Radioactive cloud

I recently wrote an article dealing with the risks of radioactive contamination associated with the consumption of food in Japan. This article was prompted by the publication of the first set of data estimating the distribution of the cesium throughout Japan's cultivable land[1]. Earlier this month, the results of another significant effort led by researchers at the University of Tokyo was published[2], this time, dealing with the doses of radioactive iodine to which the populations of the Tokyo area were exposed in the aftermath of the nuclear crisis triggered by the 11 March 2011 earthquake and tsunami and the crippling of the Fukushima Daiichi power plant. In very much the same way as last time, I propose to analyses this paper and try to extract the most significant information for the Tokyo residents or those wishing to visit Japan soon.

I am sure that most of the readers will be eager to jump to the conclusion of the paper and I do not feel like keeping up the suspense. In a nutshell, the paper concludes that the risk of death by cancer associated to the absorption of iodine 131 emitted by the Fukushima crippled reactor falls below the risk of cancer due to natural radiation. The associated mortality rate also falls below that which is due to traffic accidents and environmental pollutants such as diesel exhausts. However, before jumping with joy, I hate to remind the readers that these results only concern iodine 131, and that the outcomes of exposure to cesium must be studied in order to have an overall idea of the extend of the global exposure to radioactivity. Still, this is probably good news even though there are a few limitations that might undermine slightly the positive outcome of this study and I will do my best to go through them in this article.

Indeed, considering the novelty of the data and the eagerness of the public to know about these results, I am wondering why this paper was published in the journal Chemosphere (impact factor 3.15) instead of a more prestigious journal (the paper I referred to in the last article was published in PNAS which has a much higher impact factor of 9.771). I think that the answer to this question has to do with a certain amount of uncertainties (mostly unavoidable) and shortcuts that the authors took to make up for them.

As I mentioned in my previous article, iodine 131 (I-131) has a rather short half-life of 8.04 days, which means that the remaining radioactivity caused by this isotope is now over a year after the outbreak, quite negligible. This of course will interest those wishing to visit Japan soon and who were not here during the accident. For those who were indeed in the Tokyo area following the accident, the scientists propose to estimate the dose of radioactivity received due to exposure to this radioactive iodine according to two situations, with the application of countermeasures, and without.

Countermeasures applied to limit exposure to radioactive iodine 131

Regarding the efficacy of these countermeasures, the short half-life of iodine 131 signifies that these ought to be applied as early as possible following the suspicion of contamination. Please note that I did not list as a countermeasure the wearing of masks and blocking of windows as I discussed previously that the maximum atmospheric radiation doses never exceeded the world average of natural and artificial atmospheric radiations. The data seems solid and was monitored in real-time through various sources and this route of entry can therefore be considered as negligible[3][4].

Following the detection of radioactivity in several food products on March 21[6], the government restricted the distribution of selected food products emanating from the affected prefectures. The detection of radioactivity (210 Bq kg-1)[7] in the water of a Tokyo water treatment plant on March 22 also prompted the distribution of bottled water to infant from March 24.

Radioactive information chart produced by Keio University

Information chart published for foreign Japan residents

However, no data exists before these dates, even though it is likely that heavy radioactive downfall occurred immediately after the accident due to the considerable rain and winds. One other obvious limitation of the paper is that it assumes that the restrictions of the government[9] led to a complete absence of radioactive exposure. In a short, they imply that first, an absence of evidence is an evidence of absence, and second, that the government's regulations were duly followed by all producers and resellers. Unfortunately, the detection of radioactivity in exported products[8] proves that both assumptions are wrong (in addition to the first one being a logical fallacy).

Limitation of consumption of leafy vegetables

Did those who leave Tokyo overreact?
It will be by now obvious to the reader that those who left Tokyo for a few days or more after the disaster probably reduced their own exposure to radionuclides in a very significant manner. This, in addition to the fact that TEPCO had planned to abandon the crippled reactor, and that the government had seriously contemplated the eventuality of an evacuation of Tokyo, should put an end to the ridiculous argument that went on between those who stayed and those who left. Let's consider a few points:
  • Many foreigners were off work (many companies closed for a while) or simply lost their jobs and assuming that they left no unpaid rents of bills what was the real point of staying in Tokyo then?
  • Many foreigners do not master the Japanese language well enough to follow efficiently emergency instructions, particularly under panic. Moreover, they potentially put additional burden on the kind Japanese people who might try to help them.
  • If an emergency evacuation had been necessary, it would be rational to think that the most sensitive, less reactive part of the population is better off having left earlier.

