This post is based on a more detailed post by Dr. Jonathan Kellogg on the Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) project website. It contributes to an ongoing effort to provide scientifically derived evidence to determine Fukushima’s impact on environmental and public health. After extended testing of select 2016 salmon samples the Fukushima-fingerprint isotope was found in single fish. The maximum level of contamination observed in this sample (134Cs, 0.07 Bq kg-1; 137Cs, 0.51 Bq kg-1) is over 1,700 times lower than the Health Canada Action Level (1,000 Bq kg-1) and does not represent a significant health risk for either the fish in question or consumers of Pacific salmon including human beings. The InFORM project continues its monitoring of seawater and marine biota for signs of Fukushima derived contamination to assess the potential impact on the marine ecosystem and public health. Results are reported as they are generated here.
These results are an update to the 2016 monitoring effort where nine salmon samples (of the 123 collected) were selected for extended, 2 week testing (samples were placed in the gamma radiation detector for a total of 336 hours), after the 6 hour initial analysis detected trace concentrations of cesium-137 (137Cs, half-life ≈ 30 years). The extended detection time results in a more precise measurement of 137Cs and an increased probability of detecting cesium-134 (134Cs, half-life ≈ 2 years), the Fukushima fingerprint isotope. This longer analysis is similar (but not identical) to increasing the exposure time on a camera in low light conditions to enhance image details and is the same procedure that allowed us to detect 134Cs in one sample in 2015. The results from this analysis confirmed low levels of 137Cs in all 9 samples and detected 134Cs in one sample. That sample was from one pink salmon (Oncorhynchus gorbuscha) from the Nass River on the northern coast of British Columbia.
Results from the nine samples from 2016 which were subjected to extended counting of radioactive decays in the gamma spectrometer (336 versus 6 hours normally). Cesium-134, the Fukushima fingerprint isotope, was detected in only one pink salmon after this more rigorous analysis.
What do these levels mean in terms of dose and risk of ionization radiation to consumers?
137Cs is more readily detected in salmon, and the environment, in part due to the 30 year half-life of this isotope. Detectable quantities from atmospheric testing of nuclear weapons and the 1986 Chernobyl disaster persist in the environment in addition to the quantities released from Fukushima. However, the 2 year half-life of 134Cs means that it is essentially only present in the environment due to the Fukushima disaster as all other sources have decayed away. In 2016, when these fish were collected, five years had passed since the disaster which means that only about 18% of the original 134Cs released remains in the environment. Mathematically, we can correct for this decay to determine how much Fukushima contamination exists in the fish as 137Cs and 134Cs were released from the disaster in roughly equal quantities.
The health impact of the radiocesium concentrations can be compared to that of the naturally occurring radionuclides in the environment by converting activity (Bq kg-1) measurements to the effective dose (Sv). This is calculated by factoring in the exposure (the average Canadian eats about 8.8 kg of seafood annually) and using a conversion factor called the dose coefficient recommended by the International Commission on Radiological Protection.
The total dose that would be ingested in a year’s supply of salmon, assuming all had the contamination of this one, most contaminated fish, would be 6.62 μSv. Of that, 99% of the dose (6.55 μSv) is due to the naturally present levels of Polonium-210 (210Po) while cesium contributes just 1% of the dose (0.09 μSv). Considering the hundreds of fish InFORM has analyzed from various west coast rivers, this dose for Fukushima-derived contamination represents a very good estimate of the maximum dose that Canadians are likely to experience owing to the disaster.
Negligible contribution of Fukushima derived ionizing radiation dose to Canadians in context of variability in background dose.
210Po (half-life = 138 days) is part of the natural decay series of Uranium-238 (238U, half-life = 4.47 billion years). Uranium is naturally present in seawater owing to the erosion of U, naturally in the crust and delivery to the ocean by rivers over the history of the Earth. 210Po is known from previous research to be the most significant ionizing radiation dose contributor owing to the consumption of salmon or seafood. This is why InFORM scientist, Dr. Jing Chen of Health Canada's Radiation Protection Bureau, has been investigating the variability of this isotope in BC and Yukon salmon since InFORM monitoring began in 2013. Levels of 210Po in this fish are consistent with other results from her study, which will be featured in an upcoming post.
Seafood is just one source of the normal background radiation dose that everyone alive receives in a year. Radiation is present in the food we eat, water we drink, in the air we breathe, and bombarding us from the cosmos above. The ingested radiation dose from cesium and polonium in BC salmon is a small fraction of the worldwide average annual effective dose from exposure to natural background radiation (2400 µSv per year).
