Excerpts from an article entitled
MERCURY IN MASSACHUSETTS: AN
EVALUATION OF SOURCES, EMISSIONS, IMPACTS AND CONTROLS
by C. Mark Smith
Massachusettes Department of Environmental Protection (MA DEP)
Office of Research and Standards
E-mail: C.Mark.Smith@state.ma.us
Last Updated August 8, 1996
Chapter 1 - Introduction
Why is Mercury a Problem?
Mercury is a metal that is commonly found in the environment in several forms, all of
which are toxic. Depending on its exact chemical form and the dose received, people or
wildlife exposed to mercury can suffer serious adverse health effects. Mercury in the
environment is derived from both natural sources and human activities. Mercury is mobile
and widely dispersed in the biosphere and persists once released. "Organic"
(carbon-containing) mercury compounds, such as methyl mercury, are of particular concern
because they can become concentrated in living organisms, such as fish.
Mercury is an important environmental concern in Massachusetts and across the country.
Extensive fish monitoring programs in Massachusetts and other states have led to some
disturbing findings regarding mercury. For many waterbodies in the Northeast,
concentrations of methyl mercury in large freshwater fish were found to be above levels
currently considered to be safe for regular consumption. These findings have led several
states, including Massachusetts, to issue statewide health advisories warning pregnant
women to avoid eating native freshwater fish. Pregnant women are of special concern
because methyl mercury can cross the placenta and is particularly toxic to developing
fetuses. Warnings that citizens should refrain from eating fish from many specific
waterbodies have also been issued across the Northeast with 37 such advisories in MA
alone.
Such advisories minimize potential health risks from mercury but also indicate a need
to further reduce sources of mercury pollution. Ultimately, the only way to achieve this
is to identify controllable sources of mercury and to then take steps to reduce them.
MADEP is committed to this goal which can be achieved in many ways, including actions that
every citizen can take.
Both State and Federal regulatory agencies have taken many recent steps to better
delineate and reduce mercury risks. Several reports by the United States Environmental
Protection Agency (USEPA) have assessed possible sources of mercury in the environment on
a national scale. The potential adverse health effects of mercury have also been
extensively considered in recent scientific publications.
These efforts, including analyses presented herein, have helped to clarify our
understanding of mercury risks and sources and have pointed to many steps that can be
taken to reduce mercury releases. For example, as is discussed in Chapter 3 of this
report, disposal of mercury containing products such as batteries, certain types of
electric light fixtures, thermostats and thermometers in municipal solid waste can lead to
substantial emissions of mercury to the environment.
Chapter 2 - Mercury: Forms, Fate & Effects
This chapter provides an overview of the forms, fates, and effects of mercury in the
environment. It first discusses the forms of mercury, and their chemical and physical
properties. Next it describes the cycling of mercury, its transformations, and the sources
of mercury on a national and worldwide level. Next, the methods available and commonly
used to measure mercury are described, along with the issues that must be considered in
assessing monitoring data. Finally, the effects of mercury on human and ecological health
are discussed.
Mercury Forms and Fates
Background
Mercury is a naturally-occurring metal, traces of which occur in rocks of the earth's
crust. Mercury has three possible "valence states", or conditions of electrical
charge. The uncharged metallic or elemental mercury (Hg0), the form commonly used in
thermometers, readily vaporizes from its liquid state, and is the most common form of
mercury in the atmosphere. Long-range transport of mercury through the atmosphere consists
primarily of mercury in the elemental form (Mitra, 1986). Limited amounts of elemental
mercury may be found in soils and water. In soils and surface waters, mercury
predominantly exists in the mercuric (Hg++- with a double positive electrical charge), and
mercurous (Hg+- with a single positive charge) states, as ions with varying solubility.
Mercuric chloride, a simple salt, is the predominant form in many surface waters
(Mitra,
1986).
Mercury can form many stable complexes with organic (carbon-containing) compounds.
Methyl mercury is a toxic, organic mercury compound that is fairly soluble in water.
Dimethyl mercury, another organic mercury compound, is much less soluble. Inorganic
mercury can be methylated by microorganisms indigenous to soils, sediments, fresh water,
and salt water, to form organic mercury. Almost all of the mercury found in animal tissues
is in the form of methyl mercury (WHO, 1989).
MERCURY
CHEMICAL SYMBOL: Hg
(based on its ancient name: Hydragyrum
Elemental Mercury
Hg 0
Inorganic Mercury
Hg +1 or Hg +2
Organic Mercury
compounds such as:
Methyl mercury - HgCH3+
Dimethyl mercury - Hg(CH3)2
Thimerosal
Mercury undergoes two predominant types of chemical transformations: 1)
oxidation-reduction, and 2) methylation-demethylation (see Figure 2-1). In oxidation, for
example, mercury present in its uncharged form (Hg0) is converted to a higher
valence state (e.g. Hg+1 ). Reduction is the reverse of this process occurring
through the addition of electrons. In methylation, elemental mercury adds an organic
"methyl group" or hydrocarbon (CH3) group, which is lost in
demethylation. Both transformations can occur in either direction.
Probably the best known properties of elemental mercury are its low viscosity and its
ability to form highly mobile droplets on surfaces. Low viscosity accounts for the way
mercury droplets amalgamate into one when they collide. The high mobility of mercury may
be the origin of its nickname, "quicksilver". Early Greek civilization
recognized this metal's properties of quickness and embodied them in the messenger god
Mercury, whom they elevated to the Pantheon. The planet Mercury, with its quick 88-day
year and silver-white luster, epitomizes the reverence universally held for this element
by ancient civilizations.
Mercury has a high surface tension, forming spherical droplets when the liquid is
released. Though the mercury molecules within the droplets are stable, the molecules on
the surface of the droplet are highly unstable, and readily vaporize. The boiling point of
mercury is approximately 357°C (675°F). Essentially, all elemental mercury will exist as
a vapor at temperatures above this level. Its high surface tension, uniform volume of
expansion make mercury ideal for use in thermometers, barometers and other measuring
devices.
PHYSICAL STATES OF MERCURY
(atomic number: 80, atomic weight: 200.6)
Elemental mercury is a silver-white, heavy, mobile, liquid metal at ambient
temperatures.
Other forms of mercury such as mercuric acetate and mercuric chloride are white, heavy
powders or crystal solids. (US Dept. HHS, 1989)
Mercury is a relatively poor metallic conductor of electricity, yet it is often used in
electronic devices such as switches and thermostats, when a liquid conductor is required
and because its weight forms a positive seal. The ability of liquid mercury to conduct
heat is responsible for the use of mercury as a coolant. The strongly toxic compounds of
mercury have been exploited for bactericides, fungicides and insecticides, and its
brilliant hues have lead to mercury use in paints (Mitra, 1986; ATSDR, 1994). It is also
an excellent preservative and disinfectant, accounting for its presence in many chemical
reagents and medical applications in forms such as mercurochrome and Thimerosal.
Elemental Mercury is Insoluble
Thus, raindrops, running water and moisture are not good sinks for mercury vapor.
Mercury is Affected by Temperature
Mercury vaporizes more easily as temperature rises; at high temperatures essentially all
mercury will exist as a vapor.
Environmental Fate of Mercury: Cycling & Transformation in the
Environment
All substances undergo cycling and transformations in the environment, but this is
particularly so for mercury. Mercury's ability to exist in several physical states and
chemical forms at commonly-encountered conditions of temperature and pressure, and
propensity to undergo biological transformations, means that it is subject to complex and
difficult-to-predict changes in concentration and form. Environmental monitoring studies
thus must consider a variety of physical changes, geochemical reactions, and biochemical
interactions in an attempt to understand the specific local conditions that contribute to
mercury levels found in different environmental media and living things.
