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© 2007 CSA Released February 2007

Methylmercury Contamination in Fish and Shellfish By Laura Griesbauer

Introduction Methylmercury accumulation in seafood and fish products is a growing global concern that poses severe health risks to the public. While mercury occurs naturally, large amounts enter the envi-ronment from anthropogenic sources. In the U.S., legislation has been passed to reduce mercury pol-lution from human sources, for instance through capping mercury emissions from coal-burning power plants and banning certain products that contain mercury. Furthermore, concerns over the amount of mercury present in fish have also lead to guidelines for fish consumption and allowable mercury levels. Since fish has many other health benefits and is an excellent source of protein, it is important to understand the risks of mercury in the environment and in the seafood we eat. By understanding where mercury comes from, how it enters the environment, how it accumulates in seafood and the health effects of mercury, we can make educated decisions about the food we consume.

“Panel urges more fish-mercury safety advice” http://www.cnn.com/2003/HEALTH/diet.fitness/12/12/mercury.fish.ap/

Mercury and Its Sources

Human exposure to mercury begins with the production of many useful products. As the only metal on Earth that can be found in a liquid form at room temperature, mercury and mercury compounds have many uses. Due to its special properties, including high density and high rate of ther-mal expansion, mer-cury is often used in barometers and ther-mometers. It can also

be combined with other metals to create special alloys called amalgams. Gold and silver amalgams have been used in dentistry for fillings and tin amalgams are used to make mir-rors. Mercury can be found in many different lamps, including “black lights,” and is used

View of the Electrochemical factory at Skoghall, Sweden. The chloralkali, chlorine condensation and lye evaporation section are visibile the center. Mercury in Sediment and Fish Communities of Lake Vaenern, Sweden: Recovery from Contamination Lindestroem, L., Ambio, Vol. 30, No. 8, pp. 538-544. Dec 2001.

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Griesbauer: Methylmercury Contamination

in the industrial production of chloride and sodium hydroxide (Environment Canada, 2004). Some mercury compounds are used as ingredients in skin creams, antiseptics, diu-retics, fungicides, insecticides, and as a preservative in vaccines. Mercury compounds were even once used in the treatment of syphilis (Clarkson & Magos, 2006).

CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released February 2007

While some of these compounds are fairly inert, many mercury compounds are ex-tremely toxic. In the U.S., some products containing mercury have been banned, have usage limits, or have special disposal requirements. These include dental fill-ings, vaccines, non-industrial thermome-ters, lamps, car starters, and electronics. There are also many regulations regarding the disposal of mercury wastes (EPA, 2006). One form of mercury that is toxic and very harmful is elemental mercury. It is highly volatile and can easily be converted to mercury vapor, exposure to which can damage the nervous system, lungs, and

kidneys. This type of exposure generally only happens to industrial workers directly han-dling mercury compounds. They are exposed either by inhaling mercury vapor or through chronic contact with volatile inorganic mercury com-pounds. For most people, exposure to mercury occurs when they eat fish or shellfish contaminated with me-thylmercury, an organic mercury compound. This com-pound is found in nearly all freshwater and marine fish (EPA, 2006). Methylmercury has the ability to be ab-sorbed by the digestive tract and enter the blood stream, which, over time, can result in damage to the nervous system.

Conservation Law Foundation page http://www.clf.org/programs/projects.asp?id=575

Vermont Department of Environ-mental Conservation http://www.anr.state.vt.us/dec/ead/mercury/dispose/index.htm

Mercury vapor is emitted to the atmosphere through both natural and anthropogenic sources. Natural sources of mercury vapor include volcanoes, as well as rocks, soils and water surfaces. Mercury is also found natu-rally in cinnabar, the major ore for the production of mercury. Anthropogenic sources of mercury vapor in-clude emissions from coal-burning power plants, mu-nicipal incinerators, and through the recycling of auto-mobiles (Clarkson & Magos, 2006). It is estimated that 50 to 70 percent of the total emission of mercury to the environment is a result of human activity. About 1,000

