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Mercury in Fish and Environment: Health Effects

Mercury is a highly poisonous silvery metallic element that is included in the World Health Organisation’s list of ten chemicals of major public concern. This heavy metal is liquid at room temperature, releasing toxic, odorless vapor into the air. Metallic mercury occurs rarely in nature. Mercury is mostly present in the form of mercury sulfide (cinnabar), which is used to produce elemental mercury. Methylmercury, found mainly in fish and shellfish, happens to be the most toxic form of mercury. Most forms of mercury primarily affect the central and peripheral nervous system but they can also be toxic to the digestive system, kidneys, lungs, eyes and skin.

In the past, elementary mercury has been frequently used to make thermometers. Today, it is still used in industrial barometers and manometers, electrical switches and fluorescent light bulbs. Mercury forms alloys with other metals known as amalgams. Amalgams find applications in gold mining and in dentistry. Dental amalgam contains about 50% mercury and the rest are silver, tin and copper. Mercury compounds can also be found in some laxatives, diaper rash ointments, topical antiseptics, nasal sprays and oral drops but medicinal use of mercury is declining.

Mercury finds its way into the human body through inhaling mercury vapor (including vapor from dental fillings), drinking contaminated water and consuming foods high in mercury such as fish and shellfish. Herbal teas are surprisingly the second largest dietary source of mercury exposure after seafood. In some cases, mercury can also enter the body through the skin. Even if the amount of mercury in water or food is not high enough to cause immediate poisoning, it can accumulate in the body to reach toxic levels. Mercury also tends to build-up in the fish and shellfish and thus becomes highly concentrated in the food chain.

Human activity, including coal burning, incinerating municipal waste, and gold and mercury mining are the greatest contributors to mercury pollution. Volcanic eruptions are the second biggest source of mercury releases. Bacteria that live in the ocean readily transform elemental mercury into yet more poisonous methylmercury.

Health Hazards of Mercury

Most people get exposed to methylmercury (mainly from consuming seafood and herbal teas) or to vapor of elemental mercury (e.g., from broken thermometer or to a lesser extent from amalgam fillings – for more information about dental amalgam read the article “Mercury Poisoning from Dental Fillings”). Both these forms of mercury are highly toxic but in a slightly different way. Exposure to inorganic mercury is rare. Generally, mercury and most of its compounds are powerful neurotoxins for people of all ages but they can be particularly harmful to the developing brain and nervous system of unborn babies and little children.

Exposure to methylmercury may cause muscle weakness, hyperexcitability, lack of coordination of movement, tremors, loss of peripheral vision, impairment of walking, hearing and speech in adults and delays in cognitive development and impairment of attention, memory, language, visual spatial skills and fine motor skills in young children.

Acute poisoning from exposure to vapor of elemental mercury may produce symptoms such as metallic taste in the mouth, chills, diarrhea, nausea, vomiting, muscle weakness, cough, laboured breathing and a feeling of tightness in the chest. Chronic exposure typically causes neurological problems including emotional changes, insomnia, headaches, changes in nerve responses, disturbance in sensations, impairment of mental function, incoherent speech as well as muscle atrophy, impairment of the immune system and severe gingivitis. Other organs may also be affected including the skin, eyes, kidneys and digestive tract. Severe exposure to elemental mercury may eventually lead to kidney and respiratory failure and even result in death.

Fish, Shellfish and Mercury

For most people in the developed world, sea fish and shellfish is by far the biggest source of mercury exposure. Mercury gets into the ocean from polluted air and rainwater, contaminated rivers and naturally occurring minerals, while large amounts of mercury can be released into the ocean during volcanic activity. The North Pacific and the North Atlantic are among the ocean regions most contaminated with mercury and, in case of the North Pacific, the levels are expected to double by 2050. Since mercury accumulates in the bodies of sea life, the situation is only going to get worse.

Fish highest in mercury: Mackerel, marlin, shark, swordfish, tilefish and tuna have most mercury accumulated in their tissues and pregnant and breastfeeding women and small children should avoid eating them. In general, larger fish have higher concentration of mercury in their tissues than smaller fish. Also, predatory fish usually have more mercury per kilogram of body weight than herbivorous fish. And, freshwater fish have typically lower mercury levels than sea fish. Other seafood that should better be avoided by pregnant and lactating women includes Atlantic cod and Atlantic flatfish, bluefish, caviar, Chilean seabass, eel, farmed salmon, groupers and king crab.

Bass, carp, croaker, jacksmelt, lobster, monkfish, perch, sablefish, skate, snapper and sea trout contain moderate levels of mercury but still higher than anchovy, butterfish, catfish, clam, crab, crayfish, haddock, hake, herring, mullet, oyster, plaice, pollock, salmon, sardines, scallop, shad, shrimp, squid, tilapia, freshwater trout, whitefish and whiting, which have the lowest levels of mercury. The best advice for those who can afford it is to buy sustainable seafood products.