Mercury released into the environment can either stay close to its source for long
periods, or be widely dispersed on a regional or even world-wide basis. Mercury
concentrations in seawater, air and in human hair are higher in the northern hemisphere
than the southern hemisphere (Mitra, 1986). The greater industrialization in the north is
probably responsible for the higher levels; the stratospheric air circulation system leads
to the re-deposition of pollutants from the mid-latitude industrial northern hemisphere in
the same hemisphere.
Although the precise compositions will vary based on the locations sampled, in general,
greater than 50% of the total amount of mercury in air exists in the elemental form, with
a few percent attributable to particulate ore and the remaining percentage being comprised
of a variety of other mercury compounds (Johnson and Braman, 1974). Atmospheric mercury
concentrations have been measured in industrial, rural, residential, and aquatic areas.
Levels are higher over industrial areas. Estimates of the residence times of various forms
of mercury in the atmosphere vary from about five to ninety days (Airey, 1982;
Mitra,
1986) to as long as three years (WHO, 1990). Atmospheric mercury concentrations over
Greenland, polar regions and the open ocean exhibit little variation, indicating that
anthropogenic, or human-caused, sources contribute to the higher levels found over the
continental landmass areas (Mitra, 1986).
The vaporization rate of mercury approximately doubles for every 10 degrees Centigrade
increase in temperature. The saturation level of mercury in air increases logarithmically
with increasing temperature. Thus, seasonal, daily and latitudinal changes in ambient air
levels occur (Mitra, 1986).
Evidence consisting of before-and-after measurements suggests that rain washes some
mercury out of the atmosphere (Fogg and Fitzgerald, 1979). However, in industrial zones
that use mercury or where mercury is a by-product of manufacturing, more mercury may be
precipitated by dry fall-out than by rainfall (Dams et. al., 1970). Rain is more effective
at removing particulate mercury than mercury vapor, because raindrops, running water and
moisture are not good sinks, or storage media, for elemental mercury.
The presence of mercury in snow fields in Greenland indicates that snow also removes
mercury from the atmosphere (Weiss et. al, 1971). Mercury enters soils by way of rain and
snowfall, dry fallout from the atmosphere, the disposal of sewage sludge, improper
disposal of mercuric hazardous wastes (formerly much more common than at present),
landfilling of solid waste, and the agricultural use of mercury-containing pesticides.
Ionized forms of mercury are strongly adsorbed (held by surface particles) by soils and
sediments and is desorbed (released) slowly (Mitra, 1986). Clay minerals adsorb mercury
maximally at pH 6. Iron oxides also adsorb mercury in neutral soils. In acid soils, most
mercury is adsorbed by organic matter. Microbial activity may then metabolize some part of
the mercury, releasing it into the soil gas. When organic matter is not present, mercury
becomes relatively more mobile in acid soils, and evaporation to the atmosphere or
leaching of mercury to groundwater occurs (Mitra, 1986).
Once released to the atmosphere, mercury is distributed to the earth's surface
including soils, wetlands, lakes, and oceans. It can then undergo chemical transformations
including oxidation, reduction, methylation, and demethylation (see Figure 2-1).
Biological processes play an important part in these transformations; depending on local
conditions, bacteria may ultimately convert some of the deposited mercury to methyl
mercury, which is taken up by organisms through ingestion and absorption (Press &
Siever, 1978).
METHYLATION OF MERCURY
Methylation, the addition of (-CH3) may occur in water, sediments or soil.
Fish accumulate methyl mercury directly from the water in which they live or from prey.
In water and sediments the amount of methylation is affected by:
1.the amount of dissolved oxygen present;
2.the amount of sulfur present;
3.the pH of the water or sediment; and,
4.the presence of particles of clay or organic material.
Where the amount of oxygen is limited, as in deeper layers of the surface water or
sediments, more methyl mercury is formed. The presence of sulfur may be important because
it is thought that sulfate-dependent bacteria are involved in the methylation process. Low
pH is associated with an increase in methylation. (This means that methylation may occur
more readily in water affected by acid rain.) If clay particles are present in the water,
the mercury may attach to the particles, and may not be as available for
methylation.
Methyl mercury may also be formed in soil. As in lakes, rivers, or sediments, the
oxygen and sulfur levels and the pH may affect the amount of methylation that occurs.
Methyl mercury formed in the soil may be transported to surface water as runoff and
ultimately enter lakes, ponds or the ocean.
The concentrations of different forms of mercury found in soil, water, or air, or in
living things, is the result of the amount of releases, how they have been transported,
and how the mercury is transformed. Figure 2-2 displays the overall process of cycling of
mercury through the environment.
Figure 2-2. Cycling and Transformation of Mercury in the Environment
Erosion, rainfall and leaching transport mercury from land surfaces to streams, lakes
and oceans. Streams that cut through mercury deposits contribute elevated amounts of
mercury to the stream environment. Thermal springs and mine drainage also contain
significant amounts.
While it circulates in the environment and changes its form, mercury is persistent and
is not biodegradable. It tends to accumulate in sediments - in rivers, streams, lakes and
the ocean. Mercury can even accumulate in sewer pipes which can lead to long-term releases
of mercury to municipal wastewater that may continue even after the original source has
been eliminated. Mercury can thus be hard to control, once released. Furthermore, once
present in a biological system, mercury can be passed up the food chain,
"bioaccumulating" (increasing its concentrations) accordingly. Larger, older
individuals build it up in their tissues with increasing age and thus the total
concentration of mercury in a higher predator can be substantial. Because of mercury's
combined qualities of potential toxicity, environmental persistence, and potential for
bioaccumulation, this metal is a particularly insidious and difficult pollutant to manage.
Sources of Mercury
There are many sources of mercury inputs to the biosphere. Natural sources are
significant contributors, clearly greater than man-made inputs in some areas, especially
those where high concentrations of mercury exist in surficial ores. The contribution of
mercury to the biosphere associated with human activities is a matter of great debate. In
part, this is because it is difficult to separate mercury that was originally derived from
past human releases from new natural inputs. In any case, many scientists believe that the
flux of human-derived mercury into the atmosphere is at least on par with, and probably
exceeds, by up to two- to four-fold, natural sources of this metal (Terry Haines,
University of Maine; USEPA, 1991; Hovart, 1993; USEPA, 1995). Reports that the typical
mercury content of lakes has increased by two- to seven times since industrialization
(Nriagu, 1979; Swedish, EPA, 1991), and that the deposition of mercury has increased
significantly in the mid-continental United States (Swain et. al. 1992) support this
contention.
Natural Sources of Mercury
Mercury is one of the natural elements that make up our solar system. It is present in
the sun, solar winds and solar flares and has been detected in meteorites and moon rocks
(Mitra, 1986). On the earth, naturally occurring mercury deposits are generally found as
Cinnabar (HgS) and this is the most important mercury ore. The mercury content of cinnabar
exceeds is 86% This vermilion-red sulfide mineral ore occurs in quantity at relatively few
locations (see text box below) (Mitra, 1986). Its associations with recent volcanic rocks
and hot springs suggests a deep crustal or mantle source.
CINNABAR (Mercury Ore) DEPOSITS
Important mining localities include:
Almaden, Spain; Idria, Yugoslavia; Huancavelica, southern Peru
In the United States, large deposits occur in:
New Almaden, California New Idria, California.
Minable quantities of cinnabar are found in:
Nevada; Utah; Oregon; Arkansas; Idaho; Texas
No deposits of cinnabar have been identified in Massachusetts. (Hurlbut & Klein,
1977). However certain shales and granite that are found in MA have higher than average
levels of mercury.