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tons of mercury per year is emitted to the atmosphere by natural sources, while more than twice as much, about 2,600 tons, is emitted from anthropogenic sources (Honda et al., 2006). Mercury emitted from all these sources is then cycled through the ecosystem. Mercury in the Environment

Once in the atmosphere, mercury vapor (Hg°) is slowly converted by oxidative processes to diva-lent mercury (Hg2+), which is then returned to the earth’s sur-face by rainfall, where it accu-mulates in soils and in surface waters. Some of the mercury load is then converted back into mer-cury vapor (Hg°) and returned to the atmosphere. However, an-other fraction of the mercury load (Hg2+) is washed into rivers, streams, and eventually the ocean where it accumulates in aquatic sediments. It is here that inor-

ganic mercury is converted to methylmercury (MeHg) by microorganisms living in the sediments by a process called methylation (Clarkson & Magos, 2006).

Tennessee Department of Health, Environmental Epidemiology http://www2.state.tn.us/health/CEDS/mercury.htm

This methylmercury then enters the food chain when it is ab-sorbed by phytoplankton species. Phytoplankton are eaten by plankton consumers, which then are eaten by larger and larger fish. Methylmercury accumulates in the tissues of fish and shellfish via a process called biomagnifi-cation, through which methyl-mercury concentration increases as it moves up from one trophic level to the next. Within each organism, methylmercury bioac-cumulates as the organism con-sumes more and more organisms

containing methylmercury. Thus, smaller fish that are lower down in the food chain have lower concentrations of mercury in their tissues while larger fish that are higher up in the food chain have higher concentrations. For example, sardines contain about .01 ppm of mercury while sharks contain from 1 ppm to as much as 4 ppm (EPA, 2006). Fish with

Environment Canada, Mercury Page http://www.ec.gc.ca/MERCURY/EN/bf.cfm


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the highest levels of mercury include sharks, swordfish, and king mackerels. Large ma-rine mammals such as whales have levels similar to these fish.

Mercury in the Body and Health Effects

The regression between the mercury concentration in pike (Esox lucius) and body weight (left) or the age of the fish (right). Based on 150 fish caught in Kattfjorden in 1988, 1991 and 1994 (p < 0.05 and p < 0.001, respectively). (ww = wet weight) Mercury in Sediment and Fish Communities of Lake Vaenern, Sweden: Recovery from Contamination Lindestroem, L., Ambio, Vol. 30, No. 8, pp. 538-544. Dec 2001

Both inorganic and organic forms of mercury are highly toxic to humans; however, inor-ganic 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 nerv-ous 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, lungDivalent 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. Methylmer-cury is naturally removed from the body over time. Eventually, this methylmercury-cys-teine complex is transported to the liver where it is secreted into bile, after which en-zymes 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 CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released February 2007

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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 methyl-mercury 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 mer-cury levels to drop significantly (CFSAN, 2004). Many adverse health effects are associated with the ac-cumulation 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 ob-served in people who consumed fish that were contaminated directly by methylmercury from anthropogenic sources, not from methylmercury that accumulated through the natu-ral methylation process (Clarkson & Magos, 2006).

“Silent Latency Periods in Me-thylmercury Poisoning and in Neurodegenerative Disease” http://www.ehponline.org/members/2002/suppl-5/851-854weiss/weiss-full.html

The Minamata Bay Incident In the 1950’s, one of the most severe incidents of industrial pollution and mercury poi-soning occurred in the small seaside town of Minamata, Japan. A local petrochemical and plastics company, Chisso Corporation, dumped an estimated 27 tons of methylmercury into the Minamata Bay over a period of 37 years. Mercury was used as a catalyst in the production of acetaldehyde, a chemical employed in the production of plastics. Methyl-mercury-contaminated wastewater, a byproduct of the process, was pumped into the bay, creating a highly toxic environment that contaminated local fish. Residents of Minamata, who relied heavily on fish for food, were at risk of exposure to methylmercury with every bite of fish they ate. The high contamination levels in the people of Minamata led to se-vere neurological damage and killed more than 900 people. An estimated 2 million peo-ple from the area suffered health problems or were left permanently disabled from the contamination (McCurry, 2006). This form of toxicity in humans is now called Minamata disease. Symptoms include sensory disorders of the four extremities, loss of feeling or numbness, cerebellar ataxia, tunnel vision or blindness, smell and hearing impairments,

