Current Status of the Science of Mercury in the
Melvin E. Keener, Ph.D.|
February 10, 1999
Elemental mercury is a heavy, silvery liquid at ambient
temperatures. Mercury easily vaporizes under the normal range of
temperature experienced. The majority of the mercury in the atmosphere
is elemental. The majority of mercury in the land and water is either
in the form of an inorganic salt or an organic form of mercury (e.g.,
Mercury cycles in the environment as a result of natural and
human activities. Elemental mercury may remain in the atmosphere for up
to a year, resulting in wide dispersion from its initial source. This
makes mercury emissions an international issue, rather than a local
one. Inorganic forms of mercury are readily removed from the atmosphere
by precipitation or dry deposition. Mercury is commonly emitted back to
the atmosphere either as a vapor or associated with particles. The
complexity of the mercury cycle is not fully understood.
What is the Controversy over Mercury?
Mercury is a known human toxicant. Mercury poisonings have
occurred in Japan and Iraq following high dose exposures to
methylmercury. The controversy is based on how different public
agencies use the available data to estimate a safe exposure level to
mercury. A reference dose (RfD) is the amount of a substance that, if
ingested daily over a lifetime, will have no adverse effect on human
health. EPA has calculated a RfD for methylmercury of 0.1ug/kg body
weight/day. The World Health Organization (WHO) estimates that a RfD of
0.48 ug/kg bw/day is a more appropriate value. The U.S. Food and Drug
Administration (FDA) and the Agency for Toxic Substances and Disease
Registry (ASTDR) agree with the World Health Organization. This has set
up an interesting dilemma where different agencies within the U.S.
government are at odds over what the action levels for mercury should
Sources of Mercury
While mercury is a natural element that has been cycling
through the environment since the earth formed, there is little doubt
that increased industrial activity has increased potential human
exposure to mercury. However, there are very little historical data to
compare recent levels of mercury in the soil and water columns. The
short duration of recent data make it difficult to determine if mercury
levels in the environment are increasing, decreasing, or remaining
In a recent report to Congress (EPA, December 1997), EPA
estimated that the world-wide emissions of mercury for 1995 were
approximately 5000 Mg. These emissions come from natural sources
(release of geologically bound mercury through normal weathering),
anthropogenic sources (human activities), or re-emission (mass transfer
of mercury already deposited on the earth's surface). This report
estimated that anthropogenic sources account for between 50 and 75
percent of total yearly input to the atmosphere. The estimated U.S.
anthropogenic emissions for 1995 were 144 Mg. Thus, the anthropogenic
emissions from the United States contributed approximately 3 percent of
the total mercury emitted in the world. Based on a computer simulation
of long-range transport, EPA estimated that approximately one third of
U.S. emissions (47 Mg) were deposited within the lower 48 states and
the remainder was transported beyond U.S. borders. This simulation
estimated that an additional 32 Mg were deposited in the U.S. from the
global reservoirs. Thus, emissions from U.S. anthropogenic sources are
responsible for approximately 60 percent of the mercury deposited in
the lower 48 states.
Of the anthropogenic sources in the U.S., four specific
sources account for approximately 80 percent of the total: coal-fired
utility boilers (47 Mg or 33%); municipal waste combustion (27 Mg or
19%); commercial/industrial boilers (26 Mg or 18%); and medical waste
incineration (14 Mg or 10%). EPA estimates that hazardous waste
combustors added about 6 Mg to the atmosphere in 1995. This represents
about 4% of the U.S. anthropogenic sources and about 0.1% of the total
mercury deposited globally. It should be noted that two new Clean Air
Act rules for municipal and medical waste incinerators are estimated to
reduce mercury emissions from these two sources by 90 percent. In
addition, several states and localities have begun programs to remove
mercury containing materials from waste streams destined to be burned.
The combustion of coal in either a utility boiler or a
commercial/industrial boiler is the only identified major anthropogenic
source of mercury where mercury emissions are not regulated.
Fate and Transport of Mercury
In their report to Congress, EPA relied heavily on the use of
computer models to describe the environmental fate of mercury. The
reason stated for this was the lack of monitoring data that
conclusively demonstrates or refutes a relationship between an
individual anthropogenic source and increased mercury concentrations in
the soil or biota near that facility. Since the models have not been
validated, all results from these analyses should be treated as
qualitative rather than quantitative. Some of the important limitations
found during their analysis includes a lack of fundamental
understanding of how mercury moves from watershed soils to a local
water body and inability to explain why the relationship between the
concentration of mercury in a water body and the mercury concentration
in the fish in that water body varies from water body to water body.
