Frequently asked questions
Preliminary Dose Estimation from the Nuclear Accident after the 2011 Great East Japan Earthquake and Tsunami
Q1. What is the Fukushima Dose Assessment Report and what is its aim?
The earthquake and the tsunami in Japan on 11 March 2011 led to releases of radioactive material into the environment from the Tokyo Electric Power Company’s Fukushima Daiichi nuclear power plant (NPP). This Dose Assessment Report provides preliminary radiation dose estimates based on official information on the amount of radioactivity in air, soil, water and food supplies after the accident.
The aim of the Dose Assessment Report is to provide information on the estimated radiation exposure of populations in Japan and around the world in the first year following the Fukushima accident. The report is based on data collected and made publicly available by the Government of Japan up to mid-September 2011, as well as on computer simulations to assess doses outside Japan.
The estimates were produced by an International Expert Panel convened by the World Health Organization (WHO). This report therefore consists of the collective views of independent experts and does not necessarily represent the views of the WHO.
The Dose Assessment Report will inform the WHO Health Risk Assessment Expert Group that is currently working on an initial assessment of the health risks incurred by different populations. It is expected that a report will be published in summer 2012.
Q2. How and why has WHO been involved in this assessment?
Within the United Nations System, WHO is responsible for coordinating public health risk assessment and response in radiation emergencies (as laid out in the Joint Radiation Emergency Management Plan of the International Organizations). WHO can only assess health risks once there has been an assessment of the doses of radiation people may have received. WHO therefore established an International Expert Panel, made up of independent experts, to conduct a preliminary evaluation of radiation exposure of people in Japan and elsewhere as a result of the Fukushima accident.
Q3. What are the main aspects considered in this report?
The report was designed to provide initial dose estimates. It focuses on the exposure to radiation of people in Japan and in other countries in the first year after the accident. The data used were measurements of levels of radioactive material in the environment (e.g. levels of different radionuclides on the ground) and levels of activity concentration in tap water and food. When direct monitoring data were not available, estimates based on computer simulations were used as input for the dose models. The doses are characteristic of the average doses, and are assessed for three different age groups, and for residents of the most-affected areas of Japan, for residents of the rest of Japan, and for residents of the rest of the world.
The dose assessment considered the major pathways for the exposure of people to radioactive material including:
- external exposure from radionuclides deposited on the ground (groundshine);
- external exposure from radionuclides in the radioactive cloud (cloudshine);
- internal exposure from inhalation of radionuclides in the radioactive cloud (inhalation); and
- internal exposure from ingestion of radionuclides in food and water (ingestion).
Q4. What was the methodology used to produce the estimates?
The methodology used to calculate whole body and thyroid doses relies on state-of-the-art mathematical models to estimate radiation doses. The assessment contains a number of assumptions (for example regarding the radioactive cloud composition and dispersion, time spent indoors/outdoors). In particular, some assumptions regarding the implementation of protective measures and food consumption levels are conservative (e.g. the assumption that people in the most affected areas outside the 20 km radius continued to live there for four months after the accident whereas a proportion of the population was relocated earlier, the assumption that all monitoring data on food were obtained from food available on the market, and the assumption that all people in Fukushima prefecture consumed only food produced in Fukushima prefecture) and some possible dose overestimation may have occurred. Given the limited information available to the panel during the timeframe of its work, conservative assumptions were used to avoid underestimation of radiation doses received.
The report considers all major routes of exposure – i.e. external exposure from cloudshine and groundshine, and internal exposure from ingestion of food and water and inhalation of air.
The doses are assessed for different age groups in locations around the world. Two main categories were measured: effective dose and thyroid dose 1.
- Effective dose is used as a quantity that allows combining external and internal exposures from different types of radiation.
- In addition, experts assessed thyroid doses because the radioactive iodine released into the environment concentrates in the thyroid, which thus becomes the most exposed organ in the human body.
To validate the results of the dose estimates, the panel used a variety of dosimetric2 approaches and made comparisons with existing data on direct measurement of radioactivity within the human body.
Q5. What does the report conclude?
It can be concluded that the estimated effective doses outside Japan from the Fukushima Daiichi NPP accident are below (and often far below) dose levels regarded as being very small by the international radiological protection community. Low effective doses are also estimated in much of Japan.
In the Fukushima prefecture and in neighbouring prefectures the effective doses are estimated to be below 10 mSv, which can be considered within the order of magnitude of the natural radiation background, except in two locations. In these two locations in the most affected part of Fukushima prefecture, the effective doses were estimated to be within a dose band of 10–50 mSv. Please see table 3 for more data on effective doses, and table 8 for comparative effective dose levels in different contexts.
This report focuses on effective dose as an appropriate measure given that it takes into account both internal and external exposures. In addition, the report includes information about thyroid doses because of this organ’s capacity for iodine concentration.
It is important to note that effective doses and thyroid doses are two different quantities that cannot be compared. Thyroid doses are organ-specific equivalent doses. See table 4 for data on specific exposure to thyroid doses.
Q6. What does this mean in terms of health risks for the populations included in the assessment?
In addition to the group of independent experts who produced this report, a different group of independent experts (the WHO Health Risk Assessment Group) is currently considering this report and other information to estimate the potential health risks of the radiation exposure of different population groups. This expert group is expected to publish a Health Risk Assessment (HRA) report in summer 2012.
The HRA report will consider short- and long-term health risks for three age groups (adults, children and infants) in different geographic locations in Japan and in the rest of the world. It will:
- Examine the possible health effects of the doses received. Based on the outcome of the dose assessment, taking its conservative nature into account, the health effects of possible concern are long-term increases in cancer cases, including thyroid cancers.
- Separately consider the short- and long-term health risks for the emergency response workers.
- If deemed appropriate, provide guidance on long-term health surveillance programmes, including screening and epidemiological follow-up to support the most exposed populations.
Q7. Are there any population groups not included in the Dose Assessment Report?
This report provides dose estimates for members of the public in the Fukushima prefecture, the rest of Japan and the rest of the world. The report does not include:
- Estimates of doses within 20 km of the Fukushima site: most people in the area were rapidly evacuated. Some exposure may have occurred prior to evacuation but the assessment of this requires more precise data than those available to the panel.
- Estimates of doses received by workers: radiation doses received by workers at the site of the Fukushima Daiichi NPP were not assessed by the panel because the evaluation of occupational radiation exposure requires a different dosimetric approach from the one used for evaluating doses to members of the public. Information on workers’ exposures will be included in the Health Risk Assessment report.
1 Thyroid doses and effective doses are two different quantities that cannot be compared. Thyroid doses are organ-specific equivalent doses, while the effective doses represent the sum of the products of the absorbed doses by each organ multiplied by the respective tissue weighting factors.
2 The process and method of calculating radiation doses.