Inorganic mercury occurs in small amounts in many rocks. Granite contains about 0.2
parts per million (ppm) mercury (Press and Siever, 1978). Other crustal rocks generally
contain less than half that amount. The mercury in rocks steadily contributes small
amounts of this metal to the atmosphere and natural waters by ordinary weathering
processes. Volcanic sources also disperse mercury vapor into the atmosphere. Atmospheric
mercury levels measured at Kilauea and Mauna Loa volcanoes in Hawaii commonly show the
same order of magnitude as Icelandic volcanoes, between 10 and 25 micrograms per cubic
meter (µg/m3). Normal values in air (Mitra, 1986) are about 3 nanograms per
cubic meter (ng/m3).
Soils and sediments may also contain mercury. The mercury content of sedimentary rocks
such as shale, which were deposited long before humans existed, signifies that at least
some of the mercury in modern sediments is natural in origin. More recent sediments will
also contain mercury derived from manmade sources.
Mercury leaches into surface and groundwaters from natural sources, and it is
distributed into the oceans through the mid-oceanic ridges and rift systems. Most natural
waters contain a few parts per billion (ppb) mercury. Freshwater concentrations have been
reported as high as 0.07 ppm (Hem, 1970). Some fraction of the mercury in natural waters
may be converted to an organic form, methyl mercury which is the form most harmful to
higher organisms (WHO, 1989).
MEASUREMENT OF CONCENTRATIONS IN AIR
Concentrations of chemicals in air are measured in units of:
the mass of chemical (milligrams, micrograms, nanograms, or picograms) per volume of
air (cubic meters).
1 milligram (mg) = 1/1,000 gram
1 microgram (µg) = 1/1,000,000 gram
1 nanogram (ng) = 1/1,000,000,000 gram
1 picogram (pg) = 1/1,000,000,000,000 gram
One cubic meter (m3) = 35.31 cubic feet.
Mercury Sources Associated with Human Activities
The unique properties of mercury have resulted in a long history of use by the
enterprising human race. The mercury ore cinnabar has been found smeared on Neolithic
skulls. In about 2000 BC, mercury pigment was used on a tomb which was discovered on an
island in the Mediterranean (Mitra, 1986). Today, its presence in batteries and
thermometers establishes a place for mercury in every household.
Many thousands of tons of mercury have been mined during the past 50 years for use in
electrical equipment, chemical processing plants, chlor-alkali plants, and pesticides.
Mining essentially results in an accelerated weathering process, by which much more
mercury than normal is released from rocks. Much of the mercury used in manufacturing
subsequently escapes into natural waters and the atmosphere.
Mercury is used in a number of consumer and commercial products. Some of these products
are more commonly recognized as containing mercury than others. Mercury is found in
varying amounts in batteries, fluorescent and high intensity light bulbs, thermometers,
thermostats, and light switches. Mercury is also used to make chlorine and caustic soda
and certain types of dental fillings. Some paints and pesticides made in the United States
used to contain mercury (as a preservative and fungicide) but no longer do as a result of
voluntary and required bans. Thus, citizens, hospitals, dental offices, farmers, builders,
and certain types of manufacturing operations all use and eventually discard products
containing mercury into the municipal solid waste stream. Following disposal the mercury
in these items may ultimately be released into a landfill or the atmosphere following
combustion in a waste combustor. More detailed discussions of these various sources and
quantitative estimates of their total contribution of mercury to MSW can be found in
Chapter 3 and Appendix F.
In addition to mercury emissions associated with disposal and incineration of municipal
wastes, mercury is also released into the atmosphere by the burning of fossil fuels such
as coal and oil, medical wastes, and wood. Releases also occur:
1.when products containing mercury, such as fluorescent lights, are broken;
2.from volatilization during laboratory and industrial uses;
3.during cremation of human bodies, due to mercury use in amalgam fillings;
and,
4.in the purification, or roasting, of ores.
In addition to industrial activities, worldwide agriculture and mining have also
contributed major amounts of mercury to soils, water and air.
Measurement of Mercury Levels in the Environment
To assess how much mercury is present in the air, water, and other environmental media
samples are taken and analyzed for this metal using a variety of scientific methods. Some
of these are described in more detail in Appendix C. Sampling for mercury is not always a
simple matter, and it is important to understand some of the key sampling issues to
appropriately interpret the available data.
One important issue is ensuring that samples are "clean" - that what is being
measured is what is present in the environment, and not the result of sample contamination
(i.e. traces of mercury in the sampling or analytical containers). Improvements in
trace-metal-free, "clean hands" methods in sampling, handling and processing
materials for mercury analysis are thought to be responsible for some of the apparent
decreases in environmental concentrations reported in recent publications. Formerly,
sample contamination problems interfered with the accurate measurement of the low levels
of mercury generally found in environmental media. For example, the measured mercury
concentration in Vandercook Lake, Wisconsin, decreased from more than 200 nanograms per
liter (ng/L) in 1983, to about 50 ng/L in 1985-1986, to 0.5 ng/L in 1986 as progressively
cleaner techniques for sample collection and handling were adopted (Zillioux et. al.,
1993). Such analytical contamination of samples presents a major uncertainty when
comparing mercury concentrations between different studies (especially older
investigations) and over time. In contrast, measurements of mercury emissions from
specific sources have, in general, been less impacted by this problem since the
concentrations are usually higher.
Another important concern in sampling is the availability of testing methods that can
measure mercury in particular forms in various media, especially in trace (low)
concentrations. If an appropriate measurement technique is not available it is easy to
assume that a material is not present.
Measurement of mercury in water and soils is commonly done using methods specified by
USEPA, such as Standard Methods for the Evaluation of Water and Wastewater (USEPA, 1986).
A now-commonly-specified method used to measure mercury in water is the cold-vapor method,
which can detect mercury down to levels of one parts per billion (ppb) depending on the
features of the sample "matrix" (background medium or the soil or water from
which the sample is taken). This and other methods afford high sensitivity, but where the
sample matrix is not conducive to a low detection limit, it may not be possible to
determine with certainty if very low or trace concentrations are present.
Most monitoring studies of atmospheric mercury have focused on deposition of this metal
to water bodies and soils via dry and wet deposition (see text box on next page). For
example, the Maine Department of Environmental Protection, in conjunction with the
University of Maine, has initiated an International Toxics Monitoring Program to study
mercury deposition in snow and rain, as well as mercury in freshwater fish from
northeastern lakes (Haines, 1994).
Direct monitoring for trace levels of mercury in the ambient air is not now commonly
performed, nor are there generally accepted methods available for making such
measurements. Such techniques are just now becoming available and are seeing limited use
in research projects. The following presents a brief summary of key issues relating to
ambient monitoring of atmospheric mercury concentrations. Appendix C provides a more
detailed discussion of sampling procedures, analytical methods, quality control issues,
siting issues for ambient monitoring efforts, and approximate costs associated with such
studies.
The development of ambient air and depositional monitoring techniques for mercury is
mainly being spurred by concerns over mercury inputs to water bodies and its subsequent
uptake by fish. Until recently, most monitoring for metals in the ambient air has been
done using modifications of the Federal Reference Method for the Determination of Lead in
Suspended Particulate Matter, which is found in the Code of Federal Regulations
(CFR), 40
CFR Part 50, Appendix G. This method has been routinely employed to measure lead, which is
a criteria air pollutant, and can be adapted to measure other metals as well. Limitations
of the method include its relative insensitivity and sampling primarily of
particulate-phase mercury. The method calls for the procurement of particulate samples on
glass fiber filters using a high volume sampler with subsequent acid digestion and
analysis by an atomic absorption (AA) or inductively coupled argon plasma emission
spectroscopy (ICAP). Although the Federal Register lists 70 ng/m3 as the lower detection
limit for the standard method, enhanced analytical methods can be used for special
monitoring studies and have achieved detection limits below 2 ng/m3.
