Should I be concerned about mercury in fish and what fish are safe to eat?

Yes, mercury contamination of fish is a definite concern for all individuals, and particularly for pregnant women, women considering pregnancy, and children. Concern over mercury contamination of fish has come about for a variety of reasons. Industrial practices worldwide and in the U.S. continue to release large amounts of mercury into the air. Facilities that incinerate medical or municipal wastes and coal-fired power plants are among the greatest air polluters when it comes to mercury. Products like fluorescent light bulbs, thermostats, and electrical components containing mercury continue to be manufactured in large volumes, and these products eventually wind up in landfills and dumping sites. Once mercury has found its way into the air or the soil, it can move through naturally-occurring ecological channels into lakes, streams, rivers, and oceans where it becomes a toxic contaminant for fish.

The Problems of a Global Food Supply

Globalization of the food supply is another reason all individuals need to be concerned about fish and mercury. In certain parts of the world, like the Mediterranean Sea, naturally-occurring ore deposits serve as an ongoing source of mercury contamination. Cinnabar (mercury sulfide) ore deposits found along the bottom of the Mediterranean Sea, for example, create levels of mercury in Mediterranean tuna that are higher than levels found in either Atlantic Ocean or Pacific Ocean tuna. A February 2007 report by the U.S. Environmental Protection Agency (EPA) has shown that the geographical origin of different fish (i.e., their original habitat) can play a more important role in degree of mercury contamination than many other factors, including the size of the fish or the length of its lifespan. For example, this 2007 EPA report found Atlantic herring (a very small fish) to contain three times the mercury level of Pacific herring, or even many larger fish like cod.

Do the health benefits outweigh the mercury risks?

Fish has always been recognized to be an excellent source of protein. In more recent years, cold-water fish have also been recognized as excellent sources of omega 3 fatty acids, including DHA (docosahexaenoic acid) and EPA(eicosapentaenoic acid). Risk of mercury contamination has thrown some of these nutritional benefits into question, and the benefits-versus-risks of fish have become a matter of widespread debate. Do the nutritional benefits of fish, including their rich omega-3 fatty acid content, outweigh the risk of mercury exposure?

We believe the answer to this question is "yes" — but a conditional yes, rather than an unconditional one. Yes, the nutritional benefits of fish outweigh the risk of mercury exposure, provided that (1) lower mercury fish are chosen for consumption, and (2) total weekly intake of fish stays fairly restricted. One recent review study definitely agrees with us, and defines "fairly restricted" as 7-10 ounces per week. Here's a closer look at the details involved in this risk-benefit analysis.

Risk-Benefit Analysis of Fish in the Bristol, United Kingdom Study

A study published in the February 17, 2007 issue of The Lancet answers this question with a definite "yes" based on questionnaire data obtained from more than 10,000 women living in Bristol, United Kingdom in the early 1990s. Researchers found that women consuming over 12 ounces (340 grams) of fish per week during their pregnancy had children with higher IQs and better nervous system development than women who consumed less than this amount. While we respect the quality of the research presented in this study, we do not totally agree with the interpretation of its findings, nor do we believe that the findings are necessarily applicable to U.S. women who are trying to evaluate the advantages and disadvantages of fish. Our reasoning is fairly simple.

In this 1991-1992 study, women who ate more than 12 ounces of fish per week during their pregnancy were also women who smoked less, had greater amounts of income, were better educated, owned homes, and sustained marriages and a family environment in the home. Even though these factors were analyzed statistically by the researchers, we believe that they influenced many aspects of the children's upbringing that were not adequately analyzed by the research team. (There are many reasons we would expect children from these households to do better on IQ tests). In addition, we are concerned about the fact that no daily food records were ever kept by pregnant women in the study, and no food contents — either nutritional or toxicity-related — were ever subjected to laboratory analysis. The fact that all of the women in the study lived in one town in the United Kingdom 15-16 years ago is also of concern, given the increasingly dynamic nature of the global food supply and geographical origins of fish in the U.S. marketplace.

