Prolonged exposures to relatively high concentrations
of mercury in the air produces damage to the nervous system and
kidney, but rarely produce a fatal injury. However, accidental oral
exposures have caused death (WHO, 1990; ATSDR ,1993c). The presence
of mercury in the environment at lower levels also produces potential
for human exposure directly through breathing mercury and indirectly
through ingestion of fish contaminated with methyl mercury. Methyl
mercury is about five times more dangerous than the inorganic and
metallic forms of mercury (Casarett and Doull, 1990). Toxic actions
of methyl mercury and mercury are different and should be considered
I Elemental mercury. Inhalation
of moderate levels of mercury for prolonged periods of time produces
unique effects on the central nervous system and the kidney. The
chemical form of mercury is important in the determination of the
actual toxic effect (Casarett and Doull, 1990; ATSDR, 1993c). For
example, elemental mercury is highly charged and does not readily
pass through the blood brain barrier but directly reaches the kidney.
Therefore, elemental mercury is more toxic to the kidney than the
brain. Metallic mercury and organic forms of mercury pass easily
through the protective blood brain barrier and may cause brain damage
as well as kidney damage (ATSDR, 1993c). Once mercury enters the
body it is retained in the kidney, brain, liver and the fetus. It
may stay in the body for several months. When mercury is eliminated
from the body it appears in the breath, urine, feces, milk and hair.
Different organs accumulate and retain mercury at different rates,
with the brain and the kidney retaining mercury for periods of months
to years. Enzymatic processes in the body transform mercury to the
inorganic or metallic form.
Toxic effects of mercury are found in the brain,
kidney, skin and liver. At low exposures the toxic effects of mercury
are found mainly in the brain. The effects are much more serious
in the infant and the fetus because of interference with development
of the motor and cognitive functions (ATSDR, 1993c; Casarett and
Doull, 1990). There is a latent period between the exposures to
mercury and the onset of effects of the poisoning. This delay complicates
the diagnosis of the damage and the implementation of the treatment
At first, low concentrations of mercury seem
to have no health effects but signs of toxicity become noticeable
with continued exposures (ATSDR 1993c). Toxic signs include loss
of feeling or burning sensations in the legs, paralysis, congenital
malformations, kidney toxicity and, on rare occasions, death.
Environmental exposures do not produce the more
severe effects but subtle toxic effects are possible, especially
in the fetus. These changes would not be apparent in the newborn
but would become apparent after later development of motor functions
such as walking (Casarett and Doull 1990). Unsteadiness and tremors
may develop after long periods of exposure. Psychological effects
such as insomnia, loss of appetite, shyness, emotional instability
and memory loss are also reported in the literature. These actions
are partially reversible with termination of exposures to mercury.
Cancer is not a significant component of mercury toxicity (ATSDR,
While the toxicity of mercury itself is an important
concern, the toxic effects of methyl mercury are dangerous because
of the higher potency of methyl mercury and its effects on development
of the fetal brain.
The occupational safety and health Administration
has set a limit of 50 ug/m 3 for metallic mercury in the work place.
EPA has set limits for inorganic mercury in drinking water and surface
II. Methyl mercury. Human
health effects of methyl mercury nearly all involve the ingestion
of methyl mercury in fish, or, in one case in grain (Spyker Cranmer
J., 1996). Methyl mercury in food is nearly all absorbed into the
body and distributed throughout the body to the brain and fetus.
Mercury absorbed into the blood stream may be metabolized to the
inorganic forms in the liver and removed from the body or reabsorbed
from the intestine.
Methyl mercury in the brain and the fetus can
be transformed and concentrated. Thus, when the blood levels are
decreasing overall the mercury in the brain and fetus may remain
high or even increase. Mercury is transferred to hair where its
measurement provides a historical indication of past exposures.
Equally if not more important, methyl mercury is also excreted into
the mother's milk where it is transferred to the nursing infant.
Damage has been found in infants and in the fetus even when the
mother has shown no toxic effects of mercury exposure.
There is a large body of information on the
toxic effects of the chronic ingestion of fish containing methyl
mercury by different populations starting before Minimata in Japan
in the 1950s to present day exposures in the Amazon river basin.
Relatively widespread human poisonings have been documented from
these episodes. The actual levels of exposures at which injury begins
to occur in the fetus, infants and children remains controversial.
The findings from chronic exposures have been used to identify reference
doses for estimating the human health hazards ( Sea Food Safety,
1991). The reference doses from these different studies and exposures
have converged on values in the 1 to 5 ug/m 3 range. Because of
uncertainties in the conditions of the different exposures and in
the potential for exposure from eating fish, there is a tendency
to rely more on the lower end of the risk of 1 ug/kg/day in fish
Over 39 states have issued fish consumption
advisories to reduce the potential hazards of sport fish high in
methyl mercury. It is not certain what the actual hazard is from
periodic ingestion of fish which have high levels of methyl mercury,
therefore public health agencies have adopted a risk reduction strategy
to limit potential exposures to mercury in the more sensitive members
of the population.
Deposition of mercury into lakes, streams and
watersheds is the first step in contamination of fish (EPA, 1996).
However the ultimate level of methyl mercury found in the fish is
determined by several factors related to the age and type of fish
and the nature of other biota in the water. Bioaccumulation appears
to increase as the ecological food webs of the water body become
more complex. Thus some ponds and lakes and rivers that appear to
be pristine often have surprisingly high levels of mercury in the
fish. Other lake which have become acidified also can have fish
with high levels of mercury.
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Last Updated: 1/31/03