Mercury commonly occurs in the environment in vapor (Hg0), particulate and
organic forms. The approach noted above may underestimate total mercury somewhat as it is
of limited effectiveness with respect to vapor phase metals. Although organic mercury
species are considered to be very toxic, due to chemical characteristics, they are not
expected to be found in detectable concentrations in the ambient air and are generally not
analyzed for.
DRY DEPOSITION VS. WET DEPOSITION
Dry Deposition
Dry deposition of atmospheric chemicals refers to any physical removal process that
does not involve precipitation. These physical processes include:
- gravitational settling-settling of the chemical due to its mass or the mass of
any paticulates to which it may be adsorbed
- impaction-when air containing particles moves past a stationary object such as a
building, some of the particles collide with the object and settle out
- adsorption -gaseous chemicals may be adsorbed by liquid surfaces or by solid
surfaces such as vegetation or soil
Wet Deposition
Wet deposition of atmospheric chemicals refers to removal processes associated with
precipitation.
- dissolving in rain droplets-occurs with gaseous chemicals
- incorporation in rain droplets or ice crystals-may occur with solids
Generally, environmental or "ambient" background levels are consistently
lower than those measurable using traditionally available techniques as noted above.
Information regarding mercury at these lower concentration ranges would help to delineate
air source contributions and overall atmospheric deposition rates of mercury to
terrestrial and aquatic environments. However, no comprehensive ambient mercury monitoring
studies have been conducted by the MADEP in Massachusetts, and few such studies have been
undertaken by others in the state or nationwide.
Air monitoring and deposition studies for mercury have been performed primarily in
rural locations. These generally show vapor phase mercury to be in the 1 to 10
ng/m3
and particulate mercury to be 10 to 100 picograms per cubic meter (pg/m3). This
indicates that, in rural areas, vapor phase mercury is likely to constitute from 95 to 99%
of the total with the remainder being particulate phase mercury. A study being conducted
in the Lake Champlain Basin, Vermont, has estimated an annual wet deposition of 15
micrograms per square meter (µg/m2).
Biological Effects
Human Health Effects
Mercury compounds are of concern because of their potential to act as poisons. A large
amount of scientific data about mercury toxicity exists. Several excellent reviews have
been published on the health effects of mercury (for example, see Clarkson et al., 1988;
Goyer, 1991; ATSDR, 1992; WHO, 1976, 1989, 1990 and 1991). This section presents a brief
overview of the toxicity of mercury and is not meant to be an exhaustive analysis. For
additional information please refer to the reviews noted above or to Appendix D, which
presents a more detailed technical summary of the effects observed after human exposures
to mercury.
Depending on the chemical form and the dose received, mercury can be toxic to both
humans and wildlife. In people, toxic doses of mercury can cause developmental effects in
the fetus, as well as effects on the kidney and the nervous system in children and adults
(Stern, 1993; WHO, 1990; ATSDR, 1994). As is discussed in more detail in the following
section, wildlife such as bald eagles, kingfishers, otter and mink that feed on fish are
particularly at risk because of the potential for methyl mercury to bioaccumulate in
freshwater fish. Methyl mercury has a high bioconcentration factor which means that it
will accumulate in living organisms such as fish.
Bioconcentration factors (BCFs) are simple ratios between the concentration of mercury
in an organism and the concentration in the medium to which the organism was exposed (WHO,
1989). For methyl mercury , BCFs of from 10,000-100,000 have been reported.
In wildlife, mercury-related effects on the central nervous system and reproductive
system have been reported (Heinz, 1976; Wobeser et al, 1976), effects consistent with
those observed in humans.
The symptoms associated with mercury poisoning can be complex. In part, this is because
mercury exists in a number of different chemical forms and the toxicity of each of these
differs. Further complicating the picture is the fact that these forms can be converted
from one to another in the environment and in the body. Thus, although the exact symptoms
caused by mercury poisoning will depend on the precise chemical form involved, some
overlap in symptoms can occur, especially at higher levels of exposure.
Mercury can be toxic when inhaled, eaten, or when placed on the skin. At low
concentrations, it may seem to have no effect but signs of toxicity may develop later or
become noticeable with continued exposure. Toxicity in humans is evidenced by loss of
feeling or a burning sensation in arms and legs, psychological effects, loss of memory,
loss of vision, loss of hearing, paralysis, congenital malformations, kidney toxicity, and
death. Prenatal toxicity can result in a child with normal appearance at birth but who
later exhibits a developmental delay in the ability to walk and/or talk. Because of the
long latent period for observable effects, the need for treatment may be recognized too
late.
The amount of mercury taken into the body largely determines whether health effects
will occur following exposures. At very low exposure levels, such as those that might
occur from mercury leaching from a modest number of amalgam dental fillings or from an
exposure that might result from wiping up a spill from a small broken thermometer, no
adverse effects are usually noted (note that vacuuming mercury can lead to more
significant exposures; by breaking the mercury up into smaller droplets and increasing air
flow over them, vacuuming can increase volatilization and dispersion of mercury and thus
increase the potential for exposure).
At the other extreme, high level exposures to mercury can cause serious effects or even
be lethal. Such exposures do not typically occur in Massachusetts or elsewhere in the US
and are generally only observed in isolated poisoning incidents. Several historical
examples of epidemic mercury poisonings in other parts of the world, however, provide
classic examples of investigative epidemiology and toxicology and serve to highlight the
reasons why regulators are concerned about mercury.
For example, in a tragic episode in Iraq in 1971-1972, over 400 people died after
ingesting large amounts of organic mercury in bread that was accidentally made with grain
treated with a mercury-containing fungicide (Marsh et al, 1987; Bakir et al, 1973). In a
second well known disaster which occurred from 1953-1960, many people living near Minamata
Bay in Japan were severely poisoned by eating fish containing methylated mercury
(Takeuchi, 1975; Tamashiro et al, 1985). In this case the bay was polluted by mercury from
local industries, a practice now prevented by environmental regulations. Methyl mercury
accumulated in marine organisms in the bay, including fish. These same fish were a primary
source of food for many people in the area. In addition to many deaths, these exposures to
mercury also caused a variety of other problems including neurological and developmental
deficits in children exposed in the womb.
Effects on the brain and nervous system are frequently seen with high level exposures
to mercury and can be quite severe. In the 18th century, mercury was used in the
manufacture of fashionable felt hats. Workers involved in this trade handled mercury-laden
skins and many were severely poisoned; while handling the furs, they would inadvertently
inhale large amounts of mercury. These poisoned workers exhibited severe, and sometimes
bizarre, psychological and behavioral symptoms. The term "mad as a hatter" was
coined as a result of these poisonings.
Fortunately, exposures to mercury in Massachusetts, and the developed world in general,
are well below those associated with such acute, severe effects. None-the-less,
longer-term exposures to more modest levels of mercury can present unacceptable risks to
susceptible groups including infants and fetuses.
In the United States a potentially significant route of exposure to mercury is from
consumption of freshwater fish, which bioaccumulate methyl mercury, caught from
contaminated waterbodies (certain larger predatory saltwater species such as shark may
also contain elevated levels of mercury). Depending on how many contaminated fish one
consumes, mercury exposures via this pathway can present a significant risk.
In contrast, inhalation exposures to mercury are generally not of concern since ambient
air concentrations are typically low, ranging from 2 to 20 ng/m3 (ATSDR, 1993).
Additionally, in Massachusetts no public drinking water supplies have been identified that
are contaminated with significant amounts of mercury.
It is important to note that other potentially significant exposures to mercury can
occur which are not related to environmental contamination. For example, exposures can
occur in the home following accidental release of mercury or its intentional dispersion,
as occurs during reported ceremonial/religious uses of this metal by certain groups of
Caribbean descent including, for example, practitioners of Santeria and
Espiritisimo, who
may sprinkle elemental mercury around a dwelling or in an automobile to ward off evil
spirits or to enhance positive forces. Some groups may also use mercury as a home remedy
to treat certain ailments (Connecticut Department of Public Health, Division of
Environmental Epidemiology and Occupational Health, personal communication).
