Sorry Charlie, There’s Cesium-137 In Your Tuna

The announcement yesterday that radioactive contamination from the Fukushima Daiichi nuclear disaster has been found in bluefin tuna spread widely in just over a day. The media accounts were frequently short on details making it very hard to understand what it all really means. Dr. Fisher of Stony Brook University, one of the authors of the study was kind enough to provide us with an advance copy of the paper, Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California. The paper will come out on the PNAS website later this week.

The bluefin tuna were caught in San Diego in August 2011, five months after the start of the Fukushima disaster. The fish are estimated to have been exposed for about 1 month before they migrated across the Pacific. Based on the age of the fish caught they grew considerably during their migration, adding to the ability to lower their cesium contamination.  This is discussed as “growth dilution” that took place over the 4 months the fish migrated. The fish were back calculated to confirm they originally had cesium concentrations that matched those of bluefin tuna found off Japan that had a rate of 61–168 Bq kg. The current Japanese maximum contamination allowed in food is 100 bq/kg. The study tested for and excluded from results naturally occurring radiation sources like potassium 40. They also were able to calculate and sort out existing cesium 137 contamination left over from old bomb testing fallout. The test results found are over and above any naturally occurring radiation and any background from bomb testing. The bluefin tuna caught and tested in California in August 2011 had 4 bq/kg of cesium 134 and 6.3 bq/kg of cesium 137.

The study raised speculation among the researchers that other highly migratory species that frequent Japan such as sharks, seals, sea turtles and whales could also be subject to the same phenomena. The researchers did not make solid claims that this is happening in other species but urge wider testing to understand if this is happening on a wider scale. The researchers say they will re-test bluefin tuna in the San Diego area this August to see how levels have changed. This may give more clues about how the fish are being contaminated over a longer period of time and if the levels are increasing or decreasing.

Bluefin tuna feed on a variety of sea life. Anchovies, squid, sardine, mackerel, sand dab and other small fishes. Since some of the bluefin tuna’s diet of small fishes are not also used for human food there is a lack of radiation testing for many of the food stocks. There is a fishing ban in Fukushima so contaminated fish the bluefin tuna are consuming may not be in the testing data. What testing is going on in the area is showing a high frequency of contamination with levels in certain fish in four digit readings. Anchovies were already showing with cesium in them about 3 weeks after the accident began.

The various media accounts of the study have included some confusing statements. Statements that the findings are “below limits” or “totally safe” are misleading and confusing as they don’t give a clear understanding of what was found. The US FDA level of contamination(1) before action is taken is 1200 bq/kg, considerably higher than the Japanese level of 100 bq/kg. While the findings in the paper are low it doesn’t mean there is no risk. Eating a piece of  tuna from the study won’t make someone drop dead at the sushi bar, but it does add to your internal contamination level of a man made radioactive substance that takes 110 days for half of it to clear your body. Internal contamination is worse than external contamination. If someone is continually eating contaminated food from one or many sources every day it does add up quickly. Some people in Japan tested by the NGO group ACRO were found to have significant amounts of cesium in their urine that increased over time rather than decreased. This was due to diet, as these people became more selective in their food choices their internal contamination levels decreased. The US “safe” level is incredibly high, any report citing “below safety limits” rather than actual contamination results are being a bit dishonest.

Other confusing statements made in relation to the bluefin tuna paper such as comparing it to an airplane flight or an xray. Both of these are external exposures, this is not the same as internal exposure one would receive from eating contaminated tuna. Others have tried comparing exposure to a nuclear accident to eating a banana or brazil nuts, two foods that contain naturally occurring potassium 40. Potassium 40’s natural abundance is quite low. It also clears the body at a half life of 30 days compared to 110 days for cesium. The specific activity of potassium 40 is 0.0000071 (Ci/g) compared to 1,300 for cesium 134 and 88 for cesium 137. Potassium 40’s radiation energy is 0.52 MeV for beta and  0.16 MeV for gamma. Cesium 134 has a radiation energy level of 0.16 MeV for beta and 1.6 for gamma. Cesium 137 has a radiation energy of  0.19 MeV for beta but also decays into barium 137m that has a gamma energy of 0.60 MeV. Cesium 134 and 137 are not naturally occurring isotopes and have a much different energy and composition than potassium 40. Comparing the cesium found in these tuna to the potassium 40 in a banana is just not an accurate comparison.
See additional reading below for more information on potassium 40 and cesium 134 & 137.

These media statements are made to try to shorthand the risk level for the public but they do a horrible job of explaining the risk of food contamination or exposure after a nuclear accident. The study isn’t really cause for immediate panic. It does show that contamination higher on the food chain happened much faster than anyone including the researchers had assumed. It also shows the ability for migratory sea life to become contaminated in one location, then travel vast distances to be landed as food. The recently published study by Woods Hole researchers that looked for radiation contamination patterns in the Pacific also resulted in some unexpected findings. Both studies concluded that more testing is necessary to fully understand the impact the Fukushima disaster has had on the Pacific.

1. FDA US intervention levels USderivedsafetylevels PDF in new window

Additional Reading:
http://www.ead.anl.gov/pub/doc/cesium.pdf
http://www.ead.anl.gov/pub/doc/potassium.pdf

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