Health Effects of Mercury

The most dangerous form to human health and the environment is organic mercury, primarily in the form of methyl mercury.  Methyl mercury is formed from mercuric ions (Hg²⁺) by microbial activity in either soil or water.    As methyl mercury accumulates in our lakes, streams, and oceans, it may enter our aquatic and terrestrial food chains.  Although the concentration of methyl mercury in our waters may be low, it bioaccumulates.  Fish and other aquatic organisms absorb methyl mercury through their gills and as they feed on other aquatic organisms.  Larger predator fish are exposed to larger amounts of methyl mercury from their prey.  Average methyl mercury concentrations in predatory fish are approximately 7 million times higher than concentrations found in the surrounding waters. ¹

      Methyl mercury is bound tightly to the proteins in fish tissue.  Skinning and trimming the fish does not significantly reduce the mercury concentration in the fish fillets and cooking does not reduce the concentrations.  Humans consuming contaminated fish are therefore in danger of methyl mercury poisoning.

      Based on methyl mercury poisonings and deaths that occurred in the Minamata Bay in Japan between 1953 and 1960 from contaminated fish and in Iraq between 1971 and 1972 from contaminated grain, a “safe” reference dose (RfD) for methyl mercury was determined.  This amount for humans is 1 x 10^-4 mg/kg/day, which ranges between 5.5 x 10^-3 mg/day to 1.0 x 10^-3 mg/day for a person weighing between 120 and 220 pounds, respectively.   Based on this dose, safe consumption limits of fish have been determined.

    The U.S. Food and Drug Administration (FDA), the Environmental Protection Agency (EPA), States, and Tribes are issuing Fish Consumption Advisories.  The FDA has set a limit of 1 part per million of mercury in fish for human consumption.  Although most fish species contain mercury concentrations of 0.01 to 0.5 ppm, some predator fish, particularly swordfish, shark, and some species of large tuna exceed this limit.  As of December 1998, 40 U.S. states have issued fish consumption advisories.  Delaware has issued mercury warnings for five water bodies in the state including Beck’s Pond, the Delaware River, the Lower Delaware River and Delaware Bay, the St. Jones River, and Silver Lake.  The advisory limits consumption of finfish to less than one eight-ounce meal per year.  

        The health effects of methyl mercury depend on the quantities consumed.  Health effects of methyl mercury poisoning have been observed and documented from studies performed on the people affected by the Japan and Iraq incidents.   Methyl mercury is a neurotoxic which affects the brain and spinal cord.  It is completely absorbed by the digestive system where it enters the blood and is distributed throughout the body.  The earliest signs of methyl mercury poisoning appear as tremors in the hands and as a numbness or tingling sensation of the lips, fingers, toes, or tongue, known as paresthesias.  Greater doses of methyl mercury may result in sensory disturbances, deafness, blurred vision, decreased peripheral vision, speech difficulties, motor skill problems, and an impaired level of consciousness.  Lethal doses of methyl mercury result in paralysis and death.  The estimated lethal dose of methyl mercury for a 70 kg (154 lb) person ranges from 20 to 60 mg per kilogram of body weight.

          A study of Hg in children 5-14 yrs. old was conducted in Germany.  Blood and urine were examined from children in a highly industrialized area none to have high Hg concentrations and were compared to those from a non-polluted area.  There was no significant difference between the two.  Also, variations with in the groups were found to be directly relatable to numbers of dental amalgam fillings.

    The greastest risk of methyl mercury poisoning, however, occurs with developing fetuses.  High levels of exposure to methyl mercury during development of the fetus interferes with the way brain cells move into position in the brain and therefore results in abnormal development.  Developmental abnormalities in the fetus may be observed at levels four to five times lower than levels that have abnormal affects in an adult.  In the methyl mercury poisoning that occurred in both Japan and Iraq, mothers who had few symptons of methyl mercury poisoning gave birth to children with severe mental and physical retardation.  The effects of methyl mercury poisoning depend on the dose received.  Mild effects include deficits in language, cognitive, and motor skill development.  Severe effects include cerebral palsy, mental and physical retardation, blindness, deafness, convulsions, and even death.

      Due to the lower levels of inorganic (primarily Hg⁺² forms) and elemental mercury in the ambient of air, adverse health effects are not observed from inhalation.  The EPA has concluded that the expected death rate for exposure to mercury in the air are less than the acceptable rate of one person in one million.