Risk-Benefit Approach to Fish by the U.S. Food and Drug Administration (FDA)

Our own conclusion about the risk-benefit profile of fish is much closer to the position taken by the U.S. Food and Drug Administration (FDA) in its March 19, 2004 advisory on mercury and fish. Like the FDA, we believe that a restriction on fish intake is prudent for all individuals. While setting a 12-ounce guideline for maximum weekly intake of all fish, the FDA also recommended that this 12-ounce intake be restricted to fish and shellfish that are lower in mercury. We support this type of approach, and we like the idea of a dividing line between lower mercury and higher mercury fish — especially when it comes to tuna.

2007 EPA Update on Current Levels of Mercury in Fish

According to a February 2007 EPA report, 39% of all mercury exposure from fish in the U.S. comes from tuna. Of this 39%, 18% comes from canned light tuna, 10% from canned albacore or white tuna, and 11% from fresh or frozen tuna. (Swordfish, pollack, shrimp, and cod account for another 25% of all mercury exposure from fish). Even though canned light tuna accounts for almost double the total mercury exposure as canned albacore or white tuna, albacore/white tuna are actually much higher in mercury content. (As a nation, we just eat much more canned light tuna because of the lower price). In the EPA update report, both Pacific and Atlantic albacore tuna (all forms, including canned and fresh) contained about triple the mercury content of both Pacific and Atlantic light (yellowfin) tuna (all forms, including canned and fresh). But it should also be noted that Atlantic tuna was always higher in mercury content than Pacific tuna. The average numbers for Atlantic tuna in this 2007 EPA study were: 0.47 milligrams/kilogram for albacore and 0.31 milligrams/kilogram for yellowfin (light). By comparison, Pacific albacore only contained an average of 0.17 milligrams/kilogram of mercury and Pacific yellowfin (light) only 0.06 milligrams per kilogram.

These differences in mercury exposure from canned tuna make it clear that light tuna (especially Pacific light tuna) — is a far better choice than albacore tuna (especially Atlantic albacore tuna) when it comes to mercury exposure risk.

Mercury Content in Popular Fish

You can use the chart below to see average mercury content in your favorite fish. This data from the 2007 EPA report tells you how many milligrams of mercury (on average) were found in 2.2 pounds (or one kilogram) of fish. The "FDA" column reports averages calculated by the U.S. Food and Drug Administration in its ongoing monitoring program launched in 1990. The "Atlantic, Pacific Catch" column reports two averages. The first average is for fish caught in the Atlantic Ocean, and the second for fish caught in the Pacific Ocean.

SpeciesFDA AverageAtlantic, Pacific Catch
Domestic Samples
Anchovies0.04ND, 0.04
Cod0.100.06, 0.11
Crab0.060.26, 0.15
Flounder0.050.08, 0.07
Halibut0.250.25, 0.28
Herring0.040.04, 0.14
Lobster0.170.28, 0.17
Mackerel0.150.22, 0.09
MusselsND0.08, 0.03
OystersND0.07, 0.06
Perch (ocean)ND0.08, 0.08
Pollock0.060.02, 0.06
Salmon (fresh)0.010.13, 0.04
Salmon (canned)0.05ND, 0.04
Scallops0.050.01, 0.04
Shark0.990.75, 0.80
ShrimpND0.04, 0.03
Snapper0.190.28, 0.25
Swordfish0.980.98, 0.98
Tilefish1.451.45, ND
Tuna (fresh/frozen)0.380.28, 0.24
Tuna/albacore (all forms) ND-0.760.47, 0.17
Tuna/yellowfin (all forms) ND-0.760.31, 0.06

Data Source: Sunderland EM. (2007). Mercury exposure from domestic and imported estuarine and marine fish in the U.S. seafood market. Environ Health Perspect 115:235-242. Note: "ND" in the above table stands for "no data".

