Comparison of lifetime mortality risk, incidence risk, and DALYs of baseline cancer rates among countries as a benchmark for radiation-related cancer risk.

Statistical benchmark data are necessary when considering the basis for radiation protection criteria based on calculated risks. We herein focused on baseline mortality and incidence cancer rates as benchmark data collected from 33 countries. Furthermore, we calculated the lifetime mortality and incidence risks and disability-adjusted life years (DALYs) for all solid cancers, colon cancer, lung cancer, breast cancer, thyroid cancer, and leukemia using the baseline cancer rates and compared them among the countries. The results showed that the lifetime mortality and incidence risks and DALYs for all solid cancers differed among the countries by a factor of 2-4 for males and 2-3 for females; these were low in less-developed countries. Our study proposed that health risk based on baseline cancer rates should be the benchmark for comparing radiation cancer risks.

Temporal changes in cumulative mortality risks of cancer, by occupation, in the working population of Japan from 1995 to 2020: a benchmark for radiation risk comparison.

The purpose of this study was to provide benchmark data for discussing the tolerability of cancer risk associated with occupational radiation exposure. It focused on differences in cancer mortality risk by occupation among Japan's working population and examined baseline cancer mortality risks and its variations from 1995 through 2020. Data were collected every five years from national vital statistics sources. By focusing on the same types of cancer among radiation induced effects, cumulative mortality risks were calculated for colorectal, lung, and breast cancer (females only) for those aged 15-74. The average cumulative mortality risk for the working population in Japan has decreased by 30%-60% over the past 25 years. Service workers and male managers were at an average risk, among all workers, while clerical workers and transportation and manufacturing workers had about half the average risk. The risks were higher for professionals and female managers, about 1.5-2 times the average for professionals and up to 5 times the average for female managers. The decrease in the average cancer mortality risk in the working population as a baseline suggests that risk tolerance in society might have changed over time. Since differences in mortality by occupation were confirmed, the usefulness of occupational data as a benchmark needs further investigation, as high-risk/low-risk occupations vary by country and region. The results of this study contribute to put radiation risks into perspective with the background risk of cancer.

Uncertainty and sensitivity analyses for the reduction factor of sheltering for radiation exposures.

Sheltering is one of the countermeasures used to mitigate radiation exposure during nuclear power plant accidents. The effectiveness of sheltering for inhalation exposure is often expressed by the reduction factor, which is defined as the ratio of the indoor to the outdoor cumulative radioactivity concentrations or doses. The indoor concentration is mainly controlled by the air exchange rate, penetration factor, and indoor deposition rate. Meanwhile, the air exchange rate depends on surrounding environmental conditions: the wind speed, leakage area normalised by the floor area of the house, and gross building coverage ratio. In this study, the ranges of the uncertainty of the reduction factors for particles and iodine in reactive gas form were investigated under various environmental conditions, and sensitivity analyses were conducted to understand the parameter with the most influence on the uncertainty of the reduction factor. From the results of the uncertainty analyses, the calculated reduction factor was highly variable depending on the environmental condition and the airtightness of the houses. The median and 95th percentile of the reduction factors for the older houses were 0.5 and 0.9 for particles and 0.07 and 0.4 for iodine in reactive gas form, respectively and these ranges were smaller for newer houses. From the results of the sensitivity analyses, the wind speed was the most influential parameter determining the reduction factor. Additionally, the wind speed was less influential for the reduction factor in newer houses.

Penetration factor and indoor deposition rate of elementary and particulate iodine in a Japanese house for assessing the effectiveness of sheltering for radiation exposures.

Sheltering is one of the countermeasures for protection against radiation exposures in nuclear accidents. The effectiveness of sheltering is often expressed by the reduction factor, that is the ratio of the indoor to the outdoor cumulative radioactivity concentrations or doses. The indoor concentration is mainly controlled by the air exchange rate, penetration factor, and indoor deposition rate. The penetration factor and indoor deposition rate depend on the surface and the materials and structure of windows and doors as it is these openings in the building envelope that control penetration. We investigated experimentally these parameters of I2and particles. The experiment was performed in two apartment houses, three single-family houses, and chambers. The obtained penetration factor ranged from 0.3 to 1 for particles of 0.3-1µm and 0.15-0.7 for I2depending on the air exchange rate. The indoor deposition rate for a house room ranged from 0.007 to 0.2 h-1for particles of 0.3-1µm and 0.2-1.5 h-1for I2.

Assessment Model of Radiation Doses from External Exposure to the Public after the Fukushima Daiichi Nuclear Power Plant Accident.

Radiation exposure is one of most important factors to manage following a nuclear emergency. Actual measurement is the best way to obtain information concerning the dose received by the people in terms of accuracy and reliability. However, in practice, it is difficult to collect measurements from all people affected by nuclear accidents over the whole period of exposure from past to future. Therefore, probabilistic assessment using a model is needed. An assessment model of radiation doses from external exposures was developed based on the actual measurement of individual doses and ambient dose equivalent rates inside and outside houses in Fukushima City. A survey of behavioral patterns was also performed for the same purpose. In addition to our measurement and survey, we took into account the latest insights from the experiences of the Fukushima Daiichi nuclear power plant accident. Comparisons between the assessed and measured results revealed that the time-dependence of doses and the distribution of doses obtained using the developed models agree well with the results of actual measurements. Thus, our probabilistic approach was validated. Based on both our assessment and on our actual measurements, no participants were observed to receive doses in excess of 1 mSv y as of 8 y after the Fukushima Daiichi nuclear power plant accident in Fukushima City.

