Eight months have passed since the March 11th earthquake which triggered a tsunami that devastated the eastern coast of Japan and provoked the worst nuclear incident since Chernobyl. With the initial fears and the foreign media frenzy now over, one could think that all is back to normal in Japan. From inside Japan however, things are still far from normal, even for those who were fortunate enough to live several hundreds of kilometers away from Fukushima and its crippled nuclear power plant. One of the main problems that all people have to face is to ensure their family’s safety as regards to the food that they consume. While the entire population of Japan is concerned, foreigners are a lot more susceptible than any other as they might not necessarily be able to access or understand crucial information regarding to the provenance and relative safety of the food that they purchase.

To illustrate the consciousness raising that occurred lately, Tokyo authorities have also just started to conduct a systematic testing of the radioactivity in food sampled across the capital. This comes almost at the same time as the publication in PNAS of the first major report on radioactive contamination of Japanese soils. The time is particularly crucial as foreign residents and travelers are slowly starting to return to Japan in spite of legitimate concerns about their safety during their trip. I would like to give you a detailed account of the state of things as regards to radioactivity contamination of foods in Japan and hopefully, provide you with useful information in order to ensure the safest possible traveling and living experience.

Japan is slowly picking up the pieces of its devastated provinces and some residents start to repopulate some of the 20-30 km exclusion zones.[1] Resident foreigners are pretty much all back and tourists start to be visible again in the streets and near the major sightseeing locations. There has been much confusion over the past months with the circulation of a lot of contradictory information and I feel that it is a good time to provide to those who are thinking of traveling in Japan with some clear and concise information about the nature of radioactive contamination and the ways it can enter the body.

The basics of radioactive contamination and exposure

The half-life (T1/2) of a radionuclide is the time taken for half of the atoms to decay. In other words, it takes one half-life for a compound to become half as radioactive.
The main radioisotopes detected following the Fukushima Daiichi accident were iodine-131 (131I; T1/2 = 8.0 days), cesium-134 (134Cs; T1/2 = 2.1 years) and cesium-137 (137Cs; T1/2 = 30.1 years). These are the main sources of interest for their impact on health and the environment even though trace amounts of other short-lived radionuclide like tellurium-132 (132Te; T1/2 = 3.2 days) and iodine-132 (132I; T1/2 = 2.3 hours) were also detected.[2]

Although Iodine was much talked about in the first few days following the disaster, its short half-life (8 days) and the fact that most of the radioactive leakage has been stopped in Fukushima[3] make it less of an issue now. Indeed, the radioactivity caused by the release of this isotope eight months ago has by now reached extremely low levels. This is why current studies now focus on another radioisotope: cesium which has a much longer half-life.

Regarding the situation on site at Fukushima, decontamination of the water seems well on the way if we believe the Japanese MP Yasuhiro Sonoda who even went as far as drinking a full glass of what he presented to be decontaminated water collected at the nuclear power plant (see video below).

Japanese MP drinking Fukushima decontaminated water on November 1st 2011

Making sense of radioactivity measurements

In all sciences, units are a critical for the understanding of any assessment. The misunderstanding of radioactivity measurement units has led to great confusion in the press and in the public. All parties, from the journalists to even TEPCO officials have, at some point or another, published material containing mistakes in either the units used, or in the orders of magnitude announced.[4] Let's try to make sense of it all.[16]

Prefixes and metric system

A lot of confusion has arisen from the misuse of metric system prefixes. Micro (μ) denotes a factor of 10-6 (one millionth, 0.000001) while milli (m) denotes a factor of 10-3 (one thousandth, 0.001). If one takes the meter as an example, there are 1000 μm in 1 mm. If you mix up or invert these two, you are therefore making a misreading of the order of a 1,000 times. Some of you might be rolling their eyes but the mistake was made countless times last March, even by TEPCO themselves.[4]


The International System of Units (SI) unit for radioactivity measurement is the Becquerel (Bq) which is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The unit is equivalent to s-1.

Count per minute

Count per minute (cpm) measures the number of disintegrations per unit of time (here, a minute). Particle detectors have probes that detect the number of atoms in a given quantity of radioactive material that is fired at them.


The sievert (Sv) is a unit that assesses the dose of radiation rather than just the frequency of decay and it is for this reason that it is more relevant in the study of radioactivity effects on biological systems. Unlike the gray (Gy) which measure the amount of energy in joule of ionizing radiation per kilogram of organic tissue, the sieverts measure the equivalent dose of radiation having the same effect as an equal dose of gamma rays. There is no direct conversion from Becquerel and count per minute to sievert as the latter does take into consideration the type of isotope emitting the radiations.


