A Survey on the Concentration of Radioactive Cesium in Japanese Milk Samples (2021)
Investigating the origin of the contamination and assessing the risks of cases where the radioactive cesium level is below the Standard Limits for Radionuclides in Foods
By Tanimura Nobuko (NPO Citizens’ Nuclear Information Center) and Fuseya Yumiko (NPO Shinjuku Yoyogi Citizen Monitoring Center)
- Introduction
Radioactive contamination of food due to the Fukushima Daiichi Nuclear Power Station accident
The Fukushima Daiichi Nuclear Power Station accident occurred in March 2011, resulting in the contamination of the environment and food by radioactive materials. At that time, no standard limit of radiation levels of food distributed in Japan existed, and a temporary standard was therefore set by the government. In April 2012, about one year after the accident, new standard limits were established, in which radiation exposure due to the consumption of food was limited to 1mSv per year, thus making the standard value of radioactive cesium 100Bq/kg for general food, 50Bq/kg for milk and baby/infant food and 10Bq/kg for drinking water. While this was supposed to regulate contaminated food, there were many reports of the discovery of food that was over the standard limit being distributed. In fiscal year 2020, twenty-two cases of food over the standard limit were recognized.
Due to inadequate food contamination checks and distribution management systems, some people who refused to be unnecessarily exposed to radiation decided to select food according to the area in which it was produced in order to avoid risks of radiation exposure. The Japanese government terms this action—avoiding the food produced in the affected area of the Great East Japan Earthquake—‘reputational damage,’ something which must be eliminated if reconstruction is to proceed.
Risks of radiation and risks of chemical materials
However, is making a choice of avoiding a health risk an unfair act that prevents the reconstruction of the affected areas?
The cancer risk of chemicals in tap water and other substances is regulated to a level of 1 in 100,000 per substance, which is usually the risk for a lifetime of ingestion of such substances. But those who do not want to take the risk of developing cancer from chemicals in food are able to buy organic products at some additional cost. This consumer action is not criticized by the government as ‘reputational damage.’
The standard for radiation exposure from consuming food is set at 1mSv/year. According to the ICRP (International Commission on Radiological Protection) the fatality risk is about 5% per 1Sv and ‘the standard is based on the hypothesis that the probability of radiation-induced cancer or hereditary effects increases in direct proportion to the increase in dose.’ In addition, it estimates that the fatality risk is 0.4% if a person continues to be exposed to radiation at 1 mSv per year throughout his/her lifetime.
Adding five hundred-thousandths per year (0.005% for 1mSv/year) means that if we take 80 years as the average lifetime, four hundred people’s deaths are added per hundred thousand people, a risk that is two digits higher than that for chemical materials.
It is sometimes claimed that as the risk of radiation exposure is the total of the risks of various radionuclides, comparing the regulation for one chemical material to the regulation for radiation exposure is improper, since there are thousands of chemical materials that we may come into contact with in daily life. However, the radiation exposure standard of 1mSv per year is not the ‘total’ amount of exposure but an ‘additional’ exposure. The following are all conveniently used to explain that exposure of 1 mSv or less is safe: a) the radiation dose limit for the public from nuclear facilities under pre-accident conditions, b) the new standard for radioactive cesium in food, c) the standard of 8,000 Bq/kg for “designated waste” introduced to handle the large amount of radioactive waste generated by the Fukushima nuclear accident (exposure of workers disposing of the waste), and d) the standard for the disposal of radioactively contaminated water, which has been the focus of attention at the Fukushima nuclear power plant, now undergoing decommissioning. The fact that these exposure risks add up, as well as the effects of this addition, have not been explained to civil society by the regulators, and of course have not been discussed.
In addition, the risk assessment of carcinogenic chemical materials is based on ‘causing cancer,’ but the risk assessment of radiation exposure is based on ‘deaths from cancer’; it is impossible to compare the two risks. ICRP estimates that the risk of ‘developing cancer’ is twice as high as that of ‘death from cancer.’
Further, it is the user who decides whether or not to use chemical materials, weighing the advantages and disadvantages of use, but in the case of exposure due to nuclear power plant accidents, there are no direct advantages to anyone.
Research and objective
We would like to support people’s right of choice to avoid the risk of radiation exposure by measuring even low levels of radioactive cesium contained in milk and disclosing the areas of production and levels of contamination.