Human Health Effects
Mercury primarily effects the nervous system. Symptoms in
adults include impairment of the peripheral vision, disturbance of
sensations (numbness or "pins and needles") usually in the hands and
feet, lack of coordination of movements such as writing, impairment of
speech, impairment of walking, impairment of hearing, and mental
disturbances. Developing fetus and infants, when exposed to high levels
of methylmercury, develop symptoms similar to congenital cerebral
palsy. These include the inability to walk two steps without support by
the age of two, the inability to respond to verbal communications by
age two (with good hearing), lower neurological test scores, and
presence of seizures.
These symptoms were first recognized in 1956 in Minamata
City, Japan. A chemical factory released methylmercury into Minamata
Bay. Once released, the methylmercury accumulated in the fish (mercury
concentrations as high as 30 ppm). A second methylmercury poisoning
outbreak occurred in 1965 in Niigata, Japan, when a chemical factory
released mercury into the Agano River. These two events were long-term
exposures to mercury via the consumption of contaminated fish. There
have been two outbreaks of mercury poisoning from the consumption of
mercury contaminated grain in Iraq. The first occurred prior to 1960
and the second in the early 1970s. In the second event, mercury-treated
seed grain arrived after the planting season (at that time, seeds were
routinely treated with mercury-based chemicals to maintain seed
germination). Instead of being planted or waiting for the next growing
season, the treated grain was ground for flour and baked into bread.
Unlike the exposures in Japan, the mercury poisoning in Iraq was a
short-term, higher exposure event. However, the human health symptoms
were similar for both types of exposure.
Necessarily, the studies on these mercury exposure events
were retrospective. In retrospective studies, it can be difficult to
determine the exact exposure to mercury at critical times. These
exposures must estimated from interviews on consumption and
back-calculations for current body levels of mercury. Two prospective
studies are currently being conducted on the fish-eating populations of
the Seychelles Islands and the Faeroe Islands. These studies are
monitoring the exposure of a large number of infants to lower levels of
mercury and how this exposure effects neurological development. A
number of papers have been published in the last three years that
chronicle the development of these children. The most recent paper
(Davidson, et. al.,1998) reported no significant developmental effects
in children through the age of five and a half years old with mercury
exposure that produced 10 ppm mercury in hair samples.
Human Exposure Pathways
All forms of mercury can cause adverse health effects in
humans at sufficiently high doses. Elemental mercury is rapidly
absorbed through the lungs but is poorly absorbed in the
gastrointestinal tract. Inorganic mercury is also poorly absorbed by
the gastrointestinal tract. Because the gastrointestinal tract rapidly
absorbs methylmercury, the primary pathway for human exposure is eating
food that contains methylmercury.
The methylation of mercury is a key step in the entrance of
mercury into the food chain. This transformation can occur in the
sediment or the water column of a water body. All mercury entering an
aquatic ecosystem is not methylated. Demethylation reactions as well as
the volatilization of dimethylmercury reduced that amount of
methylmercury in an ecosystem. There is considerable variability
between bodies of water on rates of methylization, demethylization, and
conversion to dimethylmercury.
Since nearly 100% of the mercury that accumulates in the
aquatic food chain is methylated, consumption of fish is the considered
to be the primary pathway for mercury exposure to humans.
Development of Safe Exposure Levels
The current EPA RfD for methylmercury is based on neurological
changes of 81 Iraqi children who were exposed when their mothers ate
methylmercury contaminated bread during pregnancy. The data were
collected by interviewing the mothers and by clinical examination of
the children by pediatric neurologists approximately 30 months after
the poisoning. Based on this data and an uncertainty factor of 10, EPA
calculated an RfD for methylmercury to be 0.1 ug methylmercury/kg body
weight/day. This corresponds to a mercury hair content of 11 ppm and a
mercury blood level of 44ug/L.
In 1990, the World Health Organization, using the same data
base, concluded that a daily uptake of 0.48 ug/kg bw/day would not
cause any adverse effect to adults. In addition, they concluded that
methylmercury uptakes of 3 to 7 ug/kg bw/day would result in less than
a 5% increase in neurological effects in adults. These uptake rates
correspond to mercury hair levels of 50 to 125 ppm.
The U.S. Food and Drug Administration (FDA) takes a somewhat
different approach but comes to a similar conclusion as the World
Health Organization. FDA's action levels are based on a tolerable daily
intake (TDI) which combines the concentration of methylmercury in food
(primarily fish) and information on consumption. FDA's action level of
0.2 mg per person per week translates to an RfD of 0.47 ug/kg bw/day.
