Hair mercury levels in U.S. children and women of childbearing age: reference range data from NHANES 1999-2000

Environmental Health Perspectives, August, 2004 by Margaret A. McDowell, Charles F. Dillon, John Osterloh, P. Michael Bolger, Edo Pellizzari, Reshan Fernando, Ruben Montes de Oca, Susan E. Schober, Thomas Sinks, Robert L. Jones, Kathryn R. Mahaffey

Exposure to methyl mercury, a risk factor for neurodevelopmental toxicity, was assessed in U.S. children 1-5 years of age (n = 838) and women 16-49 years of age (n = 1,726) using hair mercury analysis during the 1999-2000 National Health and Nutrition Examination Survey (NHANES). The data are nationally representative and are based on analysis of cross-sectional data for the non-institutionalized, U.S. household population. The survey consisted of interviews conducted in participants' homes and standardized health examinations conducted in mobile examination centers. Distributions of total hair mercury levels expressed as micrograms per gram hair Hg and the association of hair Hg levels with sociodemographic characteristics and fish consumption are reported. Geometric mean (standard error of the geometric mean) hair mercury was 0.12 [micro]g/g (0.01 [micro]g/g) in children, and 0.20 [micro]g/g (0.02 [micro]g/g) in women. Among frequent fish consumers, geometric mean hair mercury levels were 3-fold higher for women (0.38 vs. 0.11 [micro]g/g) and 2-fold higher for children (0.16 vs. 0.08 [micro]g/g) compared with nonconsumers. The NHANES 1999-2000 data provide population-based data on hair mercury concentrations for women and children in the United States. Hair mercury levels were associated with age and fish consumption frequency. Key words: blood, child, diet, female, hair, mercury, NHANES, preschool. Environ Health Perspect 112:1165-1171 (2004). doi:10.1289/ehp.7046 available via http://dx.doi.org/[Online 27 May 2004]

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Mercury is a naturally occurring heavy metal whose presence in the environment is widespread and persistent [Agency for Toxic Substances and Disease Registry (ATSDR) 1999; National Research Council (NRC) 2000]. Hg occurs in metallic or elemental, inorganic, and organic forms (ATSDR 1999). When elemental Hg is emitted as a combustion by-product of fossil fuels, it becomes methylated in the environment and accumulates in animal tissues, including fish. Methyl mercury (MeHg) in the aquatic food chain contributes to higher tissue Hg levels among fish consumers (Boening 2000). Total Hg in the hair of fish eaters correlates with Hg in the target tissue, the brain (Cernichiari et al. 1995).

The mammalian nervous system is highly vulnerable to MeHg (Castoldi et al. 2001). Exposure to high levels of MeHg during the last two trimesters of pregnancy produces documented neurodevelopmental problems, including language, attention, and memory problems [Marsh et al. 1987; NRC 2000; U.S. Environmental Protection Agency (EPA) 1997]. Accidental poisoning incidents in Japan (Harada 1995) and Iraq (Amin-Zaki et al. 1974) demonstrated the pronounced neurologic injuries that result from high-level MeHg exposures, particularly in children who were exposed in utero.

More recent prospective epidemiologic data from New Zealand, the Faroe Islands, and the Seychelles assessed developmental effects of lower level MeHg exposure in fish-consuming populations resulting from maternal and fetal exposures to MeHg (Cernichiari et al. 1995; Grandjean et al. 1999). The U.S. EPA MeHg exposure reference dose (RfD) of 0.1 [micro]g/kg body weight/day was based on data from the Faroes and New Zealand, with supporting analyses from all three major prospective cohort studies (Rice et al. 2003). The assessments of Hg exposure for the U.S. population have included regional biomonitoring studies conducted by state and federal agencies (Pellizzari et al. 1999) and assessments of population subgroups, including sport fishermen and their families (Burge and Evans 1994; Kosatsky et al. 2000), pregnant women in selected geographic areas (Bjornberg et al. 2003; Stern et al. 2001), high-end fish consumers (Hightower and Moore 2003), and American Indian and Alaskan Native groups (Rothschild and Duffy 2001).

Total blood and hair Hg are indicators of MeHg exposure in fish consumers and others who are not exposed to inorganic and elemental Hg occupationally or incidentally (Carrington and Bolger 2002; Mahaffey 2000; NRC 2000). Exposure to MeHg increases with fish consumption [International Programme on Chemical Safety (IPCS) 1990; Yamaguchi et al. 1975]. Once consumed, 90% of MeHg is absorbed in the human gut (Miettinen 1973). Approximately 95% of measurable Hg in blood is the methylated form (Sherlock et al. 1984). After absorption, MeHg is distributed throughout the body within hours (Clarkson 1997). Peak MeHg blood levels in human subjects fed fish containing known concentrations of MeHg occurred within 4-14 hr of ingestion (Kershaw et al. 1980). The mean [+ or -] SE half-life of blood MeHg reported from a study with 20 adults whose diet included halibut with measured MeHg was 50 [+ or -] 1 days (range, 42-70 days) (Sherlock et al. 1984).

Hair Hg concentration is the preferred biomarker for evaluating Hg exposure for extended periods of time such as periods of weeks or months (NRC 2000). Hair incorporates Hg present in circulating blood during hair formation in the hair follicle (Clarkson 1983). Hair growth and analysis studies assessed hair growth rates and the relationship between MeHg intake and hair levels (Clarkson 1992; Grandjean et al. 1994). Hair growth averages 1-1.5 cm per month and provides a time record of previous Hg exposure depending on the length of the hair (Suzuki et al. 1984). Approximately 80% of hair Hg is MeHg (Cernichiari et al. 1995; Phelps et al. 1980). Total Hg and MeHg levels in hair are linearly related (Pellizzari et al. 1999), with total Hg concentrations in hair thought to average 150- to 200-fold higher than Hg concentrations in blood (Gill et al. 2002).