Ionizing radiation

Frequently asked questions on health risk assessment

Health risk assessment from the nuclear accident after the 2011 Great East Japan earthquake and tsunami, based on a preliminary dose estimation

1. What is the aim of this health risk assessment?

Soon after the accident, WHO began work to develop a formal health risk assessment (HRA) to estimate the consequences of the accident. In its role as the international authority on public health issues, WHO is responsible for assessing global health risks related to radiation emergencies.

The health risk assessment aims to estimate the health risks related to radiation exposure from the Fukushima Daiichi nuclear power station accident that followed the Japan earthquake and tsunami on 11 March 2011. Such information can support the identification of needs and priorities for public health actions.

The HRA considers health risks for the general population inside and outside of Japan and emergency workers from the nuclear station.

2. Is this report about radiation health effects?

There is a difference between radiation health risks and radiation health effects. This report refers to an assessment of radiation health risks, and not radiation health effects. Health effects are changes in the health status of an individual or population, identifiable either by diagnostic or epidemiological methods.

Health risks express the likelihood of a health effect to occur under defined circumstances and exposure to a certain hazard, e.g. radiation. Risks are estimated using available data and risk models.

3. What is the scientific basis for the assessment?

In order to assess the health risks related to these estimated radiation doses, existing scientific data and risk models were used. Much of the epidemiological information and data used to develop cancer risk models came from the Hiroshima and Nagasaki atomic bomb survivors and from populations affected by the radiation release from the Chernobyl power plant.

The risk models derived from previous radiation events and experience do not match exactly the pattern of exposure seen in Fukushima, requiring some adjustments. The assumptions and methodological choices in this assessment were consciously made to minimize the possibility of underestimating health risks, i.e. conservative assumptions were made.

For example, the risks were calculated taking into account population radiation exposure over lifetime, assuming that risks are proportional to doses (i.e. “linear non-threshold ” /LNT model), and without applying a factor often used when the dose is low or delivered over a long period of time (i.e. dose and dose rate effectiveness factor /DDREF).

A preliminary radiation dose estimation for the general population was conducted by WHO and published in May 2012. Based on conservative assumptions, effective doses were estimated to be low in most of Japan and extremely low in the rest of the world. All efforts were made to avoid any underestimation of the doses. (see question below for more information about the conservative assumptions).

Radiation dose estimates for emergency workers were provided by the Tokyo Electric Power Company (TEPCO).

4. Which conservative assumptions were included?

Some of the conservative assumptions that may have resulted in overestimation of the doses in the general population are:

  • relocation in the “deliberate evacuation area” was assumed to take place at four months after the accident (though some inhabitants of this area were subjected to relocation earlier than this);
  • consumers were assumed to only eat food produced in the area where monitoring was implemented (i.e. people living in Fukushima ate only food produced in Fukushima, though this was not always the case);
  • all the food monitored was assumed to be on the market, although the data set included the results of food samples collected for monitoring purposes, which were not allowed on the market.

5. What is the scope of the health risk assessment?

General population

Health risks in the general population were assessed considering geographical location – from the most affected areas of the Fukushima prefecture, to less affected areas in Japan and the rest of the world. Risks were calculated for men and women, and three age groups at time of exposure: 1-year-old infants, 10-year-old children and 20-year-old adults (age at the time of radiation exposure is one of the most important variables influencing radiation-induced cancer risks).

Power plant emergency workers

Health risks for the emergency workers at the Fukushima Daiichi nuclear power station were evaluated under four exposure scenarios and characterized for male workers at age 20, age 40 and age 60.

6. What are the health effects considered in this assessment?

The HRA considers cancer and non-cancer health implications for the general population and power plant emergency workers.

Some health effects of radiation, termed deterministic effects, are known to occur only when certain radiation dose levels are exceeded. The radiation doses following Fukushima Daiichi nuclear accident were below such levels and therefore such effects are not expected to occur in the general population.

Based on the estimated exposure levels, the main health effect of concern is the increased risk of developing cancer. Therefore the HRA focuses on excess lifetime cancer risks of leukaemia, thyroid cancer and female breast cancer (cancers most likely to occur after radiation exposure, with a demonstrated dependence of risk on the age-at- exposure), and all solid cancers combined i.e. tumours found in solid organs.

Because of the increasing success of treatments, cancer incidence (i.e. fatal and non-fatal cancer) was considered rather than cancer mortality (i.e. fatal cancer only), which would be comparatively lower.

Non-cancer health risks were assessed but not quantitatively estimated. The psychosocial impact and potential mental health conditions that may be associated with the accident are briefly discussed but not assessed for risk.

7. What does the health risk assessment conclude for the general population?

From a global health perspective, the results of the assessment show that health risks directly related to radiation exposure are low in Japan and extremely low in neighbouring countries and the rest of the world.

