March 02, 2001 / 50(08);140-3
Mercury (Hg), a heavy metal, is widespread and
persistent in the environment. Exposure to hazardous Hg
levels can cause permanent neurologic and kidney
impairment (1--3). Elemental or inorganic Hg
released into the air or water becomes methylated in the
environment where it accumulates in animal tissues and
increases in concentration through the food chain. The
U.S. population primarily is exposed to methylmercury by
eating fish (see Mercury
Exposure). Methylmercury exposures to women of
childbearing age are of great concern because a fetus is
highly susceptible to adverse effects. This report
presents preliminary estimates of blood and hair Hg
levels from the 1999 National Health and Nutrition
Examination Survey (NHANES 1999) and compares them with
a recent toxicologic review by the National Research
Council (NRC). The findings suggest that Hg levels in
young children and women of childbearing age generally
are below those considered hazardous. These preliminary
estimates show that approximately 10% of women have Hg
levels within one tenth of potentially hazardous levels
indicating a narrow margin of safety for some women and
supporting efforts to reduce methylmercury exposure.
CDC's NHANES is a continuous survey of the health and
nutritional status of the U.S. civilian,
noninstitutionalized population with each year of data
constituting a representative population sample. A
household interview and a physical examination were
conducted for each survey participant. During the
physical examination, blood was collected by
venipuncture for all persons aged >1 year and
hair samples, consisting of approximately 100 strands,
were cut from the occipital position of the head of
children aged 1--5 years and women aged 16--49 years.
Whole blood specimens were analyzed for total Hg and
inorganic Hg for children aged 1--5 years and women aged
16--49 years by automated cold vapor atomic absorption
spectrophotometry in CDC's trace elements laboratory.
The detection limit was 0.2 parts per billion (ppb) for
total Hg and 0.4 ppb for inorganic Hg (4). Hairs
of 0.6 inches (1.5 cm) closest to the scalp
(approximately 1 month's growth) were analyzed for total
Hg concentration using cold vapor atomic fluorescence
spectroscopy (5). The limit of detection for
total Hg in hair varied by analytic batch; the maximum
limit of detection (0.1 parts per million [ppm]) was
used in these analyses. Blood Hg levels less than the
limit of detection were assigned a value equal to the
detection limit divided by the square root of two for
calculation of geometric mean values.
The geometric mean total blood Hg concentration for
all women aged 16--49 years and children aged 1--5 years
was 1.2 ppb and 0.3 ppb, respectively; the 90th
percentile of blood Hg for women and children was 6.2
ppb and 1.4 ppb, respectively (Table 1).
Almost all inorganic Hg levels were undetectable;
therefore, these measures indicate blood methylmercury
levels. The 90th percentile of hair Hg for women and
children was 1.4 ppm and 0.4 ppm, respectively.
Geometric mean values were not calculated for hair Hg
Reported by: Center for Food Safety and Applied
Nutrition, Food and Drug Administration. US
Environmental Protection Agency. National Energy
Technology Laboratory, Dept of Energy. National Marine
Fisheries Laboratory, National Oceanic and Atmospheric
Administration. National Center for Health Statistics;
National Center for Environmental Health, CDC.
The NHANES1999 blood and hair Hg data are the first
nationally representative human tissue measures of the
U.S. population's exposure to Hg. Previous estimates of
methylmercury exposure in the general population were
based on exposure models using fish tissue Hg
concentrations and dietary recall survey data (1).
The NRC review provided guidance to the Environmental
Protection Agency (EPA) for developing an exposure
reference dose for methylmercury (i.e., an estimated
daily exposure that probably is free of risk for adverse
effects over the course of a person's life) (3).
The NRC report recommended statistical modeling of
results from an epidemiologic study conducted in the
Faroe Islands near Iceland, where methylmercury
exposures are high because of the large amount of
seafood eaten by the local population. Results of this
study were used to calculate a benchmark dose (BMD), an
estimate of a methylmercury exposure in utero associated
with an increase in the prevalence of abnormal scores on
cognitive function tests in children. The lower 95%
confidence limit of the BMD (BMDL*) was recommended to
calculate the EPA reference dose. The NRC committee
recommended a BMDL of 58 ppb Hg in cord blood
(corresponding to 12 ppm Hg in maternal hair) (3).
In the NHANES 1999 sample, there were no measurements of
blood values >58 ppb or hair values >12
ppm. A margin-of-exposure analysis (i.e., an evaluation
of the ratio of BMDL to estimated population exposure
levels) showed ratios of <10 when comparing BMDL with
NHANES 1999 estimates of the 90th percentile for blood
and hair Hg levels in women of childbearing age.