Risk-Benefit Analysis of Fish: An Opinion from the National Academy of Sciences

The U.S. National Academy of Science (NAS) - through its Institute of Medicine (IOM) - issued a major position paper in October 2006 on the risks and benefits of fish consumption. In order to come up with responsible recommendations, the IOM found it necessary to separate all potential fish eaters into four different categories. These categories were: (1) pregnant and nursing women, or women who may become pregnant; (2) children 12 years and under; (3) healthy teens and adults (with no chance of pregnancy); and (4) adults who are at risk of coronary heart disease. Here are the IOM recommendations for each population sub-group:

Category 1 — Pregnant and nursing women, or women who may become pregnant:

Category 2 — Children 12 years and under:

Category 3 - Healthy teens and adults with no chance of pregnancy:

Category 4 - Adults who are at risk of coronary heart disease:

This age-specific and gender-specific approach to seafood consumption makes sense to us, and it follows the same basic principles put forth by the FDA in its 2004 recommendations. Those principles are: (1) continue to consume fish, (2) restrict total weekly consumption, and (3) select fish that are lower in mercury.

It's worth noting here that an October 2006 research review conducted by Dariush Mozaffarian, MD, a physician affiliated with Harvard Medical School and the Harvard School of Public Health, arrived at the same conclusion stated above. Dr. Mozaffarian emphasized restricted intake of lower-mercury fish in his research conclusions, together with increased caution for children and pregnant women (or women of childbearing age).

Making Your Own Calculations

Since 1995, the U.S. Environmental Protection Agency (EPA) has set a safe daily mercury intake level of 0.1 micrograms per kilogram of body weight. This level is called the Reference Dose, or RfD. RfDs are designed as intake amounts that can be consumed every day over the course of an entire lifetime without significant risk of harm. The mercury RfD means that a person weighing 150 pounds (68 kilograms) would be able to consume about 7 micrograms of daily mercury and remain safe. A person weighing 125 pounds would be able consume 6 micrograms, and a person weighing 200 pounds would be able to consume about 9 micrograms. Or, combined together as a summary of the RfD mercury allowance, individuals weighing between 125-200 pounds can consume 6-9 micrograms of mercury daily.

With this range in mind, let's consider the microgram amounts of mercury found in fish in the EPA's 2007 update report. We will consider one low mercury fish (Pacific salmon) and one higher mercury example (canned Atlantic albacore tuna). Pacific salmon was found to contain an average of .04 milligrams of mercury in every 2.2 pounds of fish. That milligram amount translates into 40 micrograms. Since 2.2 pounds is the same as 35.2 ounces of fish, we end up with a little more than 1 microgram of mercury in every once of Pacific salmon. If our limit is 6-9 micrograms per day (because we weigh between 125-200 pounds), we can eat between 6-9 ounces of Pacific salmon per day, provided that we have no other oral intake of mercury in any form (from other foods or even non-food sources, as might occur in the case of mercury-containing dental amalgams, for example).

By comparison, let's consider the case of canned Atlantic albacore tuna. At 0.47 milligrams per kilogram, canned Atlantic albacore tuna contains about 13.35 micrograms of mercury per ounce. In order for a 125-pound person to average 6 micrograms per day (or 42 micrograms per week), only 3.15 ounces of canned Atlantic albacore tuna could be consumed during the course of the entire week. Slightly more (4.7 ounces) could be consumed by a 200-pound person who was allowed an average of 9 micrograms per day.

When making this type of personal calculation, it's important to remember that a person's metabolic vitality, nutritional reserves, and ability to detoxify small amounts of mercury all come into play when evaluating the safety of an RfD level. If you suspect that your vitality level, nutrient status, or detoxification capability are not up to par, you may want to target an mercury limit that is below the RfD level as a way of compensating for this uncertainty factor in your health safety.

