by Peggy Olive
The July 19, 2012 issue of Georgia Straight featured a full-page cover graphic showing a cartoon of three-eyed mutant fish cleverly entitled, Nuclear Fishin’. According to the article, high radiation levels in some Pacific Ocean fish have created concern among doctors at B.C. universities. Should we be worried about the health effects of consuming fish from Japan or fish that migrate here from Japan?
Sixteen months after the Fukushima nuclear accident in March 2011, the average level of radioactive cesium in fish from Japan had risen from 5 becquerels per kilogram to 65 Bq/kg. We are told that Japan’s official limit for radioactive cesium in food is 100 Bq/kg. Radioactivity in about 10% of the fish species, some exported to Canada this spring, may have exceeded this limit.
The cesium action level is the level above which food is no longer considered safe for human consumption so that action must be taken to prevent consumption. This level is ten times higher in Canada and elsewhere than in Japan. Why the difference?
According to Yomiuri Online (Dec. 25, 2011), action levels were recently set much lower in Japan in order to ensure the public’s safety and provide reassurance. Do you feel reassured to know that fish considered safe to eat last year is now considered contaminated? This article also goes on to say that the planned tightening of the limits is puzzling local government officials who are charged with monitoring radioactive cesium in food.
There is a good reason behind the choice of 1000 Bq/kg cesium as the action level for our food set by Health Canada and most other international advisory and national regulatory bodies. The cesium action level of 1000 Bq/kg in foodstuffs translates, with a few reasonable assumptions, to a dose to a person of about 5 milli-Sieverts per year. There is international consensus that exposure to 5 mSv in a year is acceptable because we already receive a natural background dose of ionizing radiation of similar magnitude and because no actions have been recommended for avoiding exposure from other natural sources at doses of 5 mSv or less. Radiation workers have an exposure limit of 20 mSv per year, and medical diagnostic procedures like CT scans can produce exposures in excess of 10 mSv.
What can we expect from exposure to 5 mSv? There are recognized limitations in trying to predict health effects from chronic radiation exposures below about 50 mSv (Brenner et al.,Proc. Nat’l. Acad. Sci, 2003). However, acceptable estimates can be made by extrapolating information on cancer risks observed after higher doses. If a population of 10,000 individuals receives 5 mSv, this would be expected to result in an additional two cancer deaths on top of a background of about 2000 cancer deaths in that population. Should we worry if we consume fish that contains 65 Bq/kg cesium radiation? Based on the above numbers, we could expect perhaps one additional cancer in 100,000 people who consume this fish. In my view, the protection afforded by avoiding food with more than1000 Bq/kg cesium is adequate.
Recently, small but measurable levels of radioactivity were found in endangered bluefin tuna that migrated from Japan to the California coast last summer (Madigan et al., Proc. Nat’l Acad. Sci. 2012). The World Health Organization had previously reported that there was no reason for concern about seafood safety outside of Japan. Even though levels of radioactive cesium in the California tuna were 200 times below our action level, the presence of even trace amounts of radioactivity still stimulated concern about whether the tuna was safe to consume. Increasing the risk of developing a radiation-induced cancer by 5% would require eating over 40 tons of this tuna!
In spite of the lack of health concerns with the current action levels in place, fear of eating radioactive fish is widespread and disproportionate to risk. An article in Forbes quotes one of the PNAS study’s coauthors as saying, “My first thought was this will do more for the conservation of this endangered animal (bluefin tuna) than nearly anything else could.” I hope he’s right because this would be the small silver lining in a radioactive cloud.