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and disequilibrium syndrome. More se-rious cases lead to convulsions, seizures, paralysis, and possibly death. In addition to the outbreak among the townspeople, congenital Minamata disease was ob-served in babies born to affected moth-ers. These babies demonstrated symp-toms of cerebral palsy (Honda et al., 2006). Doctors struggled to diagnose the myste-rious disease when it first was noticed in the early 1950’s. Local cats were seen acting strangely before falling over and birds would fall from the sky. In 1959, doctors at Kumamota University deter-mined that organic mercury poisoning

was the cause of the symptoms exhibited by so many of the townspeople. However, it was not until 1968 that the Tokyo government acknowledged that the mercury dumping by Chisso was the ultimate cause. Five years later, Chisso admit-ted legal responsibility for the dumping. Yet the environ-mental impact on the bay had already occurred. In 1977, the Japanese government took on the huge task of cleaning the sediments in the bay by vac-uuming up 1.5 million cubic

meters of mercury-contaminated sludge. After $359 million dollars and 14 years, the pro-ject was completed in 1997 (McCurry, 2006).

Corrosion Doctors http://www.corrosion-doctors.org/Elements-Toxic/Minamata-1.htm

Jeremy Sutton-Hibbert http://www.thelancet.com/journals/lancet/article/PIIS0140673606679440/fulltext

Fish Consumption Advisories and Mercury Levels While the danger of mercury poisoning may seem like a good reason to refrain from con-suming fish, the benefits of eating fish may outweigh many of the risks. Fish is high in protein, low in saturated fats, and contains important nutrients such as heart healthy omega-3 fatty acids. One fatty acid found in fish oils, Docosahexaenoic acid (DHA), is one of the most important fatty acids for normal brain development and function (Saka-moto et al., 2004). It is possible that DHA may even counteract the negative effects of mercury though this relationship has not yet been proven scientifically. Eating fish has

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also been found to reduce the risk of heart attacks, lower blood pressure, and improve arterial health (Senkowsky, 2004). Mercury Levels in Commonly Consumed Fish and Shellfish Species - Fish Mercury Concentration (ppm) Shark 0.988 Swordfish 0.976 Tuna (Fresh/Frozen, Bigeye) 0.639 Chilean Bass 0.386 Tuna (Fresh/Frozen, Albacore) 0.357 Tuna (Canned, Albacore) 0.353 Halibut 0.252 Tuna (Canned, Light) 0.118 Cod 0.095 Trout (Freshwater) 0.072 Anchovies 0.043 Sardine 0.016 Salmon (Fresh/Frozen) 0.014 Tilapia 0.01 Salmon (Canned) ND Species - Shellfish Mercury Concentration (ppm) Lobster 0.31 Crab 0.06 Squid 0.07 Scallop 0.05 Crawfish 0.033 Oyster 0.013 Clam ND Shrimp ND ND=Mercury concentration below detection level Source: CFSAN, US FDA, Feb. 2006 The greater concern for mercury exposure is not to an adult human, but to a developing fetus. As seen in the extreme Minamata case, some mothers showed no outward signs of mercury poisoning, but gave birth to children with severe brain damage (Clarkson & Ma-gos, 2006). Studies have shown a correlation between prenatal exposure to mercury and decreased ability of infants and children on neurobehavioral tests including tests of atten-tion, fine motor function, language skills, visual-spatial abilities and memory (Shea et al., 2004). This is because methylmercury readily crosses the placenta through blood circula-tion, and fetal blood concentration of mercury is slightly higher than maternal levels. Me-thylmercury can also be passed through breast milk to infants and consumed by young CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released February 2007