ORGANIC MERCURY POISONING- CASES OF CONTAMINATION OF FOOD WITH
HIGH LEVELS OF METHYL MERCURY
Descriptions and analyses of symptoms are found in reports of several poisoning
episodes where foods became inadvertently contaminated with high levels of methyl mercury.
In Iraq, seed grain treated with a methyl mercury pesticide was mistakenly used to make
bread that was a major source of food. Methyl mercury concentrations in the bread were
estimated to average approximately 9 milligrams per kilogram (mg/kg) or 9 ppm.
In Japan, fish containing high levels of methyl mercury were a major food source. In
Minamata Bay, Japan, estimated concentrations of methyl mercury in marine products ranged
from 5.6 ppm to 35.7 ppm.
In the United States, in Alamogordo, New Mexico, a farm family was poisoned by eating
meat from a pig that had been fed grain treated with methyl mercury fungicide. Exposure in
this case was likely to have been very high.
Adverse effects have been found to be persistent in survivors of all major epidemics of
methyl mercury poisoning. Effects often developed long after the exposure had ceased.
In the Iraq epidemic and in the United States family exposed by eating pork, follow-up
studies showed that serious effects (quadriplegia, mental defect, loss of vision, etc.)
persisted for the duration of follow-up or until death. Mercury remained in the brain over
this period of time as well.
In both situations, methyl mercury had been ingested for as little as 3 months (at high
levels). Medical attention, including chelation therapy, had been provided to the family
in the United States.
With respect to fish contamination, methyl mercury has been found at unsafe levels in
freshwater fish from many lakes and ponds in the Northeast. [Endnote 1] In MA alone, fish
consumption advisories have been issued by the MADPH for 37 waterbodies due to mercury and
a Statewide advisory warning pregnant women of the potential dangers of eating any
freshwater fish caught from MA waterbodies has been issued. Nationwide, more than thirty
states currently have freshwater fish consumption advisories in place for at least some
waterbodies due to elevated levels of methyl mercury.
In general, consumption of larger sized predatory fish from species such as largemouth
bass will pose greater risks- because these fish are both older and at the top of the food
chain they will have accumulated more methyl mercury than younger, smaller fish. It is
also important to note that freshly stocked trout that are part of the Massachusetts
Division of Fisheries and Wildlife fish stocking program do not contain elevated amounts
of mercury. These fish are grown in hatcheries, fed fish food containing low levels of
mercury and generally do not live long enough after release to bioaccumulate elevated
amounts of methyl mercury.
In Massachusetts, fish monitoring and fish consumption advisories currently in place
have reduced the potential for harm from this pathway. Such fish contamination may,
however, still present a risk to those who are unaware of the problem, do not heed the
warnings or depend on freshwater fish as regular source of food. Contamination also
diminishes the overall quality of the Massachusetts environment by reducing recreational
and subsistence fishing opportunities.
The health risks of mercury at low levels of exposure remain uncertain and this is an
area of considerable ongoing scientific investigation and debate (ATSDR, 1994; Stern,
1993; Marsh, et al 1995a; 1995b; Weiss, 1995). Fetuses, infants and small children,
however, appear to be particularly sensitive to mercury. For prenatal exposures, effects
may not be apparent at birth but may only reveal themselves later in childhood as delays
or deficits in language, cognitive and motor skill development. Current research suggests
that potentially important neurological and behavioral effects may be caused by exposures
of a fetus to methyl mercury during pregnancy (ATSDR, 1994; Stern, 1993; WHO, 1990). The
MADPH has established a trigger level of 0.50 ppm for methyl mercury in fish, a level at
which pregnant women are advised to avoid consuming the fish in question. [Endnote 2] For
fish with mercury levels between 0.5 - 1 ppm people are warned to limit their consumption
of the affected fish and at levels above 1 ppm MADPH urges that everyone avoid eating the
fish.
However, it is important to re-emphasize that the precise level at which demonstrably
adverse effects occur remains highly uncertain. Two recent studies of children exposed to
mercury via fish consumption have yielded conflicting results regarding the hazard posed
by mercury in fish. In both studies, children living on islands where fish are regularly
eaten were studied. No clearly adverse effects were reported among approximately 1,500
Seychelles islander children who were studied to the age of 5.5 years (see
NeuroToxicology, V16, 1995). In publications on this study, the mercury concentrations of
the fish consumed were not specifically given but they are reported to have been generally
below levels deemed to be of concern by the USFDA and MADPH (presented at the Boston Risk
Assessment Group (BRAG) seminar on May 8, 1996 by Dr. Philip Davidson one of the lead
authors of the Seychelles Island mercury study). In any case, although reassuring in that
clearly adverse effects were not seen, the reported results of this study must be
interpreted cautiously. First of all, new analyses of the data suggest potentially
deleterious effects may have occurred among some children (Dr. Philip Davidson, BRAG
seminar, 1996). Secondly, the study has yet to be completed; additional assessments of the
children, including tests that are more sensitive indicators of neurological effects,
remain to be analyzed and/or completed. Further complicating this issue is a second study
soon to be published, which has been reported to have detected significant effects in
1,000 children exposed to mercury and, potentially, other contaminants in the Faroe
Islands. MADEP as well as other State and Federal regulatory agencies will continue to
keep abreast of these studies to determine whether any changes to current health
guidelines or exposure standards are warranted.
Ecological Effects of Mercury
Many more studies have been published on the effects of mercury on human health than on
its effects on ecosystems. Ecosystems encompass the functional relationships between
organisms and their physical environment. They include energy flow through food chains,
and pathways through which chemical elements essential to life move through a complex
network (Ehrlich & Ehrlich, 1970). Groups of living organisms interact within an
ecosystem, giving it a certain amount of resilience to stress. If, for example, mercury is
present in an ecosystem at high enough levels to cause the local extinction of eagles that
live on fish, another predator species may assume the eagles' place on the food chain. The
ecosystem persists, but the populations within it are less diverse, and possibly less
specialized. Contaminants with a global distribution like mercury may cause impacts over a
widespread geographic area.
The study of ecosystem effects of mercury typically has been reduced to studying its
effects on individual species. Published studies generally fall into two categories:
laboratory investigations and field studies. Laboratory studies tend to focus on
individual species and show that organisms can absorb mercury compounds from their food as
well as directly from the water, soil or sediments in which they live. Aquatic
invertebrates bioconcentrate mercury at a much higher rate than fish, and plants have
variable rates of bioconcentration depending on the species.
Effects of mercury on organisms in the laboratory do not directly correspond to field
effects. Natural conditions introduce many variables that confound results. For instance,
sediments can partition mercury from the water, lessening exposure to organisms in the
water column. Changing temperatures and pH levels affect bioconcentration as well. The
spectrum of other chemicals that occur in nature affect mercury interactions with
sediments, water and organisms (WHO, 1989). In spite of these limitations,
such studies do, however, provide insight into the types of effects mercury may cause in
wildlife and their potential magnitude.
Mercury is accumulated by aquatic organisms of all types and, in its methylated form,
is the most common contaminant in freshwater fish (ATSDR, 1994; USEPA, 1992). Fish kills
have occurred in cases of severe mercury contamination, such as occurred in Minamata Bay
in Japan. Freshwater microorganisms can also be very sensitive to mercury contamination.
Field studies indicate that tissue concentrations of mercury in marine and freshwater
fish increase with size. Monitoring of winter flounder, lobster and bivalves from coastal
Massachusetts shows that mercury levels in these marine species are lower than
concentrations in freshwater fish (Schwartz, et. al. 1995). Marine predators, particularly
those that grow to large sizes, such as sharks, have been found to exhibit high mercury
levels. Marine species that do not grow to large sizes and have short lives (e.g. many
flounder) are generally lower in mercury than predatory freshwater species.