Peer pressure was very high on everybody and those who left often found it difficult to return. On hindsight, if we look at the situation rationally and consider the facts above, who then can argue that there was any point in staying in harm's way? To me, it was probably the most reasonable thing to do to get away from Tokyo at that time and if I had been smarter, I personally would have stayed away for longer until my work resumed. The principle of precaution applied and although things went mostly ok, there was a very serious chance that they could go awfully wrong. I hope that this puts a final nail in the coffin of the "fly-jin" vs. "stay-jin" debate. If you think otherwise, I would be really interested to read your arguments, feel free to share them in the comment section below.

Nine domestically-grown vegetables were considered in the study and the researchers based their calculations on the average intake of these in a typical Japanese diet. Now as foreigners, I think it is safe to assume that our diet might lead us to consume far less of these than the average Japanese person. Also, I reckon that most foreigners probably stayed clear from these vegetables following the first reports of contamination. By the way, I was right to point out in my last article that deep growing vegetables were less of a risk than those growing on the soil (11.1 instead of 33.4 mSv y-1).

Limitation of consumption of milk products

I explained in my last article that it is quite difficult to trace back the origin of dairy products in Japan. The reason is that for the product of a multitude of farms across the country tends to be mixed in the factories and that only the location of the factory is required by law to be written on the packaging. To give some perspective however, the I-131 concentration in milk products was about eight times lower than that detected following the explosion of the Chernobyl reactor (5,300 and 40,000 Bq kg-1, respectively).

Limitation of consumption of tap water

Knowing that a considerable amount of bottled water comes from the same place as tap water, I cannot help but wonder about the real effect of drinking only bottled water on radioactive exposure. I am not sure about the law in Japan but I know for a fact that bottled waters in Europe and the US tend to be subjected to less stringent and frequent screens that that of the mains. I would therefore be careful regarding the conclusions drawn about the efficacy of this counter measure, apart for those drinking bottled water only bought Perrier, Evian, or other foreign waters of course.

Average radioactivity doses received due to exposure to iodine 131

As expected, the doses received by infants were the highest, followed by children and adults. This is due in part to differences in diet; adults being thought to consume less dairy products (the main source of I-131 for infants) even though they consumed tap water (the main source of exposure for adults) as opposed to bottled one. The other factor is the thyroid ingestion dose coefficient, that of adults being lower than that of infants.

Results show that during the period spanning from March 18 2011 to March 20 2012, the average doses without countermeasures for adults, children, and infants were 0.55, 1.97, and 2.79 mSv respectively. The limit for intervention set by the government is 50 mSv in one year so these results fall well below that level. In addition to that, this regulation is still below the doses for which evidence is available in terms of direct effects on cancers[10]. Interestingly, counter measures set by the government on March 21 seem to have led in adults, children, and infants to a reduction in these amounts by 33, 35, and 45% respectively. Please note that these reductions do not concern the period before March 21 and that these few days earlier probably account for most of the exposure unfortunately. The bottom line is that counter measures are efficient but were taken at a much too late date. Hopefully, inhabitants had started to apply their own countermeasures well before that, especially regarding water and food products originating from the north.

Estimation of cancer and death risks

In some parts of this study, perhaps to make up for the potential limitations that I mentioned above, authors considered the worst case scenario for their predictions. In terms of the influence of radiation dose on cancer risk, such an option was used. They considered that the radioactive dose and the risk of cancer were directly and linearly correlated, which probably led to an overestimation of the risk. Although this relationship is still not well understood, the justification for this methodology seems rather arbitrary and if the worst case scenario is indeed used as reference, then the absolute value of the data loses much meaning.

What the results of the study seem to show is that the risks of developing a cancer due to the intake of I-131 was lower than that due to either the naturally occurring K-40, or diesel particles from exhausts. In terms of the mortality rate, exposure to I-131 falls well below that of passive smoking, traffic accidents, drowning in your bathtub, being victim of murder. Unrelated be still interesting, it seems that in Japan, one has greater chances to die from passive smoking than from a traffic accident...

Authors calculated the absolute maximum dose that one might have received in this period, a sort of worst case scenario which assumes that one kept eating and drinking the most contaminated aliments. In this case, the doses received between Marc 21 2011 and March 22 2012 were estimated at 5.0, 35, and 67 mSv for adults, children, and infants respectively. This means that even in the worst case scenario as imagined by the authors, only the children received a value that was above the intervention level (50 mSv y-1).