The absorbed dose is a measure of the amount of radiation absorbed by matter and it is measured in gray (Gy). One gray is equivalent to 1 joule absorbed per kilogram of matter. The dose absorbed is independent of the particle type (alpha, beta, gamma).

The equivalent dose allows to take into account the effect of the different types of radioactivity on living tissues. As an example, 1 gray of alpha radiation will have more effect than 1 gray of beta radiation. The equivalent dose is measured in sieverts which is in fact the absorbed dose multiplied by a factor of the radiation.

The effective dose allows to take into account the type of tissue submitted to the radiation. It is measured in sievert. It is in fact the equivalent dose multiplied by a factor that varies according to the type of tissue targeted. This factor is function of the radio sensitivity of each organ as well as the mortality rate of the cancers it triggers.

Routes of radioactive exposure

There are two main ways through which living organisms may be exposed to radiations; external and internal.

Exposure to an external radioactive source

External exposure is roughly the equivalent of being subjected to a light source; as long as you stay under the light source, you are exposed to radiations, but if the source is removed from you, your exposure thus ends. Also, clothes and skin make relatively good shields that protect internal organs from most ionizing radiations.

Exposure to an internal radioactive source

Internal exposure is more problematic and harder to get rid of since it consists in the radioisotopes fixating in the organism (through breathing or ingestion) and emitting radiations from the inside. Iodine was particularly feared in the first days after the accident as volatile radioactive elements flew with the wind and were breathed in by the people. Iodine (radioactive or not) fixes itself on the thyroid (it is used by the thyroid to synthesize its hormone) and therefore, although Iodine half-life is short, it fires directly its particles towards the cells, hence making a lot more damage than external exposure. It is also a lot harder to get rid of. This is why the governments usually distribute heavy doses of Potassium Iodide to populations before potential exposure to radioactive Iodine as the non-radioactive dose saturates the thyroid prior to exposure to the radioactive isotope.

The situation in Japan as regards to radioactive contamination

Airborne contamination

We mentioned earlier that in order to define radioactivity doses as regards to their effect on the health of living organisms, the unit of choice is the Sievert. The doses in Sieverts are often expressed relatively to a time period. The average annual exposure to which an average human is exposed (from both natural and man-made sources) during a year amounts to 3.00 mSv which is therefore expressed as 3.00 mSv/y.[5] This value varies greatly according to the geographical location. For example, in the city of Princeton (USA), resident receive more than double this average does i.e. 6.20 mSv/y.[6] In the France, the average natural background radiation (includes cosmic and terrestrial radiation but excluding man-made ones) is 2.2 mSv/y[7] although it can get a lot higher in certain regions including the Pernian Bassin of Lodève were values can reach a staggering 876 mSv/y.[8][9] Before the Fukushima incident, the average value for Japan was between 1.50 and 3.81 mSv/y. The average dose to which a Tokyoite was usually subjected (before Fukushima) was much lower than that; somewhere between 0.079 and 0.69 mSv/y.[10][11]

To give you some perspective, the current (November 14th 2011, 21:10) hourly dose is estimated at 0.99 µSv/h (8.68 mSv/y) in Fukushima and 0.059 µSv/h (0.52 mSv/y) in Tokyo.[11] This means that Tokyo is currently subjected to six times less radiation than the world average, a value that is within the usual normal historical readings for the city. Fukushima on the other hand is still three times over the World average but only one and a half times higher than that of Princeton.

The absolute maximum amount of radiations in Tokyo (Shinjuku) were detected by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) on March 15th 2011 between 19h00 and 20h00 and values reached 0.361 µSv/h (3.16 mSv/y).[10][12] If this level had stayed constant until now, Tokyo would still be within the average worldwide dose.

I hope that this section reassures you on the airborne exposure that Tokyo inhabitants were exposed to during the crisis. With that out of the way, I now have to focus on the other source of exposure: ingestion through food.

Radioactive isotopes contaminating farming soil

Map of the Japanese prefectures with the highest cesium soil contamination (adapted from Yasuni et al., (2011))
Figure 1. The 137Cs concentration (Bq/kg) in Japanese soil (adapted from Yasunari et al., 2011[14]). The Japanese Food Sanitation Law sets a limit for the sum of 134Cs and 137Cs concentrations (as total cesium) in soil as 5,000 Bq/kg.[15] 137Cs is considered to make up 50% of this amount and therefore, the regions in red are likely to be over this limit.

