Vegan Health ArticlesFish and Shellfish: Contamination Problems Preclude Inclusion in the Dietary Guidelines for Americans
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From Spring 2004

Understanding Mercury

Mercury is a global pollutant that comes from both natural and human-generated sources. Naturally occurring mercury is present in rock and soils. Combustion of fossil fuels is the main way mercury is released into the environment. Medical and municipal waste incinerators and coal-fired utility plants contribute much of the mercury released into the atmosphere. Once released, mercury can travel long distances and pollute the air, water, and food supply.1

In the environment, mercury exists in its elemental form and in a variety of organic forms. One of these organic forms, methylmercury, accumulates up the food chain in aquatic systems, concentrating especially in large predatory fish. The potential sources of mercury contamination for the general population are consumption of water or food stuffs contaminated with mercury, inhalation of mercury-containing vapors, and exposure to dental amalgams or medical treatments that contain mercury. Of these, the consumption of fish and shellfish contributes most to the methylmercury concentration in humans.1

Nearly all fish contain traces of methylmercury. Some fish and shellfish tend to contain higher levels either because they live in more contaminated waters or because they are larger carnivores consuming many contaminated smaller fish. Because mercury is eliminated slowly from the body, it may build to very high levels in the systems of animals—including humans—that consume it.

Shark, swordfish, king mackerel, and tile fish are known to have especially high concentrations of methylmercury (mean of samples tested: 0.73, 0.99, 0.97, and 1.45 parts per million (ppm), respectively). Other commonly eaten fish also contain high levels of methylmercury (between 0.25 and 0.55 ppm): bass, bluefish, grouper, halibut, lobster, marlin, orange roughy, canned albacore tuna, and fresh tuna. Some fish have more modest amounts on average (less than 0.1 ppm); these include anchovies, catfish, clams, cod, crab, haddock, perch, pollock, salmon, scallops, shrimp, and trout.2

Levels of contamination vary widely. Among tuna, for example, there is a three-fold difference in mean levels of contamination between canned light tuna (0.12 ppm) and canned albacore tuna (0.35 ppm) or tuna that is sold fresh or frozen (0.38 ppm).2 Contamination also varies greatly between individual fish. Therefore, even well-informed consumers have no way of knowing whether the fish they have purchased has a high or low level of mercury contamination.

In 2000, the National Research Council convened a group of scientists to make recommendations on “acceptable” levels of mercury consumption. This level, known as the exposure reference dose (RfD), is the level of daily exposure to mercury thought likely to be without risk of adverse effects for humans (including sensitive subgroups), even if exposure occurred regularly over a lifetime. This committee set the RfD at 0.1 micrograms (µg) of mercury per kilogram of body weight per day.1 This means that the weekly RfD would be about 7 µg per week for a toddler, about 14 µg per week for a five-year-old child, and about 42 µg per week for a 135-pound woman.3

Specific examples put these numbers in perspective. Two ounces of canned tuna with .36 ppm would provide 20 µg mercury—nearly three times the RfD for a toddler. Six ounces, the amount in two tuna salad sandwiches, would provide 61µg of mercury, which is more than four times the weekly RfD for a five-year-old; it would also be about 50 percent over the weekly RfD for an adult. Clearly, even modest consumption of moderately contaminated and commonly eaten fish can put consumers at risk very quickly.3

It is not surprising that the most recent surveys of methylmercury contamination (based on data from 1999—2000) found that 7.8 percent of women of childbearing age have blood mercury levels above the EPA’s “safe” limit of 5.8 µg of mercury per liter. Moreover, 15.7 percent of women of childbearing age have levels above 3.5 µg/L, which is high enough to put a fetus or breastfeeding infant at risk.4,5 The EPA estimates that about 7 million women and children are eating mercury-contaminated fish at or above levels it considers safe.4 The bottom line: Significant numbers of Americans are already over-consuming mercury-laden fish and seafood. It is inadvisable from a public health perspective to encourage further consumption of this contaminated product.

Go on to: Effects of Mercury Contamination
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We began this archive as a means of assisting our visitors in answering many of their health and diet questions, and in encouraging them to take a pro-active part in their own health. We believe the articles and information contained herein are true, but are not presenting them as advice. We, personally, have found that a whole food vegan diet has helped our own health, and simply wish to share with others the things we have found. Each of us must make our own decisions, for it's our own body. If you have a health problem, see your own physician.