A Review of Mercury in Seafood: special focus on tuna

Mercury is a toxic heavy metal released into the environment from both natural and anthropogenic sources. It is of great interest to consumers as to whether it can cause neurological effects at low dose levels. The effects of organic mercury exposure at high levels have been demonstrated in several large-scale poisonings, particularly those in Japan and Iraq in the 1950s, ‘60s and ‘70s. These epidemics showed that organic mercury, in sufficient concentrations, is a potent neurotoxin that is especially harmful to the developing nervous system. Since the most common form of human exposure to organic mercury is through fish consumption, several epidemiological studies have examined the relationship between maternal fish intake and health effects in humans, especially the fetus. Levels of mercury in fish vary depending on factors such as: trophic level in the food chain, size, and habitat location. For this reason, it is important to gather information on mercury levels in different types of fish in various parts of the world. Results of recent studies have caused the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) to issue new advisories on the fish consumption for pregnant women and young children. However, there is concern that other individuals will significantly reduce their fish consumption also, thereby decreasing the potential health benefits of seafood. This review is meant to promote understanding of current issues regarding mercury in seafood and provides a compilation of up to date information on the following topics: background information on mercury; large scale mercury poisonings; epidemiology studies and risk assessment; and studies on mercury in tuna in different geographical locations. Background Information on Mercury Mercury is a heavy metal with an atomic weight of 200.59 g/mole and a boiling point of 356.6°C. It was originally called quicksilver by Aristotle circa 350 B.C. (Hunter, 1975) because it is a dense, silvery-white liquid at room temperature. Mercury exists in three oxidation states: Hg (metallic or elemental), Hg (mercurous), and Hg (mercuric). In the mercuric state, it can bind to elements such as chlorine, sulfur, and oxygen to form inorganic compounds, or it can bind to 1-2 carbon atoms to form organic mercury. Mercury bound to a single methyl group is called methylmercury (MeHg or CH3Hg), mercury bound to two carbons is called dimethylmercury (DMHg or (CH3)2Hg), and mercury bound to an ethyl group is termed ethylmercury (EtHg or CH3CH2Hg). It was formerly thought that organic mercury existed in fish and human bodies in the form of MeHg+ or MeHgCl, but a recent study has reported that methylmercury present in fish is bound to cysteine, not chlorine (Harris et al., 2003). It was theorized that methylmercury does not bind to chlorine until it reaches the human stomach, where acidic conditions provide for many free Clions. MeHgCl has very low water solubility compared to other forms of organic and inorganic mercury (NRC., 2000), allowing it to cross membranes, such as the blood-brain and placental barriers, more rapidly . Mercury is distributed throughout the world, and cycles naturally through the earth’s crust, atmosphere, oceans, and life forms, with trace amounts in fish, plants, and animals. It can be found in natural deposits, such as the mercury bed under the Mediterranean Sea, which holds some of the richest reserves of mercury in the world (Bacci, 1989). The main ore of mercury is the red sulphide cinnabar (HgS), which has been mined throughout the world in places such as: Spain, Italy, Yugoslavia, Russia, China, Japan, Mexico, California, and British Columbia (Hunter, 1975). Mercury is naturally emitted into the air as a result of off-gassing from the earth’s surface and from volcanoes. Mercuric vapor can remain in the atmosphere for significant amounts of time and travel long distances before it cycles back to the earth in rainwater. Human activity increases the amount of mercury emitted into water, air, and soil, through coal-combustion (used to power utility plants); mining; waste disposal; preparation of fungicide seed dressings in agriculture; and chlor/alkali plants (BNS., 1999; WHO., 2000). Mercury is used in products such as batteries, vapor discharge lamps, fluorescent bulbs, switches, and thermometers. Many of these products end up in landfills or incinerators. Dental amalgams also contain mercury and are the major source of human exposure to elemental mercury (NRC., 2000). Although mercury and mercury compounds are hazardous air pollutants according to the Clean Air Act, mercury continues to be a valuable compound for industry. It has several unique properties, such as the ability to: conduct electricity, form alloys with almost all other metals, act as a biocide, and measure temperature. Mercury enters the food chain with the help of aquatic microorganisms. Released into the environment in inorganic form, mercury is methylated by bacteria in water and converted to an organic form, usually methylmercury. Organic mercury compounds are generally more hydrophobic, readily bind proteins, and are neurotoxic (Costa, 1988; NRC., 2000). Organic mercury bioaccumulates in aquatic organisms as it works its way up the trophic levels of the food chain. Although humans can come into contact with mercury through contaminated food, drink, or air, exposure to organic mercury is almost exclusively a result of consumption of fish and shellfish (Gunderson, 1995; NRC., 2000). Storelli et al. (2002) estimated that fish consumption accounts for 80-90% of the total exposure to mercury, of which 75 to 100% is methylmercury. Also, blood mercury concentrations were reported to be directly related to the amount of fish consumed (Mahaffey et al., 2004). Mercury Poisoning Outbreaks The health effects of mercury poisoning vary depending on the form of mercury and means of exposure; however, both organic and inorganic, acute and chronic exposure can cause severe, irreversible effects. Inorganic mercury poisoning was recorded as early as 415 B.C., in Almaden, Spain, when the Romans noticed it in slaves who worked to recover mercury from cinnabar mines (Hunter, 1975). Over centuries, its toxic properties have been observed in those whose professions involve working with mercury, such as: mirror manufacturers, felt hatters, amalgam platers, and goldsmiths. The first documented use of organic mercury in chemical research was in 1863, and there were reported incidents of illness and death within several years (Hunter, 1975).