The long-term ecological effects of elevated mercury in fish are not presently known.
In theory, the effects could be critical to the survival of species whose diet consists
mainly of other fish.
Fish-eating birds have higher concentrations of mercury than other birds. Studies of
mercury in feathers from Maine eagles, conducted by the United States Fish and Wildlife
Service (USFWS) show that coastal eagles have a lower body burden of mercury than eagles
that live inland and feed on freshwater fish. In areas where methyl mercury fungicides are
used, seed-eating birds and small mammals and their predators can have high mercury
concentrations (WHO, 1989).
Environmental Monitoring of Biological Effects
Extensive studies of mercury in the aquatic environment are underway at colleges and
universities throughout the world. In the United States, a large number of studies are
also supported by private industries and conducted by environmental consultants. State and
federal environmental agencies support and conduct environmental monitoring studies. Many
large ecosystems are presently being studied. For example, the Great Waters Study
represents a concerted effort led by USEPA to assess the quality of large water bodies in
the United States; mercury is one of several chemicals being considered. The International
Toxics Monitoring Program investigates mercury in waterbodies in eastern Canada and New
England. The Everglades, the Great Lakes, Lake Champlain, the Chesapeake Bay, large areas
in Alaska, the Rocky Mountains and the Canadian Shield are additional examples of places
where ecosystem-scale mercury monitoring projects have been initiated. Government agencies
on the federal and state levels, scientists, and students from colleges and universities
often join forces to perform these studies.
Tissue concentrations of mercury in fish and invertebrates are extensively available in
the literature. Associated sediment and water quality data are also often available.
Assessment of trophic pathways by means of radioisotope tagging is a recent trend in
monitoring strategies. Aquatic studies are well-developed due in part to human
exploitation of fish as a food supply. The National Study of Chemical Residues in Fish,
published by USEPA's Office of Science and Technology, reports mercury detection in fish
tissue at 92% of the 388 test locations. Measured concentrations ranged up to 1.77 parts
per million, with 2% of the sites greater than 1 part per million. Most of the higher
concentrations were in the Northeast (USEPA, 1992). Results of fish monitoring studies in
Massachusetts will be discussed in detail in Chapter 4.
Terrestrial studies of mercury in the environment include studies of birds, mammals,
invertebrates, soil microorganisms, plants, air and soils. A multitude of terrestrial
investigations have been conducted on mercury in birds. Tissue mercury concentrations,
often organ-specific, are widely available. Feathers are often used to measure mercury
levels, thereby sparing the bird; evidence shows that significant adverse effects may
occur at levels as low as 13 parts per million (WHO, 1976).
Studies of mercury levels in Maine eagles have been conducted by the US Fish and
Wildlife Service. Data from these studies (Linda Welch, US Fish and Wildlife Service,
personal communication) indicate that inland eagles have high levels of mercury in their
feathers (an average of 20 parts per million, and as high as 37 parts per million in six
to eight week-old fledgling eagles). The 'background" level in feathers is estimated
to range from 2 to 3 parts per million. Mercury in eggs was greater than 0.5 parts per
million in some cases. More than 0.5 parts per million of mercury in eggs is considered
sufficient to prevent hatching. Freshwater fish make up 75% of the diet of inland eagles.
Coastal eagles show much lower levels of mercury in feathers and eggs, suggesting that
their prey along the coast of Maine contain lower concentrations of methyl mercury.
Testing Massachusetts eagles for mercury is underway in conjunction with the highly
successful program to reestablish these raptors in the Quabbin Reservoir area, conducted
by the Massachusetts Division of Fisheries and Wildlife. No effects of mercury have been
observed, but mercury sampling is not far enough along to offer insights into possible
effects on the health and fitness of eagle populations (Bill Davis, MA Division of
Fisheries and Wildlife, personal communication). The population of eagles in Massachusetts
is young, so perhaps the birds have not lived long enough to bioaccumulate significant
levels of mercury.
One of the leading causes of death in eagles is collisions with buildings, cars, power
lines, and the like. It has been proposed that an excess of mercury, which can lead to
neurological impairment, may have contributed to an observed increase in eagle collisions
in recent years, although other factors such as an increase in the absolute numbers of
eagles in more highly developed regions of the country are also likely to be involved
(Kenneth Carr, US Fish and Wildlife Service, personal communication).
Small mammal studies in the laboratory and in field situations demonstrate that mammals
are particularly vulnerable to mercury, probably due to its neurotoxic effects and the
high trophic position of mammals in the food chain. Mink show sublethal effects on a diet
containing 5 to 10 ppm mercury, including loss of balance and coordination, anorexia, and
weight loss (Wren et. al., 1987). Some of the test animals died. Small mammals sampled
from fields sown with mercury-treated grain also died. Mercury poisoning was suspected as
the possible cause of death of at least one Florida panther, and environmental mercury may
have contributed to the severe population decline experienced by this endangered wild cat
(Roelke, 1990).
Plants have also been studied for mercury accumulation. Sensitivities were
species-specific, but in general, plants accumulate mercury as readily as other organisms.
Aquatic plants are more efficient accumulators than terrestrial plants (John, 1972; WHO,
1989).
Terrestrial invertebrates also concentrate mercury. This observation has led to the
suggestion that earthworms be used as a means to bioremediate soils contaminated with
mercury (WHO, 1989).
Assessing the amount of the risk to human and ecological health from exposure to
mercury, especially where organisms such as large fish and other species high on the food
chain concentrate mercury, is central to determining the needed level of control of
mercury emissions. The standards presently in place, how these standards are established,
and the use of risk assessment in evaluating mercury hazards will be discussed in Chapter
5. To provide a picture of what is known about mercury releases in
MA, this report next assess State-wide mercury sources (Chapter 3). Chapter 4 then
summarizes the results of monitoring studies in Massachusetts.
Endnotes:
1. In these situations the water itself does not contain harmful
amounts of methyl mercury; swimming, bathing and drinking these waters is thus generally
safe.
2. At this level, consumption of 4.6 ounces of fish a day would lead
to a maternal mercury exposure of about 65 ug/day, a level approximately equal to that
associated with a significant risk of adverse effects among those exposed in utero during
the Iraq epidemic.
APPENDIX D -Mercury Toxicity: Technical Overview
Introduction
Several review articles have been published on health effects of mercury (for example,
see Clarkson et al., 1988; Goyer, 1991; ATSDR, 1992; WHO, 1976, 1989, 1990 and 1991). The
following section is not meant to be a duplication of these extensive technical reviews;
rather, it is a brief summary of the effects observed after human exposures to mercury in
its three most prevalent forms:
The unsteadiness is seen most dramatically when the patient is asked in the clinic to
hold both arms out to the side for three minutes. The patient is unable to do so, and will
begin to flap the arms to relieve the stress (seagull sign). The psychological signs
include insomnia, loss of appetite, shyness, emotional instability, and memory loss. Some
reversal of these effects may occur upon removal from contact with mercury. With continued
exposure, more severe tremor and muscle spasms as well as death may result.
Literature reports of incidents of mercury vapor toxicity include another type seen
mainly in children--acrodynia (also known as Swift's disease or pink disease). In this
disease, which occurs infrequently even among children exposed there is weight loss, loss
of appetite, irritability, muscle weakness, learning disorders, and redness (hence
"pink") and peeling of skin on fingers and toes. Children have most frequently
shown these symptoms when calomel (a substance containing inorganic mercury) was used as a
soothing agent on their teething rings. The same symptoms have been seen in children
exposed to mercury vapor from contaminated floors or carpeting. Since the symptoms from
inhaled mercury were accompanied by high urinary excretion rates and were the same
symptoms as seen after calomel exposure, it may be that they were related to the
transformation of metallic mercury to the inorganic form. It is thought that these signs
may be the result of an autoimmune reaction against tissue containing mercury.