The estimated cancer risk is highest in the locations with the highest estimated radiation doses in Fukushima prefecture. Outside those most affected areas, no increase in cancer risk above variation in background rates is anticipated in the less affected areas of Fukushima Prefecture, neighbouring prefectures and the rest of Japan, or countries other than Japan.

In general, the results show that estimated risks are higher in those exposed at younger ages. The greatest risks are estimated for leukaemia in male infants, and for thyroid and breast cancer in female infants in the areas with highest estimated doses in Fukushima prefecture.

In terms of specific cancers, for people in the most contaminated location, the estimated increased risks over what would normally be expected are;

  • all solid cancers - around 4 % in females exposed as infants;
  • breast cancer - around 6% in females exposed as infants;
  • leukaemia - around 7% in males exposed as infants;
  • thyroid cancer - up to 70% in females exposed as infants (Due to the low background rates of thyroid cancer, even a large relative increase in the lifetime risk represents a small absolute excess risk. This is more evident when considering the relative increase during the first 15 years after the accident, because thyroid cancer is rare in children).


  • All solid cancers: the normally expected risk for all solid cancers combined in females over lifetime is 29% and the additional lifetime risk assessed for females exposed as infants in the most affected location is around 1%.
  • Breast cancer: the normally expected risk of female breast cancer over lifetime is 5.53% and the additional lifetime risk assessed for females exposed as infants in the most affected location is 0.36%.
  • Leukaemia: the normally expected risk of leukaemia in males over lifetime is 0.60% and the additional lifetime risk assessed for males exposed as infants in the most affected location is 0.04%.
  • Thyroid cancer: the normally expected risk of thyroid cancer in females over lifetime is around 0.75% and the additional lifetime risk assessed for females exposed as infants in the most affected location is 0.50%.

For people in the second most contaminated location of Fukushima Prefecture, the estimated risks are approximately one-half of those in the location with the highest doses.

In the next most exposed group in Fukushima prefecture, where estimated radiation doses were 3 to 5 mSv, the increased lifetime estimates for cancer risks over background rates were approximately one-quarter to one-third of those in the geographical location with the highest estimated doses.

This health risk assessment concludes that no discernible increase in health risks from the Fukushima event is expected outside Japan. With respect to Japan, this assessment estimates that the lifetime risk for some cancers may be elevated above baseline rates in certain age and sex groups that were in the areas with the highest estimated doses. These estimates provide information for setting priorities in the coming years for population health monitoring, as has already begun.

8. What about risks of birth defects due to the exposure of unborn children?

Based on the low doses no increases in miscarriages, stillbirths or birth defects are expected as a result of prenatal radiation exposure from the accident.

9. What about health risks for power plant emergency workers?

The greatest estimated risks are for a few emergency workers who received high doses to the thyroid gland (thyroid cancer risk). There may be an increased long-term risk of circulatory diseases among workers with the highest exposure levels.

10. Will future generations be affected?

A risk of radiation-induced hereditary effects has not been definitively demonstrated in human populations. Based on animal data, any risk of hereditary effects for the offspring of those who were exposed before they have conceived children is considered to be much lower than the additional lifetime risk of cancer assessed for the individuals exposed (about ten times lower).

11. Who took part in developing the HRA?

The health risk assessment was conducted by an independent expert group of scientists and health specialists convened by WHO. They were selected for their relevant expertise and experience in such areas as radiation risk modeling, epidemiology, dosimetry, radiation effects and public health. A list of the experts and their affiliations is provided in the report.

The expert group met in Geneva in December 2011 and March 2012. All members provided declarations of interest before they began their work.

Observers were present from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the International Labour Organization (ILO) and the Government of Japan.

UNSCEAR was selected as it is conducting a more detailed assessment of the exposure levels and effects, which will be presented to the UN General Assembly in 2013. ILO was selected as the lead organization for overseeing international labour and workers health. The Government of Japan facilitated access to data for the assessment.

The role of observers was limited to participating in discussions and sharing data.

12. What are the public health implications based on the risks identified?

This health risk assessment concludes that no discernible increase in health risks from the Fukushima event is expected outside Japan.

With respect to Japan, this assessment suggests that the lifetime risk for some cancers is elevated above background rates in certain defined age and sex groups that were in the most affected areas. These estimates provide valuable information for setting priorities in the coming years for population health monitoring, as has already begun.

On the basis of these findings, continuation of the monitoring of food and the environment that is already underway remains prudent.

13. What happens next?

A broader assessment, with additional data, is being undertaken by UNSCEAR, to which WHO is contributing. Its scientific report will be submitted to the United Nations General Assembly in 2013.

Health monitoring is ongoing in Japan. The Fukushima Health Management Survey, launched in June 2011, will monitor the long-term health of the prefecture’s 2 million residents. This initiative is expected to inform future health assessments. In addition, a special protocol on medical follow-ups for the emergency workers is being conducted.

Additional health and exposure data will allow further refinement of this health risk assessment, if warranted.

WHO will remain active in assisting and supporting Member States and relevant organizations regarding radiation and public health issues.