The Azouz and Dulai paper was published recently in the journal Pacific Science and can be found here. The authors obtained 13 different species (Ahi, Albacore Tuna, King Salmon, Cod, Dover Sole, Halibut, Mahi Mahi, Monchong, Onaga, Opah, Opakapaka, Swordfish and Yellowfin Tuna) of fish that were caught in the North Pacific (>20oN) and commonly consumed in Hawai’i at local markets. Information about the range and size of the fish are given in Table 1:
Levels of Radiocesium in Fish From Hawai’i
Samples of the fish tissue were freeze dried and homogenized before gamma emitting radioisotopes were measured using a gamma spectrometer by counting samples for a period of 7 days. Levels of 134Cs, because of its short half life, serve as a fingerprint of Fukushima in samples as previous sources of this human made isotope (e.g. 20th century nuclear weapons testing and the Chernobyl disaster) are sufficiently far in the past that all of the isotope has decayed away and is no longer present in the environment. Results of the analyses are summarized in the following figure:
In 3 fish statistically significant (>95% confidence interval) but trace levels of 134Cs was detected. Given that 137Cs/134Cs ratio in vast majority of the release from the Fukushima site was ~1 the authors were able to determine the fraction of radiocesium present in these fish owing to Fukushima versus legacy sources like atmospheric weapons testing. Maximum radiocesium levels in the fish approached 0.7-0.8 Bq kg-1 which is more than 1,500 fold lower than conservative levels thought be a health risk set by the FDA (1,200 Bq kg-1). Most fish had radiocesium attributable to weapons testing fallout. Fukushima radiocesium accounted for ~60% of the radiocesium detected in an Ahi measured by the authors.
Levels of Naturally Occurring 40-Potassium in Fish
Naturally occurring 40K decays with a half life of 1.25 billion years and in taken up into the tissue of marine fish. The levels of 40K in the fish measured by the authors are summarized in the table below:
Activity of 40K (Bq kg-1) tended be ~100 fold higher in the fish tissue than radiocesium activities.
Effective Dose of Ionizing Radiation and Health Impact to Fish Consumers
The authors determined the impact of fish consumption on the ionizing radiation dose experienced by individuals consuming an average amount of fish per year (24.1 kg per year or 53.1 pounds per year). The table below compares the dose in nanoSieverts per year (10-9 Sv yr-1) owing to historic and Fukushima sourced radiocesium and naturally occurring 40K in seafood.
Converting isotope activities in the fish to dose demonstrates that 40K is responsible for ~100 times higher dose than 134Cs + 137Cs. Doses to humans from consuming the fish owing to radiocesium were 0.02–0.2 µ Sv yr-1, while doses of 6–20 µ Sv yr-1 were contributed by the natural 40K present in the same fish. These levels of radioisotopes and the calculated doses to consumers are similar to those reported by the InFORM project who have looked at Pacific salmon returning to rivers and streams in North America over the last 3-4 years. It is important to note that the bulk of ionizing radiation dose to fish consumers normally results from 210-Polonium (210Po half life 138 days) naturally present in the fish but this isotope was not measured in the Azouz and Dulai study.
Fukushima derived radioisotopes 134Cs and 137Cs were detected (at 95% confidence interval) in 3 of 13 fish caught in the North Pacific and commonly consumed by people living in the Hawaiian islands. The radiocesium in most fish reflected contamination largely present in the North Pacific Ocean owing to atmospheric weapons testing during the last century. The levels of radiocesium in the fish were a small fraction of the levels of naturally occurring radioisotopes like 40K. The committed effective dose of ionizing radiation to fish consumers is dominated by the naturally occurring isotopes and do not remotely approach levels known to represent a significant or measurable health risk to human beings. The results of this study agree with previously published research and results of the InFORM project which focuses on the impact of the Fukushima disaster on the marine ecosystem and public health in North America.