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children in their diets. This is of concern since young children are potentially more sus-ceptible to toxicity from mercury and the brain may be more affected as it continues to develop during the first years of life (Shea et al., 2004). Based on the growing body of evidence concerning the health issues of methylmercury accumulation in the body, the U.S. Environmental Protection Agency (EPA) and the U.S. Food and Drug Administration (FDA) have issued advisories targeting consumption of fish for specific groups. Their advice to women who may become pregnant, pregnant women, nursing mothers, and young children up to age six is to avoid certain types of fish high in methylmercury and limit the amount of fish consumed each week. Specifi-cally, the EPA and FDA advise these groups not to eat shark, swordfish, king mackerel, or tilefish at all because they contain very high levels of mercury (>1 ppm). They also advise these groups to eat up to 12 ounces (or two average meals) a week of fish and shellfish that are low in mercury. Children should only eat six ounces of fish. Low mer-cury fish and shellfish include shrimp, canned light tuna, pollock, salmon and tilapia. Al-bacore tuna is a commonly eaten fish but contains moderate amount of mercury. The EPA and FDA advise eating only 6 ounces of albacore tuna a week. Also, if you exceed the suggested amount of fish or shellfish in a week, simply cut back the amount con-sumed the next week or two. Lastly, the EPA and FDA advise the public to check for lo-cal advisories on fish caught from local lakes, rivers, and streams. These fish may be more greatly affected by anthropogenic pollution sources (CFSAN, 2006). The above guidelines are not aimed at adult men, or woman past child bearing age, but individuals concerned with possible exposure to mercury should follow them as well. Conclusion While these health advisories are targeted mostly at women and children, the exact ef-fects of mercury accumulation in the body over time are not yet know. More studies are being conducted to determine the risks. Most adverse effects in adult humans have only been seen with toxic doses of methylmercury, such as with the Minamata incident where fish had mercury levels of 20 ppm. The FDA level of concern for mercury in fish is 1 ppm. Fish with levels higher than this should probably be avoided by everyone. However, the consumption of fish and shellfish should not be completely eliminated because they are an important part of a healthy diet. Balancing the risks of mercury exposure and the benefits of fish consumption is essential to proper nutrition.


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References Center for Food Safety & Applied Nutrition (CFSAN), United States Food and Drug

Administration. (2004). What You Need to Know About Mercury in Fish and Shellfish. http://www.cfsan.fda.gov/~dms/admehg3.html

Center for Food Safety & Applied Nutrition (CFSAN), United States Food and Drug

Administration. (2006). Mercury Levels in Commercial Fish and Shellfish. http://www.cfsan.fda.gov/~frf/sea-mehg.html

Clarkson, T. W., & Magos, L. (2006). The toxicology of mercury and its chemical com-

pounds. Critical Reviews in Toxicology, 36(8), 609-662. Environment Canada. (2004). Mercury and the Environment.

http://www.ec.gc.ca/mercury/en/index.cfm Honda, S., Hylander, L., & Sakamoto, M. (2006). Recent advances in evaluation of

health effects on mercury with special reference to methylmercury: A minireview. Environmental Health and Preventive Medicine, 11(4), 171-176.

McCurry, J. (2006). Japan remembers minamata. Lancet, 367(9505), 99-100. Sakamoto, M., Kubota, M., Liu, X. J., Murata, K., Nakai, K., & Satoh, H. (2004). Mater-

nal and fetal mercury and n-3 polyunsaturated fatty acids as a risk and benefit of fish consumption to fetus. Environmental science & technology, 38(14), 3860-3863.

Senkowsky, S. (2004). Fear of fish: The contaminant controversy. Bioscience, 54(11),

986-988. Shea, K. M., Perry, K. L., & Shah, M. (2004). Health effects of methylmercury. Physi-

cians for Social Responsibility publication. Washington, DC: Physicians for So-cial Responsibility.

United States Environmental Protection Agency (EPA). (2006). Mercury.

http://www.epa.gov/mercury/


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minamata bay - ms. thompson's oceanography site!!

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