When mercury is deposited in waterways, bacteria convert it to methylmercury. Methylmercury builds up in the tissue of fish, which may then be eaten by wildlife (e.g., eagles, osprey, common loons, river otters, minks) and by people. Because mercury is tightly bound to the fish muscle tissue, there is no method of cooking or preparation that will remove or reduce mercury once it is in fish. This doesn't mean that you should stop eating fish. It is a good source of protein and is low in saturated fat. You can still get the benefits of eating fish by using moderation in how much you eat.
The two organ systems most likely affected by methylmercury are the central nervous system and the kidneys. The groups most vulnerable to the effects of mercury toxicity include women who are pregnant or may become pregnant, nursing mothers, and young children. The most significant concerns regarding chronic exposure to low concentrations of methylmercury in fish are for neurological effects in the developing fetus and children.
Although human exposure to mercury occurs most frequently through eating contaminated fish, other human exposures to mercury can occur. People have been exposed to mercury from inhaling mercury vapors from broken fluorescent lamps, gas regulators, or even home fever thermometers. There have been cases of mercury exposures from accidental swallowing, but these cases are rare.
Mercury (Hg) is a naturally occurring metal found throughout the environment. It is a liquid at room temperature, combines easily with other metals and expands and contracts evenly with temperature changes. Because of these properties, mercury has been used in many household, medical and industrial products. Although mercury performs many useful functions in our workplaces and homes, it is toxic and can impair our health. Mercury is a potent neurotoxin, meaning that it interferes with the way nerve cells function.
Mercury poisoning causes a decreased ability to see, hear, talk and walk. It can cause personality changes, depression, irritability, nervousness, and the inability to concentrate. It can also cause damage to the brain, kidneys, and lungs. Mercury is a particularly serious problem for pregnant women and children. Fetuses and young children suffer the greatest risk because their nervous systems are still developing. They are four to five times more sensitive to mercury than adults.
Mercury emissions from natural and anthropogenic sources enter the global mercury cycle and are distributed in the environment locally and globally through various processes. Atmospheric emissions of mercury can enter the environment through deposition onto soils and water. When mercury enters fresh water bodies and the oceans, or settles into sediments and soils, it can become involved in biogeochemical cycles, be transformed into the highly toxic form of methylmercury, and bioaccumulate in the food chain. Predatory, freshwater fish species such as pike, bass and walleye have also been know to attain elevated methylmercury that may be almost completely absorbed by human or animal consumers.
While fish consumption is the main source of exposure to methylmercury, the Utah Department of Health advises that the benefits and risks of eating fish should be balanced, since fish are an excellent source of high-quality protein and omega-3 fatty acids, and are low in saturated fat. In order to protect Utah residents from mercury poisoning, the Health Department issues fish consumption advisories which recommend limits for consumption of fish caught in Utah waters. These recommendations are based on an assessment of the health risks posed by mercury or PCB (polychlorinated biphenyls) levels detected in fish and scientific information about the harmful effects of mercury and PCBs. There are specific advisories for pregnant women, for young children and for the rest of the population. The advisories vary according to fish species and body of water.
Other commercially available fish such as shark, swordfish and fresh or frozen tuna, generally have higher mercury concentrations. Canned "white" tuna has been found to have a higher mercury concentration than canned "light" tuna. The U.S. Food & Drug Administration and the U.S. Environmental Protection Agency are for the first time developing a single uniform advisory covering commercially caught fish and a copy of the advisory is available on the FDA's Web site. The joint FDA/EPA advisory pertains specifically to pregnant women, women who might become pregnant, and nursing mothers. Additional information on mercury concentrations in commercially caught fish species can be found on the FDA's Web site.
Based on available science, normal ambient air concentrations of mercury vapour, averaging 1.6 nanograms per cubic meter of air, do not appear to be a cause for concern (1 nanogram = one billionth of a gram).
Dermal contact is also a route of exposure to mercury with alkyl mercury compounds being particularly notorious. While few Utah residents come into direct contact with mercury or its compounds, skin absorption can be lethal. In 1997, a researcher named Karen Wetterhahn, from Dartmouth College in New Hampshire, died when a single drop of dimethylmercury passed through her protective latex glove and through her skin.
When an individual is exposed to mercury, a certain percentage is absorbed, depending on the route of exposure and the form of the mercury. About 80% of elemental mercury is absorbed when inhaled, however less than 1% of ingested liquid mercury is absorbed.
Methylmercury, on the other hand, is readily absorbed regardless of the exposure pathway. Approximately 95% of ingested methylmercury is absorbed, and absorption through the lungs and skin is also believed to be quite high. Both elemental and methylmercury can cross the blood-brain and placental barriers. The critical target organ for elemental mercury is the adult and fetal brain, and the critical target organs for methylmercury are the brain and the kidneys.
Inorganic mercury compounds do not readily migrate through the blood-brain or placental barriers, but do accumulate in the kidneys. Absorption of inorganic mercury varies with the type of inorganic salt. Within the body, the kidneys accumulate the highest concentrations of all forms of mercury, yet mercury can also concentrate in the brain, the central nervous system, the liver, and indeed in most organs in the body.Mercury is predominantly excreted from the body in urine and feces, but usually at a slower rate than that of uptake, leading to the accumulation of mercury in living tissue. Mercury is deposited in hair as it grows, and it may also be found in breast milk. This may result in high concentrations in infants whose mothers are heavily exposed. The unborn child also receives some of the maternal mercury body burden because mercury compounds cross the placental barrier, yielding equal or higher blood concentrations in the fetus than in the mother.