3.1 ppm, with a range up to 44 ppm, but the contribution of fish consumption to blood mercury levels is unknown because data were not collected on fish intake.