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 ofâ€¨ 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."
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.
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