A Little Tuna With My Mercury

All week, I've been dissecting the debate over eating seafood, including the latest health and environmental reports and the issues at play. Today, I look at mercury, a naturally occurring substance that has found its way into the oceans and into the fish we eat.

There's been a lot of discussion lately over the risks of mercury, with fervent arguments coming from both sides. Even two of the most recently published scientific papers vary in their assessment of the mercury issue. The recent Journal of the American Medical Association (JAMA) report concludes that the risks of mercury are outweighed by the heart-healthy omega-3 fatty acids obtained from eating fish. The Institute of Medicine (IOM) report is less strident, arguing that "considerable uncertainties are associated with estimates of the health risks to the general population from exposures to methylmercury and persistent organic pollutants at levels present in commercially-obtained seafood."

Known since ancient Egyptian times, mercury has a long, controversial history as a potentially lethal toxin that has been used in various industries, including dentistry, hat making and over-the-counter creams and antiseptics. Its appearance as a contaminant in seafood is relatively recent and is the result, experts say, of emissions from power plants and manufacturing.

In 1995, the US Environmental Protection Agency (EPA) established a Reference Dose (RfD) for methylmercury, which is defined as "an estimate of a daily oral exposure to the population that is likely to be without an appreciable risk of deleterious effects during a lifetime."

The reference dose was established in response to studies that had been conducted in the Seychelles Islands, the Faroe Islands and New Zealand, which showed a connection between higher blood mercury levels and neuro-developmental problems, particularly in young children.

In its explainer page on human mercury exposure, the EPA states: "Consumption of fish with higher methylmercury levels can lead to elevated levels of mercury in the bloodstream of unborn babies and young children and may harm their developing nervous system. These disabilities have been documented in ability to use language, to process information, and in visual/motor integration. U.S. EPA's 2001 Reference Dose (RfD) for methylmercury was calculated to protect the developing nervous system."

Numerically, the EPA reference dose is 0.1 ug (microgram) of mercury per kilogram of body weight per day (or 7 ug/day, 49 ug/week, or 213 ug/month for an average person weighing 150 pounds). Mercury concentration in fish is measured in parts per million (ppm), which is the unit of measure used in this story. Translated, if your mercury level surpasses the EPA reference dose on a regular basis, you're at greater risk of neuro-developmental side effects, which can be passed in utero and through breast milk. Here's some math to consider: A six-ounce portion of fish that has a mercury concentration of 1 ppm nets about 170 ug of mercury in a person weighing about 150 pounds. This is close to one's monthly mercury allowance.

In 2004, the EPA and the Food and Drug Administration (FDA) issued a joint advisory about mercury in seafood, with a particular focus on women who are pregnant, nursing or of childbearing age and children under 12.

If you fall into one of these categories, the advisory recommends avoiding four types of fish -- tilefish, swordfish, shark and king mackerel -- all of which have high levels of mercury concentration.

Here's how these fish stack up:
Tilefish (aka golden bass, golden snapper) from the Gulf of
 Mexico: 1.45 ppm
Shark: .99 ppm
Swordfish: .97 ppm
King mackerel: .73 ppm

Interestingly, these four fish are not among the top 10 seafood choices for Americans. Remember -- the top three seafood favorites for Americans are shrimp, canned tuna and salmon.

The IOM report acknowledges this gap: "Notably the four fish identified in federal advisories as those which pregnant women should avoid eating are not among those that are widely consumed by the general population."

Tuna Math

So what about canned tuna, America's number two seafood? Where does it stack up in the mercury contest? If you like chunk light (the most popular form of canned tuna in this country, according to the US Tuna Foundation), your mercury odds are lower.

With a mercury concentration of .12 ppm (or an accumulation of 10 ug in the body), chunk light tuna is safe to eat almost 10 times per month according to the EPA reference dose. (Mercury concentrations, e.g. .12 ppm, are independent of meal size. In other words, light tuna has .12 ppm of mercury regardless of the portion size.)

Thing is, most cans of tuna are six ounces, so your personal storehouse of mercury goes up to 20 ug. Still relatively low on the mercury scale, but what if you're a diehard canned tuna fan, eating it four or five times a week, and you eat other fish as well?

There's good reason that tuna is number two in this country: it's cheap, available everywhere (airports, the CVS, gas stations), low cal and ready to eat. So it's not out of the question that you or your kid may be eating enough canned tuna in a week to push your mercury levels over the EPA reference dose. (Note: these figures are subject to individual weight . To find out more about mercury levels in your weekly fish intake, check out the mercury calculator at Got Mercury? or the Contaminated Fish chart at Environmental Defense.)

Albacore tuna aka "white tuna" is, on average, about three times higher in mercury (at .35 ppm) than its chunk light counterpart. Noting the difference, the EPA/FDA advisory recommends a maximum of six ounces of albacore per week. But at this consumption level (a little more than four times per month), a 150-pound person would be 30 percent over EPA's reference dose.

Here's my question: If I eat six ounces of albacore every week as suggested, isn't that dangerously close to eating the do-not-eat king mackerel, (with a mercury concentration of .73 ppm), every other week?

Why, then is albacore tuna excluded from the list of four fish too high in mercury to eat? Try the math, and see what you come up with.