Did German and French citizens really "flee the country in a panic"?
In a word: No. According to Foreign Correspondents' Club of Japan[5] only 6% of the Germans (5,949 out of 102,831) and French (7,744 out of 131,100) citizen in Japan left between March 12 and April 1. The British were 7% in spite of the more rational communication of their embassy, and the Australians, Americans, and Canadians were 5% to leave. It was hardly an exodus and the large majority of foreign resident did stay in the country. The largest amount of people who left were the Indians (16%), followed by the Chinese (13%). If observed in Tokyo residents only, this proportion might have been larger but this is still far from the alarmist headlines see around the world[5][7].


Perhaps the most disappointing aspect of this paper is its very succinct discussion. In my eyes, the authors did not discuss enough the practical validity and limitations of their results, especially in terms of the assumptions that they had to make in order to iron out the intrinsic uncertainties. Also, very little was said about the doses potentially received between the March 11 2011 and March 17, which, considering the way the rest of the data goes, is when the peak of exposure probably occurred.

Quite interestingly, these results also show that on hindsight, the precautions taken by the most careful citizens (or those who trusted less the over optimistic reports of the government), in spite of the raised eyebrows and even mockeries from fellow commoners, successfully managed to significantly reduce the exposure to radioactivity.

The authors mentioned at the end of their paper that a similar study dealing with the intake of cesium 134 and 137 is on its way. This will prove to be determinant in order to get a clear idea of the total exposure of radioactivity that Tokyo inhabitants were subjected to. Although cesium's much longer half-life (134Cs; T1/2 = 2.1 years and 137Cs; T1/2 = 30.1 years) will make it a lot easier to trace back and measure, it also implies that still to this day and for many years to come, we will be exposed to its potential invisible threat. I will be eagerly waiting for the publication of this paper and of course, I will report about it as soon as possible.

Documentary: BBC Two - Horizon, 2011-2012, Fukushima: Is Nuclear Power Safe?


  1. Yasunari T. J., Stohl A., Hayano R. S., Burkhart J. F., Eckhardt S. and Yasunari T. (2011) Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident. Proceedings of the National Academy of Sciences; published ahead of print November 14, 2011, doi:10.1073/pnas.1112058108.
  2. Murakami, M. and Oki, T. Estimation of thyroid doses and health risk resulting from the intake of radioactive iodine in foods and drinking water by the citizens of Tokyo after the Fukushima nuclear accident. Chemosphere (2012). Doi:10.1016/j.chemosphere.2012.02.028
  3. MEXT: Reading of environmental radioactivity level by prefecture (2011/03/15)
  4. The Tokyo Metropolitan Institute of Public Health: Monitored data on environmental radiation levels in Tokyo
  5. Johnston, E., The Tohoku Earthquake and Tsunami, the Fukushima Nuclear Reactor, and How the World's Media Reported Them. The Japan Times News Digest. August 2011.
  6. Ogasawara, T. 3.11 - A Chronicle of events following the Great East Japan Earthquake. The Japan Times Special Report. June 2011.
  7. Detection of radioactive materials in tap water in Fukushima Prefecture and Tokyo. Ministry of Health, Labour and Welfare, Water Supply Division, Health Service Bureau. March 23, 2011
  8. Japanese imported vegetables in Singapore 'radioactive'. BBC News 24 March 2011
  9. Funabashi, Y. and Takenaka, H. Lessons from the Disaster – Risk management and the compound crisis presented by the Great East Japan Earthquake. The Japan Times. December 2011.
  10. Brenner DJ, Doll R, Goodhead DT, Hall EJ, Land CE, Little JB, Lubin JH, Preston DL, Preston RJ, Puskin JS, Ron E, Sachs RK, Samet JM, Setlow RB, and Zaider M. Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know PNAS 2003 100 (24) 13761-13766; published ahead of print November 10, 2003, doi:10.1073/pnas.2235592100

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About the author
Guillaume Erard
Author: Guillaume ErardWebsite:
Founder of the site in 2007, Guillaume has a passion for Japanese culture and martial arts. After having practiced Judo during childhood, he started studying Aikido in 1996, and Daito-ryu Aiki-jujutsu in 2008. He currently holds the ranks of 4th Dan in Aikido (Aikikai) and 2nd Dan in Daito-ryu Aiki-jujutsu (Takumakai). Guillaume is also passionate about science and education and he holds a PhD in Molecular and Cell Biology since 2010. He currently lives in Tokyo and works as a consultant for medical research. > View Full Profile

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