To any rational individual, this type of exposure is the main source of concern in terms of health effects. Just last week, the Tokyo Metropolitan government, through its Institute of Public Health, started making regular radioactivity measurement in the food sold in shops all across the capital.[13] Even if the initiative comes quite late (they should really have started eight months ago) and that the amount of samples taken (20 to 30 per day) is too ridiculously low for the results to have any sort of statistical significance, it shows that the problem is real enough for Ishihara and his staff to bother conducting this rather symbolic action. On top of that, a major paper by Yasunari et al. (2011)[14] was published last week in PNAS, presenting the first solid results regarding Japanese soil contamination with radioactive cesium.

Conclusion: The problem in terms of food safety IS real.

In brief, the authors of the PNAS paper estimated the quantity and distribution of Cesium-137. It is the isotope with the longest half-life (30.1 years) and therefore it makes it the main source of concern for the decades to come as regards to agriculture, livestock and human food production. The authors hope that the results of their study will help to focus the decontamination procedures and plan future regulatory measures in Japan through suggesting the areas of greater potential contamination and by extension, health risk. On a personal point of view, I think that this article provides very useful information for the consumer as regards to where the regions of greater risks are in terms of food origin.

I have modified the figures by Yasunari et al. (2011)[14] for the purpose of this article. Figure 1 shows a map of Japan with the zones of highest contamination. The authors mention that these are only average values and that radioactive "hot spots" within one region are almost certainly present and thus, they suggest that food from regions with contamination levels around 250 Bq/kg should be dealt with, even though it is below the maximum 5,000 Bq/kg recommended by the Japanese Ministry of Agriculture, Forestry, and Fisheries (MAFF).[15] Author also suggest to keep in mind that Cesium-137 only accounts for half of the total radioactivity emitted by the Fukushima power plant and therefore, while reading the map and associated tables, the safety threshold value to have in mind should really be 2,500 Bq/kg. The bottom line is that unless decontamination is painstakingly conducted, these zones will remain contaminated for many years to come and there is no chance that farmers will be able to resume their activity.

In Table 1, I have classified the eight main prefectures of concern according to soil contamination levels so you can get a good idea of the zones of production to avoid when shopping.

Table 1. The 137Cs concentration (Bq/kg) in soil in the eight most contaminated prefectures of Japan (adapted from Yasunari et al., 2011)[14].





137Cs maximum soil concentration (Bq/kg)

Fukushima 福島 Fukushima Tōhoku 25,000
Miyagi 宮城 Sendai Tōhoku 2,500a
Tochigi 栃木 Utsunomiya Kantō 2,500a
Ibaraki 茨城 Mito Kantō 2,500a
Iwate 岩手 Morioka Tōhoku 2,500a
Yamagata 山形 Yamagata Tōhoku 250
Chiba 千葉 Chiba Kantō 250
Niigata 新潟 Niigata Chūbu 250
aThe Japanese Food Sanitation Law sets a limit for the sum of 134Cs and 137Cs concentrations (as total cesium) in soil as 5,000 Bq kg−1.[15] 137Cs is considered to make up 50% of this amount and therefore, these regions are likely to be at or over this limit.

How to make sure of the origin of the product that you purchase in Japan

The first victims of the food safety concern were the restaurants. The combination of the March disaster with the all-time high of the Japanese Yen very seriously undermined the touristic activity in Japan this year (60% decrease immediately after the March earthquake) and in addition to foreigners leaving and travellers refraining from going to Japan, Japanese customers rightfully assumed that they would have much more control over the origin of what they ate if the actually cooked their own meals. The general mood was also certainly not to eating out and partying. As a consequence, most restaurants had to let some of their staff go due to lack of activity. Luckily for the consumers, Japanese shops have always displayed a great deal of information on the labels of their products and this did prove a good way to avoid certain products and certain locations. Let us look at a typical label for certain food products and try to locate the relevant information for our shopping.

Japanese law does not require the manufacturer to provide the area of origin of the food or its ingredient but they must state the region of manufacture. The difference is quite significant in the case of highly processed and complex foods containing different ingredients and therefore, you can only truly be sure of the origin of simple, unrefined products like fish, meats, flour etc. The problem for products like milk is that the pasteurisation process is significant enough to justify putting the location of the factory, not the farm, on the milk packs so things can be tricky. You can however limit the risk by choosing the milk with the huge Hokkaido label on its box as it is likely to be produced and packaged there. About vegetables, one should also keep in mind that cesium accumulates mainly in the top 5 cm of soil and therefore, crops growing under that should suffer lesser levels of contamination.

Japanese food label translated
Figure 2. Typical Japanese food labels. These are two pictures of Japanese food labels, the one on the left is a Japanese product from Shizuoka and the one on the right comes from England.