Even though many people avoided milk produced in the Tohoku district and chose milk produced in Hokkaido to avoid risks of radiation exposure. In FY2020 our research revealed the unanticipated fact that the milk produced in Hokkaido is also contaminated due to the Fukushima accident. The survey was therefore expanded to include western Japan products in order to compare contamination on a national scale.
Each measurement sample was 22kg of commercial milk which had an identifiable production location. In FY2021, the milk produced in 11 areas was included in the survey: Iwate (K), Miyagi (L), Ibaraki (M), Tokyo (N), Shizuoka (O), Ehime and Kochi (P), Miyazaki and Kagoshima (Q), Nagasaki (R), Oita (S), Shimane (T), and Ishikawa (U). (See Table 1. Areas A to J indicate areas surveyed in FY2020).
In measuring the concentration of radioactive cesium, 2kg of the sample was used as a direct measurement sample and the remainder (20kg) was used as a concentrated measurement sample. In order to detect small amounts of radioactive cesium, we performed the ammonium phosphomolybdate (AMP) method on whey after separating the whey from the milk.
The germanium semiconductor detector (BSI Co. GCD70-200) was used for gamma ray detection in these radioactivity measurements.
The results of the measurements are shown in Table 1. In FY 2021, the production areas in which cesium 137 was detected in the direct measurement were Iwate (K) and Miyagi (L). No cesium 134 was detected in any areas in the direct measurement. In the concentrated measurement, cesium 137 was detected in all areas including the Kyushu district. The most contaminated area was Miyagi (L) with 152mBq/kg. This figure was higher than the 135mBq/kg found in Fukushima (H). The second most contaminated was the milk produced in Iwate (K), 79mBq/kg. These were followed by Shizuoka (O) (16mBq/kg), Ibaraki (M) (11mBq/kg), Tokyo (N) (7.3mBq/kg), Miyazaki and Kagoshima (Q)(7.0Bq/kg), Oita (S) (5.7mBq/kg), Shimane (T) (5.4mBq/kg), Nagasaki (R) (5.2mBq), Ehime and Kochi (P) (5.1mBq.kg) and Ishikawa (U) (3.9mBq/kg). Cesium 134 was detected only in Miyagi (L) (4.4mBq.kg) and Iwate (K) (2.0mBq/kg).
Please note that about 90% of cesium in milk exists in the whey, and therefore adopting this cesium concentration method indicates figures that are 10% lower in the concentrated measurement than in the direct measurement.
- Discussion
Origin of cesium 137
At the time the Fukushima Daiichi Nuclear Power Station accident occurred, cesium 134 and cesium 137 were emitted into the environment at the ratio of about 1:1. The half-life of cesium 137 is about 30 years, and that of cesium 134 is about 2.1 years. By taking into account the half-life of cesium 134 and 137, and the length of time from the disaster to the measurement, it is possible to calculate a cesium ratio (cesium 134/cesium 137) of cesium that was emitted as a result of the accident at the time of the measurement. Five years after the accident (March 2016), the cesium ratio had fallen to 0.21, Ten years after (March 2021), it had decreased to 0.046.
The cesium ratio differs slightly according to each reactor. Thus the cesium ratios due to the accident in the fallout in each location are different. The initial cesium ratios in the atmospheric fallouts in each area from March to May in 2011 were calculated using data from the environmental radiation database.
The proportions of cesium-137 (derived from the Fukushima nuclear reactor / total in the milk sample) were derived by calculating the measured value of cesium 134 concentration and calculated cesium 134/137 ratio at the time of measurement.
In the survey conducted last fiscal year (2021), the production areas where cesium 134 was detected were Hokkaido, Fukushima, Gunma, Tochigi, Iwate and Miyagi. The cesium 134/137 ratios in the atmospheric fallouts soon after the Fukushima nuclear disaster were as follows: Hokkaido 1.05, Fukushima 0.94, Gunma 1.00, Tochigi 1.01, Iwate 1.00 and Miyagi 1.00. The cesium 134/137 ratios (at the time of the measurement) were calculated by using the figures for the half-life of each cesium radionuclide and the number of years from the disaster to when the measurements were taken. The following figures were obtained: Hokkaido 0.053, Fukushima 0.044, Gunma 0.045, Tochigi 0.045, Iwate 0.035 and Miyagi 0.036. Dividing the cesium 134 concentration of the measurement by the cesium 134/137 ratio (at the time of the measurement) of the sample production area, the results of cesium 137 concentrations that derived from the Fukushima nuclear disaster were obtained and are shown in Table 2.
