How About Tuna (with a dash of mercury?)

I search the pantry and the fridge for a quick nutritious dinner.

“How about tuna,” suggests Sophie, my youngest. Tuna is her favorite protein, besides cheese, cheese, peanut butter, and cheese.

“I haven’t had it since last weekend,” she adds, turning on the charm.

My concern is mercury. And these days, most of us are aware that there is plenty of it in both fresh water and ocean dwelling fish. For parents with young children who find tuna one of the few foods with protein that their youngsters will eat, the situation is particularly worrisome. As a mother of two and a toxicologist, environmental contaminants are of particular interest constantly intruding upon our lives, while at the same time presenting unfortunate but interesting examples of human impact upon the natural environment.

Mercury, like all metals, occurs in nature and is present in the earth’s crust. While natural sources of mercury include volcanoes and geologic deposits, as most folks know, mercury is also released into the air by other processes such as incineration of medical waste (for example, burning thermometers) and, more importantly, burning coal.

Although the role of mercury as a potent neurotoxicant (a chemical which impacts the brain) has been known for centuries, the exact mechanism by which it causes toxicity remains frustratingly elusive. The term “mad as a hatter,” for example, is thought to originate from early observations of mercury’s neurotoxicity on those in the business. In the 1800s and early 1900s mercury was used in the felting process of hat manufacturing, likely resulting in large exposures and crazy hatters. In modern days, mercury was responsible for the neurotoxic and teratogenic effects (impacts the developing fetus) observed in villagers of Minamata, Japan, in the 1950’s and now known as Minamata disease. The disease was first noticed in the village cats, consumers of discarded or dead fish. The cats apparently danced and stumbled around the village prior to dropping dead on the street. Eventually, the disease manifested in humans, and was traced to mercury which for decades had been released into the water by local industry and accumulated in the fish and shellfish of Minamata Bay.

But mad hatters and Minimata were caused by exposures to much greater concentrations of the metal than are present in the workplace and the environment today. Present concerns for public exposure to mercury involve tiny amounts - in the parts-per-million range – in fish tissues. But even at these very low concentrations (a part-per-million is approximately the concentration of ink when four drops of ink are released into a 55 gallon drum) there is increasing evidence of potential health impacts, particularly on the developing brain. And although release of mercury into the environment is more tightly controlled, it is still released by industrial incinerators and coal-burning power plants (in 2005, the EPA issued its first ever rule to permanently reduce and cap mercury emissions.)

Once released into the atmosphere mercury may travel across state and country lines before it eventually settles and is transformed from metallic mercury into other forms including highly toxic methylmercury. It is this form of mercury, methylmercury that becomes incorporated into the diet of aquatic creatures and those that eat them.

Here is where the tuna comes in. We all know the story, big fish eat little fish, and bigger fish eat those fish. Big fish include tuna, swordfish and other large ocean species, as well as some freshwater species including lake trout and largemouth bass. Methylmercury concentrates as it moves up the food chain. Generally the larger older predators tend to have the greatest concentration of mercury in the flesh. This is why the EPA and FDA suggest that pregnant or nursing mothers and young children stay away from large predatory fish. According to the EPA, ingestion of chunk light tuna should be limited to 12 ounces a week, while albacore tuna should be limited to 6 ounces. Albacore tuna is a different fish than the tuna used for chunk light, which can be skipjack tuna and in some cases yellowfin tuna. Differences in the size, age and life histories explain the difference in accumulated mercury.

I pull a 6 ounce can of chunk light from the shelf, and hand it to Sophie. It is sad that we need to consider “how much,” of a contaminant we’re willing to ingest, or expose our youngsters to, but until mercury emissions into the environment are fully controlled if fish is part of your diet, then it’s a necessary consideration.

If you want to learn more, there are many good sites that provide greater detail on mercury in fish, mercury toxicity and mercury controls that you may want to explore:

EPA sites: http://www.epa.gov/waterscience/fishadvice/advice.html http://www.epa.gov/air/mercuryrule/basic.htm; Physicians for Social Responsibility: http://www.mercuryaction.org/uploads/PSR_Hg3_FishC.pdf; MA Department of Environmental Protection: http://www.mass.gov/dep/bwp/hgres.htm. Specific fish advisories for freshwater fish in the state may be found by searching: http://db.state.ma.us/dph/fishadvisory/.