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Methylmercury Contamination in Fish and Shellfish
(Released February 2007)

 
  by Laura Griesbauer  

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Mercury in the Body and Health Effects

Contents

Both inorganic and organic forms of mercury are highly toxic to humans; however, inorganic mercury is not as easily absorbed by the body. Inorganic mercury, such as mercury vapor, is toxic if inhaled in large concentrations and can cause acute pneumonia. Inhaled gaseous mercury is absorbed into the blood. Once in the circulatory system, it can pass through the blood-brain barrier and accumulate in the brain, damaging the central nervous system. As the body tries to rid itself of these toxins, gaseous mercury is oxidized to divalent mercury, which accumulates in the kidneys and can cause kidney damage (Honda et al., 2006).

What organs does mercury target?
Chemical form of mercury Target organ
Elemental (Gaseous) Hg0 Brain, kidney, lung
Divalent Hg2+ Kidney
Methyl CH3-HG+ Brain, fetal brain

Most people are not exposed to inorganic mercury but rather absorb methylmercury through the consumption of fish and shellfish. Methylmercury is easily absorbed in the digestive tract, where it forms a complex with the amino acid cysteine. This new complex resembles a large neutral amino acid found in the body, methionine, and can more easily gain entry into cells. As with inorganic mercury, once in the bloodstream, methylmercury will accumulate in the brain and cause damage to the central nervous system. Methylmercury is naturally removed from the body over time. Eventually, this methylmercury-cysteine complex is transported to the liver where it is secreted into bile, after which enzymes break the complex down into its amino acid and methylmercury parts. Some of this methylmercury then comes in contact with the bacteria in the intestine and is broken down into inorganic mercury and carbon. The inorganic mercury is poorly absorbed in the digestive tract and 90 percent is excreted in the feces. The rest of the methylmercury that does not interact with bacteria is reabsorbed by the body and goes through the process again. It takes about 30 to 40 hours for methylmercury to be distributed to the tissues of the body (Clarkson & Magos, 2006). This cycle is the reason it takes so long to rid the body of mercury and how it can accumulate in the blood. It can take up to a year for mercury levels to drop significantly (CFSAN, 2004).

brain
Silent Latency Periods in Methylmercury Poisoning and in Neurodegenerative Disease
Many adverse health effects are associated with the accumulation of mercury in the body, though these vary depending on the amount of mercury one is exposed to, time of exposure, chemical form of the mercury, and age of the subject. Methylmercury, the most easily absorbed form of mercury, is a very potent neurotoxin that interferes with brain development. Once in the brain, it interferes with nerve cell differentiation and cell division by binding DNA and RNA. It can cause nerve cell death and scarring in selected areas of the brain (Shea, Perry, & Shah, 2004). With methylmercury exposure, paresthesia is the first and mildest symptom observed, where a tingling or numbness is felt in the hands, arms, legs, or feet, but can also occur in other parts of the body. In the case of methylmercury poisoning, this numbness is the first sign of damage to the nervous system (Clarkson & Magos, 2006). Other symptoms that may follow a higher dose of methylmercury poisoning are ataxia (stumbling or clumsy gait) and generalized weakness. Higher doses of methylmercury poisoning may lead to dysarthria, loss of vision and hearing, tremor, and finally, coma and death (Shea et al., 2004). To date, these more severe symptoms have only been observed in people who consumed fish that were contaminated directly by methylmercury from anthropogenic sources, not from methylmercury that accumulated through the natural methylation process (Clarkson & Magos, 2006).

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