How does methylmercury affect health?
Where does mercury come from and how does it get into fish?
Do some kinds of fish have more mercury than others?
Do salmon, steelhead, shad, lamprey and other ocean fish contain mercury?
How is the public protected from methylmercury exposure?
Are there ways one can reduce exposure to fish mercury?
HOW DOES METHYLMERCURY AFFECT HEALTH?
Mercury is poisonous to the human body when it reaches certain concentrations in specific organs. The nervous system (brain, spinal cord and nerves) appears to be the most sensitive to methylmercury effects. Excessive exposure can result in tremors, loss of sensation in extremities, vision and hearing loss, and developmental and behavioral abnormalities.
Mercury is especially harmful to fetuses and to small children. Women of childbearing age are at special risk because of the effect their mercury bodyload would have if they were to become pregnant. If a nursing mother has elevated mercury, her milk will carry mercury to the infant. Babies and small children are at special risk because their organ systems are developing rapidly and are more vulnerable to damage. Also, children eat greater quantities of food relative to their body size than adults.
WHERE DOES MERCURY COME FROM AND HOW DOES IT GET INTO FISH?
Mercury is a naturally-occurring element in many Oregon soils, volcanic rocks and in geothermal water sources. Mercury can be found in the environment as free metallic mercury, in a number of chemically combined soil or rock constituents, or in plants and animals where it is mostly in the form of methylmercury. In Oregon, mineral mercury is widespread in much of eastern regions and throughout the Cascade Mountains, and in smaller amounts in the Coastal Range. Streams that flow through and out of these areas carry sediments that contain small amounts of mercury, and these sediments become distributed throughout river systems. All forms of mercury are potentially hazardous to animals and humans, but the most hazardous forms are organic. For fish consumers the most hazardous form of mercury is methylmercury.
Mercury contamination in the environment can also be caused by human activities including the use or disposal of manmade products that contain mercury. Other human activities that release mercury to the environment include the burning of fossil fuels (petroleum and coal), mining activities, smelting processes, pesticide applications and industrial discharges. In Oregon, unlike much of the remainder of the US, most of the mercury in fish is from volcanic and geothermal mercury minerals rather than from manmade sources. Whether mercury is natural or manmade, it undergoes the same methylation and bioaccumulation processes in the environment.
Mineral forms of mercury pose very little direct exposure hazard to people or to the environment, but as soil and rock particles wash into streams and accumulate in stream bottoms, the mercury is transformed into methylmercury by bacterial activity in the sediments. Once this transformation occurs, mercury is readily absorbed by plants and small animal life. As small organisms are eaten by larger organisms, such as predatory fish, the mercury bioaccumulates in the tissues of these organisms. Fish also absorb methylmercury directly from the water through their skin and gills. Bioaccumulation processes produce much higher levels of mercury in organisms than in their environment. Since fish are high on the food chain, and humans eat fish, there can be a serious potential for exposure to dangerous levels of mercury.
DO SOME KINDS OF FISH HAVE MORE MERCURY THAN OTHERS?
Mercury bioaccumulates in the body of fish and is eliminated from them very slowly. Because of this slow elimination, the older a fish is, the larger the concentration of methylmercury is likely to be in that fish. Since the size of fish generally corresponds to age, it is also true that smaller fish are likely to have lower mercury loads than larger fish of the same kind. Also different kinds of fish have differing mercury levels. Fish that eat insects, small aquatic animals and other fish usually have greater mercury loads than fish that eat only plants. In Oregon the highest mercury loadings are often seen in bass, squawfish and very large trout or chars. All species of fish contain mercury, however; and consumption advisories generally apply to all kinds of fish in the waterbody.
DO SALMON, STEELHEAD, SHAD, LAMPREY AND OTHER OCEAN FISH CONTAIN MERCURY?
Ocean fish also contain methylmercury, but very little testing of these nonresident fish has been done. In Oregon most fish testing is done using resident fish because resident fish are important indicators of stream quality and health. The mercury found in migrating or spawning ocean fish has little or no relationship to stream quality. Generally Oregon fish advisories can only address the safety of resident fish.
HOW IS THE PUBLIC PROTECTED FROM METHYLMERCURY EXPOSURE? The US Food and Drug Administration is responsible for testing and regulating market fish. They have adopted an action limit of 1 ppm. Commercial fish having greater than 1 ppm are not to be sold for human food. Due to limits in regulatory capability, however, FDA acknowledges that large ocean species such as shark, swordfish, yellowfin tuna and bluefin tuna that reach the marketplace or are served in restaurants frequently exceed the regulatory limit, and should be consumed infrequently.
Most state health departments monitor sportfish resources and issue advisories to anglers when necessary. Oregon, like many other states, relies on the assessment standards of the US Environmental Protection Agency, and their reference dose ("safe exposure level") for methylmercury. Oregon fish advisories generally address three different risk levels: children, women of childbearing age, and the general population. When the average level of methylmercury in fish from a given body of water reaches or exceeds 0.35 ppm, the Oregon Health Services issues a consumption advisory recommending safe amounts of fish that may be eaten by persons in each of the three risk groups.
ARE THERE WAYS ONE CAN REDUCE EXPOSURE TO FISH MERCURY?
Mercury accumulates in the edible muscle tissue of fish and cannot be reduced by cleaning techniques or cooking techniques as some other contaminants can. The only way one can control the amount of mercury consumed is to limit the amount of fish one eats. This is why Department of Human Services advisories for mercury are based on limiting consumption.
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