Since most foreigners have trouble reading kanji, Figure 2 presents common food labels with the translation of the information contained in it as regard to the origin of the product. You just have to spot the origin of the product on the label and check the prefecture kanji against that of the table. For your convenience, I also produced a little memory card containing a summary of that information which you can download as PDF, print, and keep in your wallet to have with you when shopping. Try to spot the kanji 産 which means production and this one 原 which means origin. Since we are particularly concerned about the Japanese products, and therefore, you will want to spot the kanji 県 which means prefecture. Most foreign products will have the kanji 原産国 meaning country of origin (原: Original, 産: Production, 国: Country) while Japanese products will display the kanji 県産 meaning prefecture of origin (県: Prefecture, 産: Production).

A few rules for safe shopping

  • Buy simple. Raw, unprocessed products have origins that are easily verifiable. I fund that it ends up a actually be a good thing since it avoids me eating too much junk food and forces me to do my own cooking.
  • If you crave for processed foods. I always go for foreign products such as for example my favourite camembert cheese which I know it is made in France. Same goes for my favourite English tea and Belgian beer.
  • When eating out, go cheap. Kaiten sushi and Izakaya just can't afford buying the more expensive, higher quality, Japanese-made products and they always rely on import to stay competitive. While it did not use to be a sale argument, it has definitely become one for me. I know that I have no chance to eat Japanese meat at my favourite Korean yakiniku place and I am sure that the beef used at Matsuya is not from Japan.
  • Go for tap water. In major cities, tap water is far more monitored and regulated than any of their bottled counterparts. It is bar far the safest water to be found anywhere in Japan unless you can afford to drink Perrier.
  • Store what you can. If you are living in Japan or staying for a while, buy big bags of rice or pasta that was produced before March 2011 and go through these before you buy the newer stuff. Give time to the shit to hit the fan and the scandals to break to decide whether to eat the stuff produced after the incident.

Concluding remarks

I hope that this information will help you feel more at ease while you are staying in Japan. Once again, I really don't think there is anything to be alarmed about in terms of airborne contamination anywhere else but in the 20 km exclusion zone. The main concern is concentrated in the food produced within Japan. Remember that the advice I give in terms of the locations to avoid is very pessimistic; it takes into account even extremely low cesium concentrations compared to the official guidelines and one could argue that there is no real reason to avoid food coming from prefectures such as Chiba, Yamagata or Niigata. I guess my response would be: better safe than sorry but even I must confess occasionally eating vegetables from Chiba and Niigata.

It is really great to see tourists come back to Japan and streets of Tokyo be lively again. I am convinced that with a little bit of care and common sense, a trip to Japan is very safe. If you feel that you have additional information to provide to this article, please feel free to comment below and I will add your suggestions to the body of the text. Please come and visit this fantastic country that is Japan and help us make it again the wonderful place that it used to be!


  1. Euronews: 20-30km exclusion zone lifted around Fukushima
  2. Masson, O., et al. (2011) Tracking of Airborne Radionuclides from the Damaged Fukushima Dai-Ichi Nuclear Reactors by European Networks Environmental Science & Technology, 45, 18, 7670-7677
  3. TEPCO: Radiation dose measured in the Fukushima Daiichi Nuclear Power Station
  4. BBC: Japan nuclear plant data error was 'unacceptable'
  5. UNSCEAR (2008) Sources and Effects of Ionizing Radiations, United Nations
  6. Trustees of Princeton University (2011) Open Source Radiation Safety Training Module 2: Background Radiation & Other Sources of Exposure
  7. Billon, S. et al. (2004) Évaluation de l'exposition de la population française à la radioactivité naturelle Radioprotection 39 (2) 213-232
  8. UNSCEAR (200) Annex B: Exposures from 84 natural radiation sources
  9. Delpoux, M. et al. (1996) Experimental study of the genetic effects of high levels of natural radiation in South-France. High Levels of Natural Radiation 1996: Radiation Dose and Health Effects. Z. Tao, T. Sugahara and L. Wei, Exerpta Medica, Elsevier: 397-406.
  10. MEXT: Reading of environmental radioactivity level by prefecture (2011/03/15)
  12. The Tokyo Metropolitan Institute of Public Health: Monitored data on environmental radiation levels in Tokyo
  13. Japan Today: Tokyo starts radiation checks on city's food
  14. Yasunari T. J. et al. (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.
  15. Ministry of Agriculture, Forestry, and Fisheries (MAFF) (2011) A point of view on planting rice plant.
  16. Jennings, W. A., (1972) SI units in radiation measurement. British Journal of Radiology 45 (538) 784-785

To go further

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

comments powered by Disqus


Enter your email address:

to receive an Email at each publication.