Reproductive & Developmental Hazards:
Spotlight on Mercury
By Noah Chalfin
Maternal exposure to mercury via diet and airborne sources, especially in its organic
forms, is associated with reproductive toxicity, and in the case of methyl
mercury, profound neurobehavioral damage in the developing fetus.
Organic mercury (methyl mercury) is the most dangerous
form of mercury because it is the most easily absorbed in the maternal gastrointestinal
tract following ingestion, and it is readily transferred into the brain of
developing fetuses. Levels in fetal circulation may be significantly higher
than levels in maternal blood, and methylmercury appears in significant levels
in breast milk.
Bacteria in the environment transform other forms of mercury into organic
mercury, which is then taken up in algae eaten by fish, making its way into
the human diet. Contaminated fish, particularly carnivorous fish such as swordfish,
tuna, shark, and pike, are a major source of organic mercury exposure for
Elemental mercury is a significant hazard only when inhaled. People may be
exposed to mercury in the air from waste incinerators that are burning batteries,
fluorescent bulbs, or medical waste. Oil and coal burning facilities also
emit mercury into the atmosphere, since mercury is a contaminant of these
fuels. Once elemental mercury is in the body, it passes easily into the brain
and across the placenta to the fetus.
Organic mercury exposure has resulted in two large epidemics of poisoning
in recent history. One episode, in the area around Minamata Bay in Japan,
occurred in the 1950s, and the second series of outbreaks occurred in Iraq
in the late 1950s, early 1960s, and early 1970s, when imported seed grain
was treated with organic mercury to retard fungal growth. Instead of being
planted, the grain was used for bread making, and thousands of people were
poisoned. Although some adults developed symptoms, including constricted visual
fields, numbness of the fingers and toes, and even poor coordination, the
main victims of the exposure in both epidemics were children exposed before
and after birth.
Organic mercury selectively damages the developing brain. In the outbreaks
of poisoning in Japan and Iraq, infants had cerebral palsy, mental retardation,
incoordination, weakness, seizures, visual loss, and delayed development.
Often a child exposed to organic mercury in utero appeared fairly normal at
birth, with only slight abnormalities of reflexes and muscle tone, but later
had seizures, long delays in learning to walk and talk, and severe clumsiness.
At lower-dose levels, the only observed effects were abnormal muscle tone
and reflexes and mild developmental retardation when retested at an older
Health effects of organic mercury are similar in animal studies and the human
population. It is one of the best-understood developmental toxicants. Organic
mercury interferes with cell division and migration of cells in the developing
brain. Studies in mice have shown that cells in the developing brain stop
in the middle of cell division when exposed to organic mercury.
In addition, methylmercury binds to DNA and interferes with the copying of
chromosomes and production of proteins, processes that are essential to life.
Two major ongoing studies of people who eat a lot of fish - one in the Seychelles
Islands and one in the Faroe Islands - are attempting to evaluate the low-dose
effects of methylmercury on brain development. Preliminary results are conflicting,
with the Seychelles study showing little or no effect, and the Faroe study
showing subtle but significant impairment of brain function.
Based on the aforementioned Iraqi study, the U.S. EPA projected that the highest
chronic exposure to methylmercury tolerable without likely health effects
is 1.0 µg/kg body weight/day, and on that basis set a reference dose (RfD)
of 1.0 µg/kg/day.
The evidence of adverse health effects from elemental and inorganic mercury
exposure is not clear. These forms of mercury do not appear to affect the
developing brain like organic mercury does. Although animal studies indicate
that elemental mercury can damage male fertility, men occupationally exposed
to elemental mercury vapor did not have any apparent decrease in fertility
compared to a group of unexposed men, nor did their children have a greater
risk of malformations. Animal studies, however, have shown that elemental
mercury can be toxic to the fetus.
A different study of exposed male workers found a twofold-increased risk of
spontaneous abortion among their wives. Studies in women, mostly dental assistants,
have found conflicting results as to whether elemental mercury increases the
risk of spontaneous abortion. One large cohort study demonstrated spontaneous
abortion and other pregnancy complications in exposed women. Several additional
studies suggest that women occupationally exposed to elemental mercury may
have an increased risk of menstrual disorders, particularly heavy bleeding
and severe menstrual cramps.
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