By Nancy-Ann Hall
Those of us who were born and raised in the southern Nevada area have been aware of and dealt with concerns about radiation in our environment for over a half a century. With our proximity to the Nevada Test Site and Yucca Mountain, many Nevadans have spent a greater amount of time considering the environmental and health impacts of radioactive contamination than residents in many other states. Whether you have actively worked with radioactive materials, as I have, or have a great fear of them, rational or phobic, it is most concerning when there is a chance that man-made radioactive isotopes have entered our food supply. Many people are unaware that foods already contain low levels of naturally-occurring radioactive elements from the soils where those foods grow or from where the source animals graze or swim.
Recently, a study published in the Proceedings of the National Academy of Sciences of the United States of America, entitled Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California by Daniel J. Madigan, Zofia Baumann, and Nicholas S. Fisher discusses their findings regarding bioaccumulation of radioisotopes in Pacific bluefin tuna, Thunnus orientalis. The two main radionuclides of concern that were found were both isotopes of the same element, cesium. Cesium-134 and cesium-137 were found in white muscle tissue samples collected from Pacific bluefin tuna. These isotopes were traced to the incident called the Fukushima Dai-ichi release that resulted from the earthquake and tsunami on March 11, 2011, which caused flooding of the Fukushima Dai-ichi nuclear power plants in Japan. The isotope with the longer half-life (the amount of time it takes for a radioactive isotope to decay to half its original energy), cesium-137, still had detectable levels in Japan due to the nuclear fallout from weapons testing. At the time prior to the new release, cesium-134, the isotope with the shorter half-life, was undetectable in Pacific surface waters and in marine animals and plants. Following the earthquake, both isotopes were at levels up to 10,000-fold in coastal waters and 1,000-fold in the waters up to 600 kilometers east of Japan. This was after an already-significant dilution into the seawater and a time period of two to three months after the initial release.
The science of this event is interesting, but what does it mean? A few salient points: Tuna, like many other large migratory fish, bioaccumulate substances such as radioactive isotopes and heavy metals. This occurs due to their feeding habits and the wide range of waters through which they travel. Big fish eat smaller fish, thereby taking in and retaining all of the chemicals that the smaller fish had consumed. This process is called biomagnification. Humans, especially in coastal areas such as Japan and the Pacific coast of the United States, enjoy good seafood. In many areas, seafood such as tuna is consumed raw, retaining all biological and chemical contamination that they may have gathered during their normal lifetimes. In any case, heavy metals and radioactive materials cannot be cooked out of fish. Most of us are aware of governmental recommendations to limit the intake of large fish, such as tuna or swordfish, due to the risk of mercury intoxications. But how many people know how much radiation is too much? To most Americans, any radioactive isotopes in their food are too much. Luckily, there are actual guidelines because, as with most things, there are safe and dangerous levels of consumption.
Other concerns that people had after the Japanese accident was how much radioactive contamination would remain in the environment, how it would reach us, and how much would be harmful to humans. The detection in tuna answers some of the questions of how fast, by what means, and how far radioisotopes can travel from one location on earth to another.
This particular study does a very good job of pointing out that while the levels are elevated, they are still orders of magnitude below what would be harmful to the average person. Since translating levels in food directly to radiation doses in people is tricky, it is important for continued monitoring of sea life to maintain a safe seafood supply.
So, for now, consuming tuna caught in the Pacific Ocean poses no unusual risk to humans due to the release of radioactive isotopes from the Japanese nuclear power plant accident. However, it is most certainly worthy of continued study and carefully monitored reporting to the public at-large. Such ongoing studies are planned by the authors of the scientific journal article. At this point, in keeping with the physics of radioactive decay, the likelihood that levels will increase to a point where the tuna is dangerous to people is minimal. And that is good news.
Nancy-Ann Hall, REHS, HHS
Registered Environmental Health Specialist
Certified Food Safety Professional
Subject Matter Expert-Radiological Science