by Jay T. Cullen

The purpose of this post is to report measurements of radioactivity in fish caught off the west coast of Canada based on the work of InFORM team member Dr. Jing Chen.  A collaborative effort between Health Canada, Department of Fisheries and Oceans Canada, and the University of Victoria was published in May 2014 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 due to the triple reactor meltdowns at Fukushima Dai-ichi Nuclear Power Plant 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 post 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 is freely available online. 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, scientists use a variety of units to measure radioactivity. A commonly used unit is the Becquerel (Bq for short) which represents an amount of radioactive material where one atom decays per second and has units of inverse time (per second). Another unit commonly used is disintegrations per minute (dpm) where the number of atoms undergoing radioactive decay in one minute are counted (so 1 Bq = 60 dpm).

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. 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:

Doses are calculated in a unit called the Sievert which is explained in detail here. The dose from the combined 134-Cs and 137-Cs in fish was ~18 times lower for adults and 96 times lower for children than the dose they would receive from 210-Po naturally present in fish. To put these numbers into perspective we can compare these doses to others experienced by Canadians in the following figures. The dose to human beings is calculated from activities of isotopes present in the environment by taking into account, the activity, the type of radiation emitted, internal vs external exposure and the tissue affected by the energy according to established methodology (see Fisher et al. (2013) and references therein).

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 where a microSv is one-millionth of a Sv (0.000001 Sv). In subsequent figures the circles will be rescaled to allow comparison to other common radiation exposures.

We have all read and heard from our loved-ones that smoking is bad for our health causing 1 in 5 deaths and a majority of lung cancers. One reason why this is so is that tobacco plants concentrate naturally occurring radionculides in their leaves whereby smokers introduce these potentially harmful isotopes (for example 210-Po and 210-Pb) directly to the lungs. A pack a day cigarette smoker will experience a dose of ~360 microSv per year which compares as follows to the consumption of 20 kg of fish with Fukushima Cs and natural 210-Po.

To receive the same radiation yearly dose from isotopes that were released to the Pacific from Fukushima as someone who smokes cigarettes one would need to eat over 40 lbs of west coast fish per day (conservatively) for an entire year.

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.

There is roughly a factor of 3 variation in dose from background across Canada from ~1200 to ~4000 microSv per year. These doses are 1000’s of times greater than what one would experience from consuming 20 kg of fish harvested from west coast waters per year at present.