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 many people.

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 age.

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.

Citation of references has been excluded for the sake of brevity. Please contact the author noah@cetos.org for fully cited article reprints, or with questions pertaining to a specific point.