Fish Are Part of a Bigger Mercury Picture

One of the problems in determining the safety of fish consumption is the possibility of mercury exposure from other sources. Many job settings place workers at significant risk of mercury exposure. These jobs can include battery manufacturing, ink manufacturing, chlor-alkali processing, fish canning, mining, electrical component manufacturing, fluorescent lighting and vapor lamp manufacturing, bronzing, photo engraving, and many others. Home location can also act as a mercury risk factor, since persons living near fertilizer, paint, or chlor-alkali manufacturing plants are likely to be exposed to airborne release of mercury from those facilities. Even choice of household products can influence mercury risk. Long-acting nasal sprays and decongestants, antibiotic eye solutions, skin lightening creams, waterproof mascara, and hemorrhoidal ointments may all contain mercury. So can yard fungicides, pesticides, and some drain cleaners. If your household, occupational, or environmental exposure to mercury were significant, you would want to exercise extra caution in your fish selection and intake. The overall goal, of course, is to keep your total mercury exposure from all sources (including fish) below the danger level.

How much total mercury exposure is safe?

Safe levels of mercury exposure (including consumption of mercury-contaminated fish) are controversial because "safe" really depends on who is trying to stay safe and the specific health dangers they are facing. A very unhealthy person, perhaps in the hospital from weakness and poor nourishment, can withstand very little toxic exposure, including exposure from mercury-contaminated fish. An extremely healthy person, full of vitality, with good nutrient reserves, and a robust ability to get rid of toxins, would be very likely to remain fully healthy while consuming a moderate amount of mercury-contaminated fish. Exactly how much could such a person eat? Here the answer would depend on the person's age, physical activity level, body size (height and weight), and other factors, including immediate performance goals. An athlete facing endurance training might not want to deplete his or her nutrient supplies at all, and might not want to ask his or her body to engage in any unnecessary detoxification of mercury. In this case, the choice might be to avoid any mercury-contaminated fish. A well-nourished, healthy person just wanting to stay generally healthy, i.e., stay safe from premature aging or premature onset of chronic disease, might choose to eat canned light tuna twice a week and simply stay with the FDA general health guidelines.

Mercury Toxicity Isn't Easy to Detect

Even if you suspect that mercury exposure is a problem in your own health, it isn't always easy to determine exactly how problematic this heavy metal actually is. To begin with, most symptoms of chronic mercury overexposure are fairly vague. Because these symptoms have a wide variety of possible causes, they aren't particularly helpful in singling out mercury as the main culprit. This list of broad symptoms includes:

There are also blood tests and hair tests available for measuring mercury toxicity. If you are considering mercury testing, we recommend that you do so in consultation with your healthcare provider.

Practical Tips for Fish Eaters

Larger, Longer-Lived Fish Can Be Higher Risk

Larger fish, since they are longer-lived, typically have much more time to accumulate mercury than smaller, shorter-lived fish. Mercury therefore tends to be more concentrated in these fish. While this relationship does not always hold true, it can be a good rule of thumb when other details about fish are not known.

Specific Examples of Higher Risk Fish

Larger, longer-lived fish like swordfish, king mackerel (ono), shark, and great northern tilefish are specific examples of high-risk fish when it comes to mercury content. These fish often contain more than 1 milligram of mercury per kilogram of weight.

Canned Tuna Terminology

You'll find a variety of different terms used to describe tuna on canning labels. While the market name "albacore" actually refers to a biological species of tuna Thunnus alalunga , "light" tuna may refer to many different species of tuna, including skipjack, bluefin, yellowfin, and tongol. All types except tongol may be labeled "chunk light" or "solid light", while tongol may only be labeled "chunk light".

Some Tuna Manufacturers Test for Mercury

It's actually possible to find canned tuna that has been tested for mercury content by the manufacturer. If you are able to obtain canned tuna that has been tested and determined to contain very low levels of mercury, we encourage you to do so!

Specific Examples of Lower Risk Fish

Wild Pacific salmon, flounder, scallops, and shrimp have consistently shown up in research studies as the lowest risk fish when it comes to mercury contamination. Some farm-raised trout and some farm-raised catfish also seem to fall into this lowest-risk category.

Freshwater Fish and Fish Caught by Family or Friends

Freshwater fish may also be contaminated with mercury, either from natural or industrial sources. For freshwater fish caught by family or friends, check with your state and local health departments for current advisories regarding mercury, or call toll-free 1-888-SAFEFOOD.

References

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Hibbeln JR, Davis JM, Steer C et al. (2007). Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. The Lancet 369:578-585.

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