The NHANES 1999–2000 total hair Hg data provide national hair Hg reference data for U.S. children 1–5 years and women 16–49 years of age, including three major race/ethnicity subgroups. These baseline data will be used to monitor hair Hg levels in the U.S. population over time. These results were compared with reports across studies that used different biologic matrices to estimate Hg exposure.
The total hair Hg levels of NHANES children and women were generally lower than the levels reported in other studies of U.S. and international populations. GM hair Hg level in fish-consuming children 7 years of age in the Faroes was 2.99 μg/g (Grandjean et al. 1999
) compared with a GM (SE) value of 0.16 (0.02) μg/g among frequent fish consumers in the NHANES sample 1–5 years of age. The mean ± SD and median maternal hair Hg levels of women in the Seychelles where frequent fish consumption occurs were 6.85 ± 4.5 μg/g and 5.94 μg/g, respectively (Cernichiari et al. 1995
). The median total hair Hg level of women in the Faroes birth cohort study was 4.5 μg/g; 12% had levels > 10 μg/g (Grandjean et al. 1992
). Recent hair Hg data for Japanese adult females residing in five districts showed an overall total hair Hg GM of 1.43 μg/g (range, 1.23–2.50 μg/g; Yasutake et al. 2003
). The GM (SE) and median values for frequent fish consumers among women of childbearing age in NHANES were 0.77 (0.09) μg/g and 0.33 μg/g, respectively. The arithmetic mean hair Hg level reported in a probability-based sample of U.S. Great Lakes region residents ≥ 21 years of age was 0.29 μg/g (Pellizzari et al. 1999
), compared with an arithmetic mean ± SE hair Hg value of 0.47 ± 0.06 μg/g for NHANES females 16–49 years of age.
We report that the GM total hair Hg among pregnant women was 0.21 μg/g and did not differ from hair Hg levels of nonpregnant women (GM = 0.20 μg/g). The mean ± SE hair Hg level reported in a study of 189 New Jersey pregnant women was 0.53 ± 0.07 μg/g (range, < 0.2–9.1 μg/g; Stern et al. 2001
); the NHANES pregnant females had an arithmetic mean ± SE hair Hg of 0.43 ± 0.089 μg/g. Median hair Hg levels for 127 pregnant Swedish women were 0.35 mg/kg (range, 0.07–1.5 mg/kg; Björnberg et al. 2003
). Prenatal assessments of women from the Seychelles women reported mean ± SD total hair Hg of 6.85 ± 4.5 ppm and a median value of 5.94 ppm (Myers et al. 2003
These hair Hg values provide an estimate of exposure over an approximate 1 month period, as recent exposure is not yet incorporated into the hair growth outside of the scalp. The steady-state hair-to-blood MeHg concentration ratio reported by the World Health Organization for adults was approximately 250:1, compared with a total hair-to-blood Hg ratio of 234 for NHANES females (IPCS 1990
). Values for NHANES children were higher (ratio value of 342 overall), and this may reflect the higher percentages of children with blood Hg levels below the LOD. The hair-to-blood ratios were highly variable, in part, because hair and blood measurements are not comparable regarding the time period of exposure. Additionally, Hg exposure in this study is low for most respondents, and these results may not be comparable with correlations and ratios predicted in groups with higher levels of Hg exposure.
The advantages of hair Hg assessment include the fact that hair collection is noninvasive, and good response rates can be achieved in population subgroups that are difficult to obtain blood specimens from, such as children. For example, during NHANES 1999–2000, blood Hg data were collected on 56% of selected children, whereas hair collection was completed on 67% of selected children. Additionally, hair is a time record marker of MeHg exposure in individuals and can be used to estimate Hg exposure over extended periods of time such as fetal exposure during gestation (Cernichiari et al. 1995
Several considerations for interpreting the NHANES 1999–2000 hair Hg results are provided. The NHANES 1999–2000 sample, although nationally representative, does not permit estimation of MeHg exposure in population groups with potentially high dietary exposure such as subsistence fishers, residents in specific geographic regions of the United States, sport fishers, and members of racial and ethnic population subgroups (e.g., Asians and Pacific Islanders). These subgroups may consume more fish than the general U.S. population and have higher MeHg exposure. Seafood consumption among Asian Americans and Pacific Islanders (AAPI) in King County, Washington, averaged 117 g/day (Sechena et al. 2003
), compared with mean intakes of 10–14 g/day for the total U.S. population ≥ 20 years of age (U.S. Department of Agriculture 1999
). Significant variation was observed in consumption rates and food preferences of the 10 AAPI groups. Second, three extreme hair Hg values were analyzed and confirmed in NHANES and were discarded in developing the distributions because it was not possible to determine the contributing factors that resulted in these values. This underscores the complexity of Hg assessment and exposure in populations. Finally, the NHANES 1999–2000 sample design was composed of a small number of primary sampling units (PSUs; 26 unique PSUs total). This feature limits regional or geographic area comparisons and statistical comparisons between population subgroups.
The NHANES data may be useful for assessing the prevalence of health risks in the U.S. population when the associated risks of low-level Hg content are better defined and may be used to support diet and health research, policy, and monitoring activities. Diet remains the primary contributor to MeHg exposure in populations. More than 50% of NHANES participants consumed fish during a 30-day reference period. Annual seafood consumption projections for the U.S. population indicate that 75–93% of adult women and 58–72% of children 2–5 years of age consume seafood (Carrington and Bolger 2002
Hair Hg analysis in national samples of U.S. children and women of childbearing age provide a useful biomarker for long-term Hg exposure. Acceptance of the hair collection procedure was high among survey participants and excellent method precision was achieved, allowing for of detection of hair Hg in approximately 84 and 89% of children and women, respectively. Total hair Hg is associated with age, race/ethnicity, and fish consumption frequency. Among women of childbearing age, total hair Hg levels of pregnant and non-pregnant women were the same.