The Food and Drug Administration has published recommendations that warn pregnant women, nursing mothers, women who might become pregnant, and children not to ear swordfish, shark, tilefish, and king mackerel due to high methylmercury content. They also warn women and children to limit their consumption of tuna (DHHS/EPA, 2004). However, if guidelines are followed, these women and children are told that they can eat limited amounts of low mercury fish.
What Levels of methylmercury are safe?
There are many factors that help to determine how much mercury is considered to be safe. The EPA reference dose (RfD) is the amount of mercury that a person can be exposed to on a daily basis over a lifetime without appreciable risk of effects from it. The EPA RfD is 0.1 ?g mercury per kg body weight per day. This level translates into a blood mercury level 5.8?g/L or 5.8 parts per billion (ppb). Blood levels below this level are considered to pose no risk to human health under current guidelines (EPA/IRIS, 2007).
A study by the Turtle Island Restoration Network found that mercury levels in swordfish in 68% of the swordfish sampled were above the FDA action level of 1 ppm mercury (TIRN, 2004). However, the EPA discounts the danger of this level, claiming that the part of mercury in fish is actually lower in the human blood. There is a dilution factor that must be taken into account. However, this is often not mentioned in reports that seek to highlight the dangers of mercury exposure.
Mercury in the Body
The real danger to humans is not the amount of mercury found in the fish, but the amount that is absorbed by the humans body (EPA, 2001). When methylmercury contaminated fish is consumed, it is rapidly absorbed into the body. The estimated rate of absorption from the gastrointestinal tract is 95-100% (EPA, 2001). From there it rapidly finds its way into lipid cells, where it can travel to a number of body tissues. The highest levels of mercury in humans are found in the kidneys (EPA, 2001).
Methylmercury is relatively stable in the human body with an estimated half life of 44-80 days (EPA, 2001). Mercury is easily passed to the placenta and easily crosses the blood/brain barrier (EPA, 2001). A lethal dose of mercury is 10-60 mg/kg. Acute exposure to mercury can cause damage to the central nervous system (EPA, 2001). At these levels it cause kidney damage, kidney failure, gastrointestinal damage, and cardiovascular deterioration (EPA, 2001). If one consumes a high level of contaminated fish within a short period of time, toxic levels could accumulate, due to the body's inability to eliminate it rapidly.
Most of the data concerning danger to the developing fetus in humans is derived from studies on mice and rats. For this reason, there are some that claim the concerns are alarmist. However, the EPA feels that there is sufficient evidence to express the same concerns regarding the harmful effects of mercury on humans as well (EPA 2001). Chronic exposure to small doses of mercury appears to produce chronic neurological effects (EPA, 2001).
Mercury in the Grocery and at Restaurants
Lobbying groups have their own special agenda when it comes to mercury exposure through consumption of seafood. The Turtle Island Restoration Network monitors mercury levels in seafood being served at restaurants, particularly targeting sushi bars in California (Mandelbaum, 2007). Although this group represents a special interest, it does bring up the important question of how much fish containing large amounts of mercury actually reach consumers. There are even handy calculators available to help monitor your personal mercury exposure level according to the fish that one consumes during a given week.
One of the key criticisms of this calculator is that it is based on averages. Individual results may vary, but this is not addressed on the website. A portion of fish is considered to be 6 oz. The typical can of tuna is 6 oz, which means that there is one portion per can (Mandelbaum, 2007). Fish sticks and fish sandwiches are typically made from types of fish that are considered to be low in mercury content (DHHS/EPA, 2004). These levels represent average mercury levels found in fish. However, the individual levels in fish can vary drastically (Sunderland, 2007). The waters that they are in represents an important factor in the amount of mercury found.
When one considers the entire body of evidence regarding mercury levels in fish, it is difficult to determine what action to take. Opinions on the dangers of mercury levels in fish range from extremely dangerous to hardly dangerous at all. The problem for the consumer is sifting through the sources to find the most reliable information upon which to base their decisions about eating fish, or feeding fish to their children.
Many articles about mercury in fish are biased to reflect the personal feelings of the author. It would be easy to become alarmist about the topic if one only chooses to read articles by lobbyists and environmentalists. It is difficult to deny that some extremely high mercury levels can be found in fish. Mercury's high absorption rate in the digestive tract is of particular concern. However, this is tempered by the effect of dilution by volume in the blood stream. The following will examine conclusions and recommendations regarding mercury in seafood.
Chapter 3: Conclusions and Recommendations
The dilemma faced by consumers, particularly those in the danger group of women and children, is whether the dietary benefits of consuming fish outweigh the risks of absorbing high levels of mercury. No one can deny the presence of high levels of mercury in fish. However, there is conflicting evidence as to the actual risk posed to the general public.
According to the CDC, there have been no women and children reported to have blood levels above the exposure limits since they began recording this data in 1999 (NHANES, 2004). It is not known whether this is due to limits on consumption, or whether the dilution factor in the bloodstream played the most important role in these findings. This finding would suggest that, regardless of the reason, the high levels of mercury found in the fish specimens are not making it into the human body and would pose little risk. One has to consider all of the mechanism and the pharmacology of mercury in the human body before determining the risk factor to the general public.
It is easy to jump to the conclusion that if the fish consumed by humans contain high levels of mercury, then these same high levels will translate to humans that consume them. However, there are many pieces of relevant evidence that suggest that the mechanism is not this simple. In a reply to arguments posed by Senator Collins proposal to take drastic measures to reduce mercury in the food supply, it becomes apparent that there is more than one side to the issue and that credible empirical evidence tends to minimize the risk to humans. The threats to humans are considered "hypothetical" when one consider the hard evidence (Ferguson, 2005).
Science has challenged the alarmist approach to dangers of eating seafood. There is an obvious division between scientists and politicians regarding the issue of the dangers of consuming seafood. According to Ferguson (2005) current estimates of the total annual air-borne sources of mercury account for as little as 0.5%. He also points out that there are natural sources of mercury in the ocean itself, such as geothermal events. That which is human produced accounts for as little as 0.002% of the entire world mercury emissions. Ferguson references a study that found eight-560-year-old mummies from Alaska that had mercury levels twice as high as pregnant women in Alaska today. Therefore, he recommends that drastic measures to reduce mercury emissions are not needed. However, we do not know if the 560-year-old mummies suffered from neurological symptoms. There is simply no way to know if these high levels had an effect or not.
When considering whether or not the health benefits of eating fish outweigh the potential risks from mercury, one must consider all of the evidence available. One must not only consider the evidence, but the source of the evidence as well. The NHANES report has been cited by many environmental groups as reason to take drastic measures to reduce environmental pollution from industry. However, the NHANES report is considered to be poorly extrapolated data by a single EPA author (Ferguson, 2005). Therefore the data in it can be considered biased.
Before we consider recommendations regarding the consumption of fish, let us consider one final point. Children from grades 4-12 in China have mean mercury hair levels of 2.2 ppm. The U.S. EPA RfD is 1.2. Yet these children outperform U.S. children on international standardized tests for math and science. Ferguson (2005) suggests that this is due to the health benefits of the nutrients found in fish on…