By Kim ODonnel |  November 30, 2006; 4:42 PM ET Food Politics , Seafood
Previous: No More Shrimp Cocktail? | Next: Santa Wants a Low-Fat Cookie


Please email us to report offensive comments.

Kim, why isn't your blog linked on the home page?

I have to admit some confusion about all this. Maybe the answer is just to eat white tuna less often.

Posted by: SS | December 1, 2006 9:49 AM

Your link to Got Mercury is broken.

Posted by: j | December 1, 2006 10:35 AM

"(Mercury concentrations, e.g. .12 ppm, are independent of meal size. In other words, light tuna has .12 ppm of mercury regardless of the portion size.)"

But "Thing is, most cans of tuna are six ounces, so your personal storehouse of mercury goes up to 20 ug."

I'm afraid I don't follow. If light tuna has that amount of mercury regardless of portion size (an accumulation of 10 ug), but a can of tuna makes that accumulation go up to 20 ug, how can light tuna have a constant ppm?

I'm pregnant and am concerned about getting too much mercury. I therefore eat only one can of tuna a week, which I believe is far within the limits considered safe. But all of this ug/ppm is confusing.

Posted by: I'm confused | December 1, 2006 11:43 AM

Interesting that we are always concerned about mercury in seafood (as well we should) yet no one questions mercury in vaccines. A 20 lb. infant getting a flu shot gets 12.5 mcg of mercury injected, bypassing the protective effects of the digestive system. That equates to over six full 6 oz. cans of regular tuna with 100% of mercury absorbed IN ONE SITTING, and repeated one month later.

Many suspect the vaccine mercury preservative Thimerosal is not as safe as reported by the CDC. The vaccine schedule at the peak of the rise in neurological disorders in children exposed a 12.5 lb. two month old to 62.5 mcg of mercury on one day (mine got 250 mcg by 18 months). A 200 lb. person would need to eat 49 cans of tuna (0.12 ppm vs. 50 ppm in vaccines)in one sitting, 100% absorbed, to equal that.

Many will tell you that ethyl mercury from vaccines is "safer" than methyl mercury from fish. Ask them to PROVE IT - they can not. What they will find is that vaccine mercury accumulates 2 to 4 times faster than fish mercury in the brains of monkeys transformed into inorganic mercury, which does the neurological damage.

One more thing, any of you who will undoubtedly contest my point, have the courtesy of signing your name - most who support thimerosal do not.....I wonder why?

Tim Kasemodel
Wayzata, MN
father of mercury damaged son

Posted by: Tim Kasemodel | December 1, 2006 11:47 AM

I would like to have more info about fresh tuna fish vs canned tuna. As I live by the Mediterrenen sea, I am a consumer of this kind of tuna as well as the so called "blue fish" such as fresh sardines and anchovies. what about these as to mercury exposure?

Posted by: elena santini | December 1, 2006 12:10 PM

Confused, I just got off the phone with Timothy Fitzgerald, a scientist at Environmental Defense, and he this is what he had to say in response to your confusion about ppm vs ug measurements:
"Ppm measures the concentration in the fish, but the serving size of fish, coupled with the concentration of the fish, affects the amount of mercury you ingest. There are two important pieces of information: One is the kind of fish, which will have a given mercury concntration, so light tuna is .12ppm, albacore is. 35, and so on. Each species has a given average mercury contentration.
When you couple that with the serving size and frequency , that's what ultimately determines you mercury intake.
One meal of light tuna per week is well within the EPA's guidelines per week. One meal of albacore a week, however, is pushing the EPA guidelines, depending too on how much you weigh. "

Posted by: Kim O'Donnel | December 1, 2006 12:15 PM

Tim, please check your facts. The CDC says that children's vaccinations have been thimerisol (mercury) free since 1999. Some flu vaccines contain it, but the amounts would be well below dangerous levels and you can get thimerisol free flu vaccines.

The exposure has gone down from 187 ug (normal max exposure) to 3 ug, today.

Contrary to your claim that ethyl mercury is 100% absorbed, a recent study by the National Institute of Allergy and Infectious Diseases revealed that the body eliminated ethyl mercury (thimerisol) more quickly than methyl mercury (like the kind found in fish).

http://www.cdc.gov/nip/vacsafe/concerns/thimerosal/ faqs-mercury.htm

Posted by: Anonymous | December 1, 2006 6:31 PM

Like you said, the FDA and EPA have issued advisories about which fish to avoid, targeted at women who are or might become pregnant and children. Oceana, an ocean conservation group based here in DC, is working with grocery chains to make the FDA advice available at their seafood counters. Some chains, including Whole Foods and Safeway have voluntarily agreed to keep their customers informed via these advisories. However, Giant, Super Fresh, Food Lion and WalMart, among many others, have refused to do so.

In Oceana's recent report, DC was shown to be one of the leading states in making this information available in grocery stores. The District has 71% of stores posting signs, second only to Hawaii. That's particularly impressive because both Maryland and Virginia have much lower percentages, with 24% and 8% respectively. It's still confusing to make seafood decisions, but the easy access to the FDA advice within the city is a step in the right direction.

Posted by: Anonymous | December 5, 2006 11:13 AM

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