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Oregon's Fish Advisories for Methylmercury Contact: Environmental Toxicology, (971) 673-0440 Oregon is one of 35 states with fish consumption advisories for mercury. Currently, mercury advisories are in effect for seven Oregon waterbodies. In Oregon, Health Services has primary responsibility for issuing fish advisories and, predictably, we receive numerous inquiries regarding these advisories. The purpose of this paper is to explain why these advisories exist and how they are developed. A common question is why Oregon fish contain mercury. As with numerous other waterbodies throughout the country, deposition of atmospheric mercury, derived in part from waste incineration and other combustion processes, is no doubt a significant contributing factor. However, much of the mercury in Oregon fish originates from natural deposits in rocks and soils, with some influence from historic mining practices. As with other metals, the chemical form of mercury has a very large influence on its behavior in the environment and in biological systems. Though inorganic forms of mercury (metallic mercury and mercury salts ) are only minimally absorbed by fish, one form of mercury, methylmercury, is readily taken up by fish. The significance of this fact became apparent in the 1960s, when it was discovered that certain types of sediment bacteria can transform inorganic mercury to methylmercury (Clarkson, 1992). Fish readily absorb methylmercury from their food and from water as it passes over their gills. This process, whereby a chemical becomes more concentrated in a biological organism than in the surrounding environment, is termed bioconcentration. Bioconcentration can result in a very large increase in methylmercury in fish. In some cases, methylmercury concentrations in fish can be millions of times higher than the concentration in the surrounding water. Though fish absorb methylmercury very readily, they excrete it very slowly. Thus, the older a fish is, the higher its methylmercury concentration tends to be. This is why advisories for fish mercury (and many other bioconcentrating contaminants) often identify larger fish as posing the greatest risk. Because small fish and other aquatic organisms bioconcentrate methylmercury, they in effect provide concentrated doses of methylmercury to fish that prey on them. Thus, piscivorous fish (fish that eat other fish) tend to have higher methylmercury concentrations than nonpiscivorous fish. The increase in chemical concentration through higher trophic levels is called biomagnification. Biomagnification can result in a significant methylmercury exposure to humans, bald eagles, and other top level predators. Fish consumption is the primary pathway of human exposure to methylmercury (Wheeler, 1996). Humans do excrete methylmercury, though only very slowly. As with fish, if humans take in methylmercury faster than they get rid of it, the amount remaining in their body will increase. Thus, with this type of chemical, limited short-term exposure is generally less important than cumulative exposure over time. The total amount of a chemical in the body is called the body burden of that chemical. In evaluating potential health impacts, it is generally the body burden which is of primary concern. A question we are sometimes asked is why our mercury advisories target women of reproductive age as a sensitive group. Mercury advisories generally identify pregnant women as a sensitive population. The reason for this is that the developing fetus has been shown to be very sensitive to the toxic effects of methylmercury. But what the fetus is actually exposed to is the amount of methylmercury that is in the mother's body, or her methylmercury body burden. So if a woman consumes fish that is high in methylmercury for an extended period and then stops when she becomes pregnant, she can still expose her fetus to an unacceptable level of methylmercury. Serious methylmercury poisonings have occurred in the past. The most well known of these happened in the 1950s at Minamata Bay, Japan. Here, a chemical company discharged large amounts of methylmercury into Minamata Bay, which resulted in very high methylmercury concentrations in fish (as high as 40 ppm). As fish formed a large part of the diet of the local population, significant methylmercury exposure occurred. In addition to other serious effects, including numerous deaths, infants born to exposed mothers experienced severe central nervous system toxicity. These effects were observed in many cases where the mother exhibited only minimal symptoms. A second well known methylmercury poisoning incident occurred in Iraq. In this case, seed that had been treated with a methylmercury fungicide was used for making bread instead of being planted. Severe, widespread poisoning resulted (Watanabe and Satoh, 1996; Galli and Restani, 1993). Though it is difficult to consider positive aspects of tragic events such as these, these incidents did provide a substantial amount of information regarding the human toxicity of methylmercury. Importantly, these incidents contributed substantially to our understanding that the developing fetus is exquisitely sensitive to the effects of methylmercury. An important general principal in toxicology is that a chemical must be absorbed by the body and must reach its target organ to exert its toxic effect. The primary reason methylmercury is so toxic is that its form allows it to enter the body and get distributed to critical tissues. Of primary importance is the fact that methylmercury is able to cross the blood-brain barrier, where it can damage the brain. Methylmercury can also cross the placenta where it is concentrated in fetal blood and is also excreted in breast milk (Galli and Restani, 1993). The severe toxic effects of methylmercury are primarily a consequence of its entering the CNS. Though neurotoxic effects do occur in adults, effects on the developing fetal brain are the most severe, sometimes resulting in mental retardation and paralysis. Fish mercury advisories are quite common around the country. In fact, more fish advisories are issued for methylmercury than for any other toxicant (EPA, 1996). The main reason for this is no doubt that mercury is ubiquitous in the aquatic environment. A second reason may be that, unlike many other contaminants such as PCBs and dioxins, information regarding safe levels of methylmercury is fairly well established. Still, there is some disagreement among states regarding the significance of methylmercury in fish, and a level considered safe in one state might be considered unsafe in another state. Even federal agencies cannot agree on safe mercury levels in fish. FDA allows up to 1 ppm and indicates that consumers can safely eat up to 7 ounces of fish containing 1 ppm methylmercury per week. For a 60 kg person, this amounts to a methylmercury intake rate of 30 micrograms/day. However, according to EPA, a 60 kg person should only take in 6 micrograms/day. According to EPA, fish containing 1 ppm methylmercury should only be eaten at a rate of 1.4 ounces per week (Wheeler, 1996). Most people are aware of the fact that most fish, particularly canned tuna, contain some methylmercury (Yess, 1993). Fortunately, concentrations in most market fish are relatively low. But large tuna and other predatory fish such as shark and swordfish can have fairly high levels of methylmercury. FDA's higher standard is based on the assumption that people eat a variety of fish from a variety of sources (Foulke, 1994). However, if a person really likes shark or swordfish or large tuna and eats it often, that person could be getting a pretty good dose of methylmercury. As FDA's standard assumes that the consumer obtains his fish from a variety of sources, it is inappropriate to apply this standard to fish from a particular waterbody, as some states do. Something that is helpful in deriving safe consumption rates of mercury-containing fish is that EPA has recently published a revised methylmercury reference dose, based on human developmental effects. (The reference dose is basically the amount of a chemical that a person can safely be exposed to every day for a lifetime.) In Oregon, we apply EPA's revised reference dose (0.1 microgram/kg/day) to women of reproductive age, pregnant and nursing women, and young children. We continue to use the old methylmercury reference dose (0.3 microgram/kg/day) for the general population, as it is based on adult neurotoxicity. As indicated above, the most difficult aspect of fish consumption advisories generally is communication. This is no doubt the most important part of the whole process and we have been surprised sometimes at people's reactions to our advisories. One extreme reaction comes from people who don't read the details of the advisory, but conclude that the fish are "poisonous" and should be completely avoided. This is an unfortunate reaction, considering the known benefits of eating fish. We also have reactions at the opposite extreme. These individuals believe that if they don't see people obviously sick or dying than there can't really be a problem. Obviously, what we attempt to communicate is something between these extremes. Our message is generally pretty simple: catch and consume the fish but limit your consumption according to the guidelines indicated in the advisory. References: |
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