Development of an External Radiation Dose Estimation Model for Children Returning to Their Homes in Areas Affected by the Fukushima Nuclear Accident.

On 1 April 2017, around 6 y after the Fukushima Daiichi nuclear power station accident, evacuation orders for large affected areas were lifted, and areas to which people could return were expanded. In the current study, a dose estimation model based on a probabilistic approach has been developed to estimate the external radiation doses children would receive after returning to these areas. The target groups are children from infants to high school students, and the target areas are nine municipalities including evacuation areas as of 5 September 2015. The estimation period is for 4 y starting 1 April 2017. Validation of the model in an area for which individual personal dosimetry measurements were available showed that it is valid for infants, kindergarteners, 3rd and 4th grade elementary school students, 5th and 6th grade elementary school students, and junior high school students. Considering the statement of the International Commission on Radiological Protection, the estimated radiation doses for these five age groups were taken to be the 95th percentiles of the predicted distributions as an index of conservative judgement. As a result of our estimations, the 95th percentile doses to all age groups were less than 20 mSv y in all periods and in all areas. The 95th percentile doses in some areas were less than 1 mSv y, which is the long-term dosimetric target set by the Japanese government. It should be noted that our results are preliminary, being based on several assumptions and limitations regarding environmental contamination conditions and the behavioral patterns of children. To estimate the children's doses precisely, further considerations for these assumptions and limitations will be needed.

Dose-reduction Effects of Vehicles against Gamma Radiation in the Case of a Nuclear Accident.

Self-evacuation by a private vehicle is one of the most commonly used methods of public evacuation in the case of a nuclear accident. The aim of this paper is to evaluate the dose-reduction effects of vehicles. To achieve this aim, a model for calculating the dose reduction factor was developed based on the actual shape and weight of Japanese vehicles. This factor is defined as the ratio of dose rate inside a vehicle to that outside. The model was developed based on weight of vehicle to take into account the dose-reduction effects due to not only the steel plate of the vehicle body but also the other assemblies. In addition to model calculation, the dose reduction factors were evaluated by actual measurements in the areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. A comparison between the simulated and the measured results revealed that the dose reduction factors obtained using the developed models were in good agreement with the results of actual measurements. Using this model, we also evaluated the dose reduction factors for cloudshine and groundshine in the case of a nuclear accident. The evaluations were performed for four vehicle models whose weights were 800-1,930 kg. The dose reduction factor for cloudshine with photon energy of 0.4-1.5 MeV was 0.66-0.88, and that for groundshine from Cs was 0.64-0.73. Although these results were obtained under the assumption that Cs is placed only on the ground surface, according to these considerations, if Cs migrated into the ground corresponding to the relaxation mass depth of 10 g cm, the dose reduction factors would be almost 8% less than those for the ground surface.

Bioaccessibility of Fukushima-Accident-Derived Cs in Soils and the Contribution of Soil Ingestion to Radiation Doses in Children.

Ingestion of contaminated soil is one potential internal exposure pathway in areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. Doses from this pathway can be overestimated if the availability of radioactive nuclides in soils for the gastrointestinal tract is not considered. The concept of bioaccessibility has been adopted to evaluate this availability based on in vitro tests. This study evaluated the bioaccessibility of radioactive cesium from soils via the physiologically-based extraction test (PBET) and the extractability of those via an extraction test with 1 mol/L of hydrochloric acid (HCl). The bioaccessibility obtained in the PBET was 5.3% ± 1%, and the extractability in the tests with HCl was 16% ± 3%. The bioaccessibility was strongly correlated with the extractability. This result indicates the possibility that the extractability in HCl can be used as a good predictor of the bioaccessibility with PBET. In addition, we assessed the doses to children from the ingestion of soil via hand-to-mouth activity based on our PBET results using a probabilistic approach considering the spatial distribution of radioactive cesium in Date City in Fukushima Prefecture and the interindividual differences in the surveyed amounts of soil ingestion in Japan. The results of this assessment indicate that even if children were to routinely ingest a large amount of soil with relatively high contamination, the radiation doses from this pathway are negligible compared with doses from external exposure owing to deposited radionuclides in Fukushima Prefecture.

Statistical characterization of radiation doses from external exposures and relevant contributors in Fukushima prefecture.

In areas contaminated by radioactive materials, well designed dose assessment is necessary in order to protect people from radiation exposure and manage the exposure situation appropriately. Probabilistic dose assessment is a useful method for providing a more complete characterization of information on dose distributions in the population and requires statistically characterized data on pathway-relevant contributors. The objective of this paper is to determine statistical features of contributors to external exposures, as well as to identify causes of variabilities of individual doses to the populations living in areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. To achieve these objectives, measurements of individual doses and ambient dose rates, as well as surveys of behavioral patterns, were performed between February and April 2012. These were made with the cooperation of indoor workers, outdoor workers, and pensioners living in Fukushima prefecture. On the basis of these results, statistical analyses were performed in order to identify variabilities of contributors. In addition, a multi-regression analysis was done to explore a significant relationship between individual doses and relevant contributors. Results showed that the ambient dose equivalent rate also distributed with lognormal form, and it had variabilities attributable to the spatial distribution of deposited radionuclides. The distribution form of time spent outdoors depends on the characteristics of occupation, and the distributions for indoor workers and outdoor workers were lognormal and normal, respectively. Results of a multiple-regression analysis suggested that such variabilities of contributors give rise to significant differences in individual doses among the populations.