The purpose of this diary is to report new measurements of radioactivity in fish caught off the west coast of Canada. A collaborative effort between Health Canada, Department of Fisheries and Oceans Canada, and the University of Victoria is now published in the peer-reviewed, open-access scientific journal Radiation Protection Dosimetry (link). The authors examined the activities of cesium radioisotopes (134-Cs half-life ~2 years and 137-Cs half-life ~30 years) that were released in large quantities from the Fukushima Dai-ichi Nuclear Power Plant disaster in 2011 as well as a naturally occurring polonium isotope (210-Po) that can pose radiological health concerns for human consumers of marine fish. Samples of chum and coho salmon, halibut, sablefish and spiny dogfish were analyzed and none were found to contain detectable levels of Fukushima derived radionuclides. Radiation doses to human consumers were determined by assuming a conservative worst case scenario where Cs isotopes were present at detection limits of the measurement and found to be 18 times lower than doses attributable to the naturally occurring, alpha-emitter 210-Po. The authors conclude that the radiation dose from Fukushima derived isotopes present in fish caught in Canadian waters represent a very small fraction of the annual dose from exposure to natural background radiation. Based on these measurements, at present, Fukushima derived radionuclides in fish do not represent a significant radiological health risk to Canadians.
This diary is part of an ongoing series to report scientific studies on the impacts of the Fukushima Dai-ichi nuclear disaster on the North Pacific and residents of the west coast of North America. The paper in question by Dr. Jing Chen and colleagues was made available today on-line. The study examined the radioactivity of 62 fish caught off the coast of British Columbia, Canada between June and November 2013. To test for the presence of Fukushima derived radionuclides in the fish the investigators measured 134-Cs which because of its relatively short half-life of ~2 years is an unambiguous tracer of release from the March 2011 disaster as legacy sources from Chernobyl (1986) and atmospheric weapons testing in the 20th century are no longer present in the environment. Because of the significant amounts released, its longer half-life and its propensity to concentrate in marine organisms given its similar chemistry to potassium (K), 137-Cs was also measured as this isotope represents the most likely radiological health risk to marine organisms and human consumers. To provide perspective on the radiation dose attributable to Fukushima derived Cs, the naturally occurring, alpha-emitter 210-Po and the radiation dose to human consumers attributable to its presence was determine in a subset of fish samples.
For those not familiar with the scientific units used to report radioactivity in the environment please refer to this summary diary.
Fukushima 134-Cs and 137-Cs were not detected in any fish samples given a detection limit of 1.9 Bq/kg and 1.8 Bq/kg respectively. Recently reported measurements of Fukushima 134-Cs and 137-Cs in albacore tuna using more a more sensitive analytical approach were 0.02-0.36 Bq/kg and 0.23-0.82 Bq/kg respectively. To calculate the radiation dose to human consumers of fish harvested off of British Columbia the authors of the Canadian study assumed, conservatively, that 137-Cs was present in fish samples at activities equal and that 134-Cs was present at half of the detection limits of their analyses. This likely overestimates the activities and, therefore, the radiation dose associated with Fukushima Cs isotopes in consumed fish.
Radiation dose from Cs isotopes and naturally occurring 210-Po were calculated for adult and children consumers of the fish and are reported in Table 2 shown below:
The following graph shows the relative dose experienced by an adult consumer of 20 kg (44 pounds) of Fukushima contaminated fish in a year from Cs isotopes in blue and natural 210-Po in red. The size of the circles corresponds to the dose in microSv per year. In subsequent figures the circles will be rescaled to allow comparison to other common radiation exposures.
Finally, the dose from fish and smoking can be compared with the sum of all other sources (air, soil, food and water) experienced by individuals living in different parts of Canada. These regional differences reflect differences in altitude and local geology that impact exposure and dose from radionuclides in the environment.
To summarize, similar to other peer-reviewed studies published to date (for example Fisher et al. 2013 and Neville et al. 2014), the activities of Fukushima derived radionuclides in fish harvested off the North American west coast do not result in radiation doses that pose radiological health risks to human consumers at present. Ongoing monitoring and measurements of radioisotopes in seawater and marine organisms will be important to provide the public with ongoing determinations of health risks. More information will be made available here as it becomes available.