Studies of workers exposed to mercury have found that tremors and an abnormal walking
gait occurred after chronic (1-5 years) exposure to 0.076 mg vaporized mercury per cubic
meter of air. Mild tremors occurred at 0.026 mg/cu.m. Immune deficiency occurred in those
exposed to as little as 0.106 mg/cu.m. (effects summarized by ATSDR). These numbers
indicate that the toxic effects of inhaled mercury can occur at low concentrations.
A current epidemic of metallic mercury poisoning is going on now in the Amazon Rain
Forest (described in Branches et al., 1993) among native people employed as gold miners.
In Brazil alone, over a million miners are directly exposed to mercury vapors in the gold
extraction process. Many others are exposed in the refinement and working of gold
contaminated with mercury. Both sets of workers display signs of metallic mercury toxicity
and excrete mercury in urine, but the gold shop workers have higher blood levels. Medical
investigators studying these workers suggest that they may suffer increased levels of
exposure resulting from vaporization of mercury as the contaminated gold is heated
indoors.
The exposed miners and gold workers studied to date have all been adult men. No
instances of exposed pregnant women have been described. In one case an individual
studied, who did not work with gold at all, was found to have had a high blood mercury
level. It was subsequently discovered that he lived above a gold shop and was almost
certainly poisoned by mercury vapors from that source. Since these residents are also part
of the exposed population and could include pregnant women, future investigations may
extend to these families.
Inorganic mercury
Inorganic mercury toxicity can result from ingestion or direct skin contact with
inorganic mercury or it can occur as a result of transformation of metallic mercury to
inorganic mercury in the body. Poisoning has also resulted in the past when mercury
containing calomel was used on teething rings; when mercury soaps and creams were applied
as skin lighteners; or when laxatives containing inorganic mercury were taken chronically.
Somewhat different signs of toxicity result depending on whether the mercury is in the
mercuric (+2) or mercurous (+1) form.
Taken in a high dose (over 10 percent in water), mercuric chloride produces severe
abdominal cramps, bloody diarrhea, and suppression of urine. Death of important tubule
cells in the kidney also occurs after exposure to this form of mercury. Loss of these
cells results in kidney malfunction including release of essential plasma proteins into
urine (albuminuria) and excessive retention of water in the body tissues (edema). Death
can result from shock and kidney failure within 24 hours, but if the patient is otherwise
stabilized and placed on dialysis, the kidney may eventually repair itself using the
surviving cells.
Ingestion of lower concentrations of mercuric chloride in water or food can result in
an autoimmune reaction to kidney cells altered by exposure to mercury. The first signs are
an inflammation of the glomerulus (the location where plasma fluids are filtered to the
urinary tract); the body then further reacts immunologically to the degraded cells,
causing further damage.
Mercurous compounds are less toxic than mercuric compounds. Calomel (mercurous
chloride) was used in medicine; placed on gums of teething children to reduce pain; and
was used as a skin lotion. Adverse responses to this form of mercury is thought to result
from an immune reaction in the skin. Symptoms include a reddish skin and rash (leading to
the common name of "pink disease"), fever, swollen lymph nodes and spleen, and
peeling hands and feet. Mercurous compounds have also been used in the treatment of
syphilis, as purgatives, and as both internal and external disinfectants. Toxicity and
even death, generally as a result of kidney failure, have resulted from long-term use or
misuse of these substances. Current regulations on prescription drugs and consumer
products have decreased this type of exposure.
Organic mercury
Poisonings by organic mercury have occurred primarily as the result of contamination of
food with methyl mercury. Extensive descriptions and analyses of symptoms have been
described in reports on several widespread poisoning episodes where foods became
inadvertently contaminated with high levels of methyl mercury. Studies have also been made
of people exposed to more modest levels of methyl mercury in food. Some of the more
extensive documentation of mercury effects in people include studies on populations in the
following regions:
Iraq (Bakir, et al., 1973), where grain treated with a methyl mercury pesticide was
mistakenly used to make bread that was a major source of food;
Japan (Takeuchi, 1975), Canada (McKeown-Eyssen et al., 1983 reports) and New Zealand
(Kjellstrom, et al. 1986, 1989) where fish containing methyl mercury was a major food
source;
the Faroe Islands (Grandjean et al., 1992; Dalgard, 1994), where mercury containing
fish and whale meat are important sources of food;
the Mediterranean Basin (Franchie et al., 1994) where fishermen and their families are
exposed to varying amounts of mercury from fish;
the United States (Davis et al., 1994) where a farm family was seriously affected by
eating meat from a pig that had been fed methyl mercury treated grain.
In all cases, the severity of symptoms was increased when the food was either more
highly contaminated or eaten in larger quantities. In adults, the first signs of toxicity
included abnormal sensation (tingling or numbness) in arms and legs. This effect was
correlated with a cumulative intake of 25-40 mg methyl mercury and 5 ug of mercury in a
gram of hair (hair to blood ratio of approximately 250 to 1). An average daily intake of
3-7 ug methyl mercury per kilogram body weight could be expected to produce such effects.
Other early effects included blurred vision and a general feeling of malaise.
At higher mercury exposure levels and correspondingly higher body burdens, additional
symptoms appeared. These included: loss of coordination of gait (ataxia); slurred speech
(dysarthria); loss of peripheral vision; loss of hearing; coma; kidney failure; loss of
memory; abnormal blood sugar; and quadriplegia. Symptoms were due to toxic effects on the
brain, peripheral nerves, pancreas, immune system, and kidneys. In addition, in some
people, genetic changes were observed in lymphocytes, suggesting that such changes could
also occur in other tissues, including the reproductive organs.
The evidence from numerous epidemiological studies indicates that the fetus is very
sensitive to mercury. The children of women exposed to methyl mercury during pregnancy may
show signs of toxic effects either at birth or later in childhood. Some mothers who had a
hair concentration of 6 ug of mercury per gram of hair (6 ug Hg/g hair) or higher during
pregnancy had children who, compared to those not so exposed to mercury, started walking
and talking later in life and who scored lower in tests designed to measure other physical
and mental development. Children whose mothers had even higher maternal exposure levels
(hair mercury concentrations of up to 400 ug/g), were affected with a greater frequency
and suffered more severe symptoms. These included mental retardation, cerebral palsy and a
high degree of irritability and sensitivity to touch.
Comparison of the doses needed for adult toxicity and fetal toxicity is difficult since
the fetus preferentially accumulates methyl mercury; the ratio of mercury in fetal blood
to maternal blood is about 5:1. Thus, the fetus is exposed to a greater overall
concentration of methyl mercury than the mother. Additionally, there may also be a greater
rate of transfer of mercury to the brain in the fetus. Pregnant women may therefor show
little if any adverse effect following mercury exposure but still have an affected child.
Because methyl mercury is secreted into breast milk, nursing infants of mothers exposed
to mercury only after pregnancy can also be exposed to methyl mercury. Children exposed in
this way have been shown to have methyl mercury in their blood; since few children were
observed in these studies, and none followed through full development to adults, it is not
possible to determine the effects of this type of exposure. Available data, however,
suggest that effects of exposure after birth are less severe than effects from a prenatal
exposure.
Adverse effects have been found to be persistent in survivors of all major epidemics of
methyl mercury poisoning. In the Iraq epidemic and in the United States family exposed by
eating pork, follow-up studies showed that serious effects (quadriplegia, mental defect,
loss of vision, etc.) persisted for the duration of follow-up or until death; mercury
remained in the brain over this period of time as well. In both situations, methyl mercury
had been ingested for as little as 3 months (at high levels); medical attention, including
chelation therapy, had been provided to the family in the United States.
