A Toxological Analysis of Mercury

The most notable cases of severe methylmercury poisoning are from Minamata Bay, Japan in 1956, and Iraq in 1971. The Minamata Bay exposure was caused by an industrial release of methlymercury into the bay (a common site for fishing), and the Iraq exposure was due to wheat treated with a methylmercury fungicide. In both cases, hundreds of people died, and thousands were effected, many with permanent damage.

Human Exposure

• Trembling hands
• Numbness or tingling in their lips, tongues, fingers or toes
• Affected:
  

  Walking Coordination Vision Hearing Speech

*These effects can begin long after the exposure occurred, and are often reversible once exposure has ceased. Given sufficient quantities, methylmercury can be fatal.

Adults hair mercury concentration of less that 50 ppm (typically considered a safe level) is associated with disturbances of the visual system, and neuromotor deficits. This suggests that the current reference dose for adults based on 50 ppm in hair might not be sufficiently protective.

Carcinogenic Effects

The carcinogenic effects of methylmercury are inconclusive. Renal tumors have been seen in lab studies involving rats, but only at very high concentrations.

Immune System Effects

The effects of methylmercury on the immune system are still poorly understood, but studies are continually being conducted. The studies suggest that exposure to methylmercury could increase human susceptibility to infectious disease and autoimmune disorders by damaging the immune system.

Reproductive Effects

The reproductive effects stemming from methylmercury exposure are not fully evaluated yet. The Great Lakes Human Health Effects Research Program has found evidence that suggests that:

• Conception rate and the incidence of a live birth are lower in some women who are sport fish consumers.
• An association was found between men who consumed large amounts of sport fish and the risk of delayed conception in their spouses.
• Significant menstrual cycle reductions were indicated in women who reported consuming more than 1 meal per month of contaminated Great Lakes sport fish.

Kidney Damage

Organic and inorganic forms of mercury have been shown to damage the renal tubules and nephrons of the kidney. Similar effects were seen in animal studies.

Cardiovascular Effects

The cardiovascular system appears to be a target to methylmercury toxicity in both humans and animals. These effects have been shown to occur at very low exposure levels traditionally thought of as safe.

• Alters blood pressure regulation. This is seen in both adults and children at varying exposure levels and durations.
• Men who consumed at least 30 grams of fish a day, or had a hair mercury concentration of less than or equal to 2 ppm had a higher risk of acute myocardial infarction and increased risk of dying from heart disease.

Genotoxicity

The evidence that human exposure to methylmercury causes genetic damage is inconclusive. One study found higher rates of chromosomal aberrations in workers exposed to methylmercury and inorganic mercury. Unfortunately this result needs further study because it may have been under the influence of confounding factors.

Neurological

Methylmercury is a known teratogen to the fetal brain. Mothers can be asymptomatic to methylmercury exposure, but give birth to severely affected infants. This is because fetuses are 5-10 times more sensitive to methylmercury than adults are. These children often seem normal at birth, but develop psychomotor retardation, blindness, deafness, and seizures over time. The Great Lakes Human Health Effects Research Program has observed neurobehavioral and developmental deficits in newborns (12 to 24 hours after birth and again 25 to 48 hours after birth) of mothers who consumed approximately 2.3 meals per month of contaminated Lake Ontario fish.
The exposed children are now three years of age and initial test results for memory, verbal, and perceptual performance indicate their score is lower than children from mothers who consumed low amounts or no fish.

Animal Exposure

In its 1997 Mercury Study Report to Congress, the U.S. Environmental Protection Agency concluded that mercury also may pose a risk to wildlife populations that consume large amounts of fish that is contaminated by mercury. In studies in Wisconsin, reductions in loon chick production has been found in lakes where mercury concentrations in eggs exceed concentrations that are toxic in laboratory studies. These effects can become even more harmful to bird embryos due to the synergistic action of selenium when it is present in the mother’s diet.