Mercury in Fish
Human Exposure to Mercury Due to Fish Consumption
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.
The poster on the right shows the relative levels of mercury found in fish you catch in the waters of Vermont and fish you buy at your local
grocer (see press release "State Departments Work Together To Educate
Vermonters About Mercury"). Copies of the poster can be obtained by clicking on one of the two links on the right or by calling 800-974-9559
or, make your request via email.
While fish consumption is the main source of exposure to methylmercury,
the Vermont 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 Vermonters from mercury poisoning,
the Health Department issues fish
which recommend limits for consumption of fish caught in Vermont
waters. These recommendations are based on an assessment of the
health risks posed by mercury or PCB (polychlorinated biphenyls)
levels detected in fish, tests of hundreds of fish caught in Vermont
waters in the past 10 years, 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.
In addition to the poster showing the relative mercury levels of
certain fish species, the Vermont Departments of Health, Fish &
Wildlife and Environmental Conservation have developed the fish
advisory card shown on the right. This 6" x 9.25" card
conatins all of the information on the above poster and also includes
Vermont Department of Health and U.S. Food and Drug Administration
fish consumption advisories.
Although the Vermont Department of Health does not issue advisories on commercially caught fish species such as salmon, cod, pollock, sole,
shrimp, mussels, and scallops, these species generally have lower mercury levels and are therefore considered safe for consumption.
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 (see the poster above for
relative mercury levels in locally caught and commercially caught fish). 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 draft
version 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
Other Human Exposure Pathways to Mercury
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 Vermonters 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 irrespective 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.