Margin-of-exposure measures of this magnitude indicate a
narrow margin of safety (3) and suggest that
efforts aimed at decreasing human exposure to
methylmercury should continue.
The findings in this study are subject to at least
three limitations. First, the ratio of Hg in cord and
maternal blood is uncertain. The NRC committee
summarized some studies that suggest that cord blood
values may be 20%--30% higher than corresponding
maternal blood levels. However, other studies suggest
that the ratio is closer to 1:1 (3); therefore,
the NHANES values may not be directly comparable to BMDL
recommended by NRC. Second, NHANES cannot provide
estimates of Hg exposure in certain highly exposed
groups (e.g., subsistence fishermen and others who eat
large amounts of fish). Published data from studies of
highly exposed U.S. populations indicated that some
persons attain Hg tissue levels above BMDL (1).
Third, the sample size of NHANES 1999 was small and the
1999 survey was conducted in only 12 locations. More
data are needed to confirm these findings.
The long-term strategy for reducing exposure to Hg is
to lower concentrations of Hg in fish by limiting Hg
releases into the atmosphere from burning
mercury-containing fuel and waste and from other
industrial processes. On the basis of data from EPA's
National Toxics Inventory, air emissions of Hg decreased
approximately 21% during 1990--1996, largely because of
regulations for waste incineration (7). EPA
expects this trend to continue as regulations are
implemented for waste incineration and chlorine
production facilities and are developed for electric
power utilities (8,9). Fish is high in protein
and nutrients and low in saturated fatty acids and
cholesterol and should be considered an important part
of the diet. The short-term strategy to reduce Hg
exposure is to eat fish with low Hg levels and to avoid
or to moderate intake of fish with high Hg levels.
State-based fish advisories and bans identify fish
species contaminated by Hg and their locations and
provide safety advice (http://www.epa.gov/ost/fish†).
The Food and Drug Administration advises that pregnant
women and those who may become pregnant should not eat
shark, swordfish, king mackerel, and tile fish known to
contain elevated levels of methylmercury. Information is
available at http://www.fda.gov/bbs/topics/ANSWERS/2001/advisory.html
U.S. population estimates of Hg tissue levels by
race/ethnicity, region, and fish consumption will become
available after 2 additional years of NHANES data
collection. NHANES will provide the opportunity to
measure tissue Hg levels and to monitor the
effectiveness of continuing efforts to reduce
methylmercury exposure in the U.S. population.
- Environmental Protection Agency. Mercury study
report to Congress. Washington, DC: Office of Air
Quality Planning and Standards and Office of
Research and Development, Environmental Protection
Agency, December 1997.
- Agency for Toxic Substances and Disease
Registries. Toxicological profile for mercury
(update). Atlanta, Georgia: Agency for Toxic
Substances and Disease Registries, US Department of
Health and Human Services, March 1999.
- National Academy of Sciences. Toxicologic effects
of methylmercury. Washington, DC: National Research
- Chen HP, Paschal DC, Miller DT, Morrow J.
Determination of total and inorganic mercury in
whole blood by on-line digestion with flow
injection. Atomic Spectroscopy 1998;19:176--9.
- Pellizzari ED, Fernando R, Cramer GM, Meaburn GM,
Bangerter K. Analysis of mercury in hair of EPA
Region V population. J Expo Anal Environ Epidemiol
- Budtz-Jorgensen E, Grandjean P, Keiding N, White
RF, Weihe P. Benchmark dose calculations of
methylmercury-associated neurobehavioral deficits.
Toxicol Lett 2000;112--113:193--9.
- Environmental Protection Agency. National toxics
inventory. Washington, DC: Office of Air Quality
Planning and Standards, Environmental Protection
- Environmental Protection Agency and Environment
Canada. Mercury sources and regulations: draft
report, 1999 update. Binational toxics strategy.
Environmental Protection Agency and Environment
Canada, November 1999.
- Environmental Protection Agency. Regulatory
finding on the emissions of hazardous air pollutants
from electric utility steam generating units.
Federal Register 2000;65:79825--31.
*A BMD of 85 ppb Hg in cord blood or 17 ppm Hg in
maternal hair was estimated to result in an increase in
the proportion of abnormal scores on the Boston Naming
Test for children exposed in utero from an estimated
background prevalence of 5% to a prevalence of 10% (6).
BMDL recommended by NRC is the lower 95% confidence
bound of the BMD.
† References to sites of nonCDC
organizations on the World-Wide Web are provided as a
service to MMWR readers and do not constitute or
imply endorsement of these organizations or their
programs by CDC or the U.S. Department of Health and
Human Services. CDC is not responsible for the content
of pages found at these sites.