Because of the seriousness of the effects associated with methyl mercury poisoning,
their insidious onset, and the persistence of symptoms, environmental and public health
professionals have focused their efforts on preventing exposures, especially of the fetus.
As early as 1976, the World Health Organization (WHO, 1976) recommended that no more than
0.3 mg total methyl mercury be ingested per week. Other agencies have recommended limits
for allowable daily intakes of mercury in its various forms or have set limits for
concentrations in air, water, food, and other environmental media. A recent evaluation of
data on methyl mercury resulted in the suggestion that the reference dose (the daily dose
likely to be without significant adverse effects) for a chronic (long-term) exposure to
this organo-metal should be somewhat lower than the previous value recommended by the
USEPA (Stern, 1993). This and other data on mercury intake were evaluated by USEPA which
recently lowered its recommended reference dose to 0.1 ug/kg/day from its earlier value of
0.3 ug/kg/day.
It is important to note, however, that the hazard of low doses of mercury, especially
attributable to fish containing methyl mercury, is a matter of considerable controversy.
Two recent studies on fairly large numbers of children exposed to mercury in utero are
especially relevant. Results, which have not yet been published, from a study conducted on
children living in the Faroe Islands, report an association between mercury exposure and
developmental effects in the children studied (Science Scope, p. 10045, Science [271],
1996). In contrast, a second study, discussed below, did not detect any clearly adverse
effects among children living on the Seychelles Islands. This extensive study, although
not conclusive, is reassuring with respect to consumption of fish containing low levels of
mercury. However, additional analysis of data derived from further follow-up of these
children remains to be completed, making it premature to draw final conclusions regarding
this investigation. Because of the quality and potential significance of this study, it is
more extensively discussed below.
Seychelles Island Study
Previous reports by others (e.g., Grandjean, et al., 1992; Kjellstrom et al., 1986,
1989) have suggested that children born to women who eat fish or whale meat contaminated
with organic mercury during pregnancy run a risk of delayed neurological development.
Other studies showed that the average daily intake of organic mercury could be estimated
by analyzing the amount of mercury deposited in the growing hair shaft during the exposure
period (e.g., Cox et al., 1989). Data from an epidemic of organomercury poisoning caused
by ingestion of pesticide-treated grain was used to determine that a 5% risk of
developmental defects was associated with maternal hair concentrations as low as 10-20
ug/g (WHO, 1990).
The results of these studies were confounded because exposure to mercury from other
sources could affect the outcome. Scientists at Rochester University conducted an
international search for a population that was free of exposure to mercury from industrial
sources, in which women were not exposed extensively to other factors influencing rate of
fetal development (such as alcohol and tobacco), which had accessible quality medical
institutions, which had a high rate of schooling and tracking of the children, which was
stable and amenable to the study, and which had a high rate of local fish consumption. In
addition, the investigators looked for a population that had methylmercury in hair below
20 ug/g.
The Seychelles Islands was chosen as the optimal site for the study. An extensive
epidemiological study on the relationship between mercury content of mother's hair and
fetal health was conducted in the Seychelles Islands by an international team of
scientists. The details of the design of a pilot and full study; results after 66 months
of observation in the pilot study; and results after 29 months in the full study are
reported in several articles of one issue of the journal NeuroToxicology (1995;
Issue16(4)).
At the Seychelles Islands, fish from reefs are eaten daily as a source of protein.
These fish have relatively low mercury content, many species being below 0.1 ug Hg per g
fish (wet weight). However, most women in this study ate 10-14 fish meals per week a
considerably higher rate than seen with most US citizens. No estimates of the average
daily dose of mercury were provided in these reports. In the pilot study on 789
mother-infant pairs, the methylmercury deposited in mother's hair during pregnancy ranged
from 0.59 ppm to 36.4 ppm, with a median concentration of 6.6 ppm.
International neonatal physical developmental indices were used, including birth weight
and other physical measurements, Apgar scores, and gestational age to investigate
potential mercury related effects. The Revised Denver Developmental Screening Test
(DDST-R) was given to children between the ages of 5 and 109 weeks of age; this measures
motor, perceptual, and cognitive development at an early age. Results of the DDST-R are
scored as normal, questionable and abnormal. If the questionable category is grouped with
the abnormal scores, a positive association between mercury levels and developmental
effects is observed. If grouped with the normal scores the study is negative for mercury
effect.
Two hundred and seventeen children whose mothers had a median hair mercury content of
7.1 ppm were again evaluated at 66 months using additional psychological testing
procedures for that age. Standard assessment procedures (Endnote 1) on cognitive, sensory,
language, and comprehension abilities were adapted to avoid cultural and language bias.
Physical examinations were conducted periodically as usual during childhood and the
medical records were available for inclusion in the study.
The pilot study provided suggestive evidence of an association between mercury content
of hair and fetal development. A slight negative correlation was found between hair
mercury and these scores; it was not statistically significant. Five children had General
Cognitive Index scores less than 60; 3 had clinical deficiencies in fine motor
coordination, hearing, and language similar to what was seen in other children exposed to
methylmercury prenatally when the mothers ate contaminated fish (Minamata study). The
authors considered the possibility that a examination of a larger population might give
different results.
The main study on 740 children over 66 months is still in progress. In this group, the
mothers' hair mercury ranged from 0.5 to 26.7 ppm, with a median of 5.9 ppm, during
pregnancy. Developmental tests with additional endpoints were added to those previously
administered in order to expand the sensitivity of the testing. (Endnote 2) Observations
at 6.5, 19 and 29 months were analyzed and are discussed in the currently published
reports.
There was no association between mother's hair mercury content and childhood
development scores at any of the observation times to date. At 19 and 29 months, one
endpoint-- activity level of the male children-- was decreased as maternal hair mercury
level increased. Other factors influencing this measurement, such as parental attention,
other children in the family, etc., are hard to control and the authors are cautious in
assigning weight to this in the absence of other findings. A highly significant
association was, however, recently reported for this endpoint among certain women in the
study (Philip Davidson, Boston Risk Assessment Group seminar, May 8, 1996).
Other evidence on mercury exposure of the infants in the Seychelles comes from autopsy
data. Brain tissue has been routinely preserved for mercury analysis. Although the
neonatal deaths are not strictly from the study group, they represent children of the same
general population. Brain tissues from autopsies on infants in Rochester were also
analyzed as controls. Neither the Rochester nor the Seychelles children had displayed
symptoms of neurological deficiencies before death.
Tissue from 32 Seychelles children was examined for content of mercury. The data ranged
from 50 to 250 ppb, representing mainly methylmercury. Brains from 12 children from
Rochester were similarly analyzed and found to contain less than 50 ppb mercury (with one
unexplainable exception). Histological observations on the Seychelles tissues showed no
abnormality in cerebral or cerebella cortical organization; other changes were not
indicative of mercury effect since the same features were present in the Rochester control
tissue. These results, although limited in extent, suggest that the islanders were in fact
exposed to greater overall amounts of mercury compared to US citizens. It is not clear,
however, that the metabolism and distribution of methylmercury in this population would be
the same. Toxic effects may relate to the dose rate and timing of exposure rather than the
total dose received.
In conclusion, the Seychelles Island study has not detected any clearly significant
association between mercury and developmental effects in a large population of children
exposed via consumption of fish bearing low levels of this metal. Follow-up of these
children continues and the investigators will be reporting additional results in the
future. The fact that the study is not yet completed combined with questions over the
applicability of the study to episodic exposures to fish more heavily contaminated with
mercury suggest that it is premature to draw any firm conclusions based on this work at
this time.
For another good review on the fate of mercury
released into the environment see mercury.pdf.
Unfortunately, this review does not consider the
contribution of the mercury amalgam fillings on the
human body burden nor does it discuss the toxic effects
mercury amalgam fillings on the human body.
Dental
Information 