This level was based on data from the Japanese poisoning episodes. FDA
has chosen not to include the data from the Iraq study but to wait
until the data from the Seychelles Islands and the Faeroe Islands are
available before revising their action levels.
Control Mechanisms. There is no single control
mechanism that can eliminate anthropogenic mercury emissions. Since
mercury is highly volatile, any elimination of mercury before
combustion will reduce the amount emitted to the atmosphere. The
municipal and medical incineration industries are attempting to remove
as much mercury in the feed as possible to meet the new MACT standards.
EPA is currently evaluating techniques to remove mercury from coal in
an effort to reduce the amount emitted from the largest sources.
End-of-the-pipe controls for mercury are not very effective and simply
transfer the contamination from the air to another medium (solid
New Clean Air Act rules will result in significant reduction
in emissions from municipal and medical waste incinerators. Additional
reductions can be expected when the Clean Air Act rules for hazardous
waste combustors are finalized. However, since these units as such a
small source, the amount of reduction in mercury emissions will be
small. There are no currently proposed regulations to reduce mercury
emissions from coal-fired utility boilers or coal-fired
However, even if the U.S. were capable of eliminating mercury
emissions, it may not significantly impact the amounts of methyl
mercury in fish, the primary source of human exposure (EPA Mercury
Study Report to Congress, 1997) because the U.S. emissions represent
less than 3 % of the global emissions of mercury. Fitzgerald and Mason
(1996) estimated that if all anthropogenic emissions of mercury were
ceased, it would take about 15 years for the ocean and soil reservoirs
to return to pre-industrial levels. When the Science Advisory Board
panel reviewed the EPA's Mercury Study Report to Congress, they
concluded that the process would take much longer.
Revisions of RfD. When the Science Advisory Board
panel reviewed EPA Mercury Study Report to Congress, they advised that
the current RfD not be revised with the new data from the Seychelles
Islands and the Faeroe Island. This was based on the concept that time
was needed to further analyze the data to determine its exact meaning.
The panels suggested that EPA add the new data and re-estimate the RfD
within the next two years.
In December 1998, the ASTDR indicated that they would revise
their recommendation for daily exposure to 0.4 ug/kg bw/day, based on
the most recent data from the Seychelles study. In January 1999, ASTDR
decided to postpone that decision for a couple of months until EPA
could comment on their methods of determining an RfD.
While on the surface, it appears that the Iraqi and
Seychelles data sets give totally different answers, it is probable
that when all are examined together, some consistency will be found.
The development of past action levels was based on high exposure events
and estimating levels where no adverse effects would be seen. The
populations of the newest studies were selected based on identifying
subtle effects at the levels of mercury exposure that were thought to
be problematic. The preliminary results from the new studies indicate
that the estimations of no adverse effect levels by EPA were too
conservative. The new studies also more closely mimic normal exposure
pathways rather than short-term, high exposure events.
Rectification of U.S. Agencies Positions. The
rectification of the differences between the agencies within the U.S.
government will be difficult primarily because there is so much at
stake. Should everyone agree that FDA's and ASTDR's RfD is correct,
this would eliminate most of the fish advisories in the United States
with a stroke of a pen. It would also reduce the need to regulate
mercury emissions from coal fired boilers, the largest source of
mercury emissions in the U.S. EPA staff believes that mercury emissions
from these units need to be regulated to protect human health and the
environment. In all likelihood, resolution of this difference will take
place at the highest levels within the U.S. Government.
Davidson, P.W., G. J. Myers, C. Cox, C. Axtell, C. Shamlaye,
J. Sloane-Reeves, E. Cernichiari, L. Needham, A. Choi, Y. Wang, M.
Berlin, and T. W. Clarkson. 1998. Effects of Prenatal and Postnatal
Methylmercury Exposure from Fish Consumption on Neurodevelopment:
Outcomes at 66 Months of Age in the Seychelles Child Development Study.
Journal of the American Medical Association, 280:701-707.
EPA. Mercury Study Report to Congress. December 1997.
Fitzgerald, W. F., and R. P. Mason. 1996. The Global Mercury
Cycle: Oceanic and Anthropogenic Aspects. Pp. 185-208. In Baeyens, W.,
R. Ebinghaus, and O. Vasiliev, eds. Global and Regional Mercury
Cycles: Sources, Fluxes, and Mass Balances.
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