Mercury is a potent neurotoxin that causes serious adverse health effects from exposure to either the inorganic or organic forms.

Exposure to inorganic mercury 

Inorganic mercury can enter the body through inhalation or, less commonly, through absorption through the skin.  As much as 80 percent of inhaled mercury can be absorbed into the bloodstream.  Some ways that people are exposed to inorganic mercury include:

• Dental amalgams – inorganic mercury used in dental amalgams is released in vapor form and then can be inhaled.

• Accidental exposure – can occur through a broken mercury thermometer or when kids play with metallic mercury, as was recently the case in Rosemount (September 2004).  Metallic mercury is liquid, so it breaks down into smaller and smaller quantities that can then be inhaled.

• Occupational exposure – Dentists, dental hygienists, nurses, lab technicians, miners, plumbers, and machine operators may be exposed to higher levels of inorganic mercury.

Human health effects of inorganic mercury exposure

Inorganic mercury can attack the liver and kidneys.  Short-term high exposures can cause poisoning, respiratory and gastrointestinal problems, mood changes, and nerve damage, which manifests in tremors, neuromuscular changes, headaches, and memory loss.  Long-term, low-level exposure can cause kidney and nerve damage, resulting in muscle tremor, irritability, personality changes, and gingivitis. (1)

Exposure to organic mercury (methylmercury)

The most common way that people are exposed to methylmercury is through fish consumption.  When we eat fish, the gut absorbs 95 percent of the methylmercury. (2)  Fetuses are exposed to methylmercury in the mother’s body through the mother’s diet.  Because mercury is tightly bound to proteins in all fish tissue, including muscle, there is no method of cooking or cleaning fish that will remove mercury. (3)  Humans can eliminate mercury from the body over a period of months.  However, when the amount of mercury ingested exceeds the ability of the body to eliminate it, mercury builds up in the body over time.  Thus, even small amounts of mercury in the diet over a long period of time can potentially cause health effects. 

Human health effects of methylmercury exposure

Methylmercury is a potent neurotoxin, meaning it is toxic to the brain.  Chronic exposure to methylmercury is associated with toxicity to the central nervous system.  It affects the brain, spinal cord, kidneys and liver.  Methylmercury passes through the placenta and is excreted into breast milk, so it can interfere with normal fetal and infant development, preventing the brain and nervous systems from developing normally.  Even low-level exposure over time, as from maternal fish consumption, can affect a child’s learning and abilities, including reduced intelligence, impaired hearing and memory, poor coordination or delayed motor and verbal skills.

Three long-term studies examined health effects from chronic low dose exposure from maternal fish consumption.  Two studies, in the Faroe Islands and New Zealand, found that offspring of mothers who ate more fish before and during pregnancy experienced adverse effects on learning and development. (4), (5)  A third study in the Seychelles Islands did not find neurodevelopmental effects. (6)  The National Academy of Sciences affirmed the Faroe Island study as the critical study on which to base health-based guidelines for fish consumption, which are used by the EPA and the states. (7)

Wildlife effects of methylmercury exposure 

Methylmercury similarly affects fish-eating wildlife like loons, osprey, turtles, otters, mink and eagles.  Health effects in wildlife from eating mercury-contaminated fish are impaired reproduction, weight loss and neurological impairment. (8) The latter can result in early death, because reduced speed and coordination puts animals at a predatory disadvantage.

Exposure to organic mercury (ethylmercury)

The main route of exposure to ethylmercury is through vaccines with the sterilizer Thimerosal, which contains 49.6 percent mercury.  Thimerosal breaks down to ethylmercury in the human body.  Because of concerns that some infants were receiving excessive doses of mercury early in life, in 1999, the Food and Drug Administration asked pharmaceutical manufacturers to voluntarily remove Thimerosal from childhood vaccines.   It has been removed from many vaccines, but it is still present in most flu vaccines and most tetanus vaccines.

Human effects of ethylmercury exposure

Like methylmercury, ethylmercury is toxic to the brain and crosses the blood-brain barrier. (9)  "Higher-dose exposure to ethylmercury from Thimerosal results in toxicity comparable to that observed after high-dose exposure to methylmercury." (10)  High doses of ethylmercury can cause mercury poisoning, renal failure, and nerve damage. (11)  Other health effects from high exposures can include mental retardation in children; loss of coordination in speech, writing, and gait; stupor; irritability; and bad temper progressing to mania. (12)  Lower exposures at the level received by infants from Thimerosal in vaccines may be associated with neurological damage, as demonstrated in animal studies (13) and may be contributing to the increasing incidence of autism in young children. (14), (15)

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Notes

(1)     U. S. Environmental Protection Agency (EPA) http://www.epa.gov/ttn/atw/hlthef/mercury.html

(2)     National Research Council, 2000. Toxicological Effects of Methylmercury, National Academy Press, Washington, D.C.

(3)     Minnesota Department of Health, http://www.health.state.mn.us/divs/eh/fish/eating/tips.html

(4)     Grandjean P, et al. 1997. Cognitive deficit in 7-year old children with prenatal exposure to methylmercury. Neurotoxicol Teratol 19(6):417-428)

(5)     Crump, KS, et al. 1998. Influence of prenatal mercury exposure upon scholastic and psychological test performance: benchmark analysis of a New Zealand cohort. Risk Anal 18(6):701-713)

(6)     Davison PW et al, 1998. Effects of prenatal and postnatal methylmercury exposure from fish consumption on neurodevelopment: outcomes at 66 months of age in the Seychelles Child Development Study. JAMA 280(8):701-7.

(7)     National Research Council, 2000. Toxicological Effects of Methylmercury, National Academy Press, Washington, D.C.

(8)     Heinz G et al, Hazards of mercury to wildlife, Patuxent Wildlife Research Center, U.S. Geological Survey, available at http://www.netl.doe.gov/publications/proceedings/03/valuing-ext/abstracts/Heinz-abstract.pdf

(9)     Slikker, W, 2000. Developmental neurotoxicology of therapeutics:  Survey of novel recent findings.  Neurotoxicology 21(1-2): 250.

(10)Ball, LK et al, 2001. An assessment of thimerosal use in childhood vaccines.  Pediatrics 107(5):1147-1154.

(11)Goldman L et al, 2001. Technical report: Mercury in the environment: Implications for pediatricians, Pediatrics 108 (1): 197-205.

(12)U.S. Department of Health and Human Services, In nomination of Thimerosal to The National Toxicology Program, National Institutes of Health’s National Institute of Environmental Health Sciences

(13)Hornig, M, 2004. Neurotoxic effects of postnatal thimerosal are mouse strain dependent, Molecular Psychiatry 9(9): 833-845.

(14)Bernard S, 2001. Autism: a novel form of mercury poisoning, Medical Hypotheses 56(4): 462-471.

(15)Geier, MR, Geier, DA, 2003. Neurodevelopmental disorders following Thimerosal-containing vaccine.  Experimental Biology and Medicine 228: 660-664.

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