The following is one calculation example for Iwate. The cesium 134/137 ratio measured on November 19 in 2021 which was derived from the Fukushima nuclear disaster was calculated as 0.035 in Iwate. As the result of the measurement, 2.0±0.1mBq/kg of cesium 134 was detected and therefore cesium 137 derived from Fukushima reactor should be 56±3.9mBq/kg on the basis of the cesium 134/137 ratio. However, the measurement result of the concentration of cesium 137 in the sample was 79±0.8mBq/kg. The reason why it was higher than expected is because it contained cesium 137 that traces back to nuclear weapon tests and other sources. Thus, out of the total cesium 137 contained in the milk produced in Iwate, it was concluded that the ratio of 0.71±0.05 was derived from the Fukushima nuclear accident.
The origins and concentrations of cesium 137 in the samples were compared based on the production areas (Figure 2). In western Japan (P-U), which was supposed to have been mostly uninfluenced by the Fukushima nuclear disaster, the concentrations of cesium 137 were below 7mBq/kg. This cesium 137 was thought to be derived from various nuclear weapon tests and the Chernobyl disaster.
By contrast, in eastern Japan (except Hokkaido) H-O, the cesium concentrations derived from nuclear weapon tests and the Chernobyl accident were 7-19mBq/kg, and in Hokkaido the cesium concentration tends to be higher (15-66mBq/kg) than those in eastern Japan.
Consideration of health risk
Assuming that the radioactive contamination level of milk is 50Bq/kg of and also assuming that radiation exposure from all food is up to 1mSv per year, what is the additional risk of cancer death caused by radiation exposure through food based on the concentration of Cs137 in the milk measured in this study?
The concentration of cesium in milk in this survey was found to be 4-150mBq/kg, and when milk is at the contamination level of 150mBq/kg, this is equivalent to 0.003mSv per year. Under these conditions the lifetime risk of dying from cancer increases by 1.2 people per 100,000.
As noted at the beginning of the article, in general, carcinogenic chemical materials are regulated so that their concentration causes one person per 100,000 to develop cancer in his/her lifetime. If people are to face an equivalent risk of cancer death from consuming radioactively contaminated food, the detection limit should be lowered to 0.1Bq/kg (100mBq/kg), and this information should be published to allow citizens who wish to avoid exposure to make choices on what products to buy. A sufficient number of detections are also required to support this choice of citizens to avoid exposure.
- Summary
The Fukushima Daiichi nuclear accident has caused serious environmental radiation contamination to Fukushima and surrounding areas. Since then, some people have selected and purchased food that is produced in western Japan and Hokkaido in preference to food produced in the affected area in order to avoid radiation exposure through food.
The concentrations of radioactive cesium in milk produced in specific areas across Japan were measured using the AMP method and the germanium semiconductor detector. This procedure made it possible to compare contamination in each area.
In all measurements, figures were considerably lower than the new standard limit of radioactive cesium contained in food (50Bq/kg), but the commercially available milk produced in Miyagi measured in 2021 was more highly contaminated than that produced in Fukushima measured in 2020, which suggests that Fukushima products are not necessarily the most contaminated. The milk produced in Hokkaido tended to contain more cesium than that produced in western Japan.
The risk of dying from cancer caused through food intake 10 years after the Fukushima Daiichi nuclear accident in the contaminated area was calculated based on the measurement values obtained this time, and was found to be of around the same order as the management standards for carcinogenic chemical substances, even though we are talking about a risk of suffering from cancer with chemicals and a risk of dying from cancer with radiation exposure.
Every person’s sense of values, what he/she thinks is the highest priority and what risk he/she wants to avoid, should be respected. The option of avoiding the risk of exposure to radiation should be thought of as important as the option of avoiding the risk of chemical substances and a mechanism must be established to allow this.
Western Japan, which was not so much affected by the Fukushima nuclear disaster, has also been contaminated by radiation from a historical angle; the harsh fact is that the past contamination caused by atmospheric nuclear weapons tests is still contained in food. We must become more aware that the mistake which the current generation has made by causing serious environmental radiation contamination from the Fukushima Daiichi nuclear accident will continue to affect generations in the future.
