Blood Mercury Level

Mercury is a naturally occurring metal. However, through many industrial processes (e.g., chemical manufacturing operations, coal combustion), mercury is widespread and persistent in the environment. It is found in elemental form and in various organic compounds and complexes. Organic methylmercury—the form of mercury that is of primary public health significance—can accumulate in the food chain in aquatic systems and lead to high concentrations in predatory fish. The major source of human exposure to methylmercury in the U.S. is consumption of contaminated fish (mostly predatory and larger species) and shellfish (Caldwell et al, 2009; NRC, 2000).

The human health effects of mercury are diverse and depend on the forms of mercury encountered and the severity and length of exposure. Fetuses and children may be more susceptible to mercury than adults, with concern for the occurrence of developmental and neurological health effects (NRC, 2000). Prenatal exposures interfere with the growth and migration of neurons and have the potential to cause irreversible damage to the developing central nervous system. In adults, mercury exposure has been associated with neurological effects such as sensory and motor impairment, particularly at high doses (NRC, 2000). Possible associations with adverse cardiovascular effects continue to be researched.

Because of specific concerns about exposure to women of child-bearing age and young children, this indicator reports the total blood mercury levels (includes organic and inorganic) among U.S. women age 16 to 49 and children age 1 to 5, using data from the 1999-2012 continuous National Health and Nutrition Examination Survey (NHANES). NHANES began reporting blood mercury data for all eligible participants age 1 and over in 2003, and this indicator reports data for this population group from 2003 to 2012. NHANES is a series of surveys conducted by the Centers for Disease Control and Prevention's (CDC's) National Center for Health Statistics that is designed to collect data on the health and nutritional status of the civilian, non-institutionalized U.S. population using a complex, stratified, multistage, probability-cluster design. CDC's National Center for Environmental Health conducted the laboratory analyses for the biomonitoring samples. Beginning in 1999, NHANES became a continuous and annual national survey. The data presented here cover seven different survey periods: 1999-2000, 2001-2002, 2003-2004, 2005-2006, 2007-2008, 2009-2010, and 2011-2012.

Exhibit 1 presents the geometric mean and four percentiles (50^th, 75^th, 90^th, and 95^th) of blood mercury for women age 16 to 49, by race and ethnicity, sampled during each of the continuous NHANES survey periods. Among women age 16 to 49 years geometric mean blood mercury levels fluctuated throughout the survey period, ranging from 0.700 micrograms per liter (µg/L) (2011-2012) to 1.02 µg/L (1999-2000). Decreases from 1999-2000 to 2011-2012 occurred for all four percentiles, but were most pronounced at the 90^th (45.7 percent) and 95^th (47.4 percent) percentiles.

Among the three racial/ethnic groups reported in Exhibit 1, black non-Hispanic women age 16 to 49 consistently had the highest geometric mean blood mercury levels over time, followed by white non-Hispanics and then Mexican Americans.

Exhibits 2 and 3 display geometric mean and four percentiles for children age 1 to 5, by sex and race and ethnicity, respectively. The geometric mean remained largely the same during the first three survey periods: 0.343 µg/L in 1999-2000, 0.318 µg/L in 2001-2002, and 0.326 µg/L in 2003-2004. The geometric mean for females was consistently higher than for males during the first three survey periods, with a range of 0.329-0.377 µg/L for females compared to 0.302-0.317 µg/L for males (Exhibit 2). For each of the first three survey periods, non-Hispanic blacks had a higher geometric mean than Mexican Americans and non-Hispanic whites (Exhibit 3). The geometric means for total, sex, and race/ethnicity for Mexican Americans and non-Hispanic whites during 2005-2010, and for non-Hispanic blacks from 2005-2008, were not reported because the proportion of results below the limit of detection was too high to provide a valid result. Geometric means were reported for all groups for the 2011-2012 survey period; however, in all cases, the geometric means were lower than those during 1999-2004.

Exhibits 4, 5, and 6 present the geometric mean and four percentiles for the U.S. population by sex, race and ethnicity, and age, respectively. The geometric mean among the total population ranged from a high of 0.863 µg/L during the 2005-2006 and 2009-2010 periods to a low of 0.703 µg/L during 2011-2012 (Exhibits 4, 5, and 6).

When comparing blood total mercury concentrations by sex (Exhibit 4), the geometric mean was consistently lower for females than for males, with the exception of the 2005-2006 survey period when the geometric mean was the same for both sexes (0.864 µg/L). Overall, geometric means for females ranged from 0.694-0.864 µg/L and for men ranged from 0.712-0.883 µg/L.

Comparing mercury concentrations by race and ethnicity reveals that, in most years, geometric means were lowest for Mexican Americans, ranging from 0.483-0.613 µg/L. The highest geometric means varied between non-Hispanic blacks and non-Hispanic whites, with geometric means lower among non-Hispanic whites during 2003-2004 and 2007-2010, with a range of 0.688-0.891 µg/L. Geometric means were lower for non-Hispanic blacks during 2005-2006 and 2011-2012, with a range of 0.679-0.928 µg/L. For each group presented in Exhibits 4 and 5, the mean blood mercury concentrations were lowest during the most recent 2011-2012 survey period.

Exhibit 6 displays geometric mean and four percentiles for the U.S. population by age. Based on the survey periods when data were available for all age groups—2003-2004 and 2011-2012—geometric mean concentrations appear to increase with age. For example, in the 2003-2004 survey period, the geometric means were 0.326 µg/L for 1-5 years, 0.419 µg/L for 6-11 years, 0.490 µg/L for 12-19 years, and 0.979 µg/L for 20 years and older. In all cases, the lowest measured concentrations occurred during 2011-2012, with geometric means ranging from 0.262 µg/L for 1-5 years to 0.863 µg/L for 20 years and older.

• The relatively small number of samples collected in a two-year cycle (e.g., 1999-2000 or 2001-2002) may, in some cases, result in measures of central tendency that are unstable from one survey period to the next.

• Health-based benchmarks for blood levels of mercury have not been established.
   

Data used for Exhibits 1 through 3 of this indicator were generated with Stata statistical software utilizing the NHANES laboratory files available online in SAS^® transport file format (CDC, 2014a). Data used for Exhibits 4 through 6 were obtained directly from CDC's Fourth National Report on Human Exposure to Environmental Chemicals, Updated Tables, August 2014 (CDC, 2014b).