Health risks following the 2011 Fukushima nuclear accident

What happened exactly on 11 March 2011?

On 11 March 2011, a magnitude 9 earthquake, the largest ever recorded in the country, created a series of large tsunami waves that struck the east coast of Japan, causing widespread damage to infrastructures, including to several nuclear power plants (NPPs). In most cases these power plants were successfully shut down. However, at the Fukushima NPP from the Tokyo Electric Power Company , the earthquake and tsunamis knocked out the power supply to the facility, and consequently the means to control and cool the reactors. In the days that followed, reactors meltdown, venting and hydrogen gas explosions released radionuclides into the environment.

A 3-km evacuation zone was put in place around the site, which was then quickly increased to a 20-km evacuation zone, with a sheltering zone between 20 and 30 km. As the availability of environmental monitoring data increased, other protective actions were implemented to reduce exposure doses in the longer term, including the relocation of people in some areas. Stable iodine for thyroid blocking was distributed but it is thought that only a small number of persons actually consumed it. Food monitoring was conducted at the local level.

What are the potential consequences of a radioactive material release?

A major release of radioactivity to the environment is always of concern, owing to potential acute and long-term health effects ¹. Evidence from historic events confirms that any major uncontrolled release of radiation should be cause for immediate response and scientific assessment of potential health effects.

¹ See the WHO fact sheet: Ionizing radiation, health effects and protective measures http://www.who.int/mediacentre/factsheets/fs371/en/ 

What is the purpose of this report ?

This is the first analysis of the global health effects due to radiation exposure after the Fukushima nuclear incident done with the aim to estimate its potential public health impact, so that future health needs can be anticipated, and public health actions can be taken. This assessment is based on a preliminary estimate of radiation doses, as described in a WHO report published in May 2012.

What are the main risks identified one year after the events?

Adverse health effects of ionizing radiation result from two distinct mechanisms:

• cell killing, which may cause functional impairment of the exposed tissue or organ if a sufficient number of cells are affected;
• non-lethal changes in molecules of a single cell, most commonly in the DNA molecule, which may result in an increased risk of disease, long after exposure.

The exposure data upon which this report is based are preliminary and include only data that were available as of September 2011. It is important to note that the dose estimates and assumptions used in this assessment were deliberately chosen to minimize the possibility of underestimating eventual health risks. In view of the estimated exposure levels, an increased risk of cancer is the potential health effect of greatest relevance, and age at the time of irradiation is one of the most important biological variables influencing both short- and long-term effects of ionizing radiation.

Lifetime organ doses were estimated for the general population within geographical locations ranging from the most affected areas of Fukushima prefecture to the rest of the world. The lifetime risks were estimated for both sexes and three different ages at exposure (1 year [infant], 10 years [child], and 20 years [adult]). Health risks for male emergency workers were estimated for three different ages (20 years, 40 years, and 60 years.

• Among Fukushima Daiichi nuclear power plant emergency workers, based upon plausible radiation exposure scenarios, the lifetime risks for leukaemia, thyroid cancer and all solid cancers are estimated to increase over baseline rates. A few emergency workers who inhaled significant quantities of radioactive iodine may develop non-cancer thyroid disorders.
• On the two most affected locations of Fukushima prefecture, where the preliminary estimated radiation effective doses for the first year ranged from 12 to 25 mSv, the estimated lifetime risks increase over baseline cancer rates 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

The normally expected risk of thyroid cancer in females over lifetime is 0.75%, and the additional lifetime risk assessed for females exposed as infants in the most affected location is 0.50%. These percentages represent estimated relative increases over the baseline rates and are not estimated absolute risks for developing such cancers.

• In the highest dose location, the estimated additional lifetime risks for the development of leukaemia, breast cancer, thyroid cancer and all solid cancers over baseline rates are likely to represent an upper bound of the risk as methodological options were consciously chosen to avoid underestimation of risks.
• For the people in the second most affected location, the estimated additional lifetime cancer risks over baseline rates are approximately one-half of those in the highest dose location. The estimated risks are lower for people exposed as children (10 years of age) and adults (> 20 years of age) compared to infants (1 year of age).
• Outside the geographical areas most affected by radiation, even in locations within Fukushima prefecture, the predicted risks remain low, and no observable increases in cancer above natural variation in baseline rates are anticipated.
• In the next most exposed group of locations in Fukushima prefecture, where preliminary estimated radiation effective doses were 3–5 mSv, the increased lifetime estimates for cancer risks over baseline rates were approximately ¼ to 1/3 of those for the people in the most affected geographical location.
• The radiation doses in Fukushima prefecture were well below certain radiation dose levels which, when exceeded, may produce some health effects of radiation, (tissue reactions termed “deterministic effects”), which are known to occur at such levels, therefore such effects are not expected to occur in the general population.
• The estimated dose levels in Fukushima prefecture were also too low to affect fetal development or outcome of pregnancy and no increases in spontaneous abortion, miscarriage, perinatal mortality, congenital defects or cognitive impairment, are anticipated as a result of antenatal radiation exposure.
• With respect to the whole Japan, this assessment estimates that the lifetime risk for some (which ones) cancers may be somewhat elevated above baseline rates in certain age and sex groups that were present in the most affected areas.
• Outside Japan, this health risk assessment concludes that no discernible increase in health risks from the Fukushima event is expected.

Health effects other than cancer include thyroid diseases (nodules, dysfunction), visual impairment (lens opacities, cataracts), acute skin reactions, hematopoietic, gastrointestinal and neurovascular abnormalities, depending on the dose. No acute effects of radiation exposure such as acute radiation syndrome or skin injury, have been observed among the general population. However, there may be an increased risk of long-term circulatory disease among workers exposed to the highest doses, which is likely to be substantially smaller than any additional cancer risk.

What are the uncertainties around these risk estimates?

This health risk assessment is based on the current state of scientific knowledge. The assessment models used were derived from previous radiation events and experience, which do not exactly match the pattern of exposure seen in Fukushima; thus, adjustments were required. The values presented in the report should be regarded as inferences of the magnitude of the health risks, rather than as precise predictions.

The relationship between radiation exposure and lifetime risk of cancer is complex and varies depending on several factors, mainly radiation dose, age at time of exposure, sex and cancer site. These factors can influence the uncertainty in projecting radiation risks, in particular when assessing risks at low doses.

Because scientific understanding of radiation effects, particularly at low doses, may increase in the future, it is possible that further investigation may change our understanding of the risks of this radiation accident.

How was this study conducted ?

This health risk assessment was conducted by independent international experts who were selected by WHO for their expertise and experience in radiation risk modelling, epidemiology, dosimetry, radiation effects and public health. At two meetings in December 2011 and March 2012, observers from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the International Labour Organization, and the Government of Japan were present, but they were not involved in the decision-making process.

The risk assessment was made following a four step approach:

• Identification of the specific radiation sources, such as different radionuclides and pathways of exposure (hazard identification);
• Identification of the types of harmful effects that could result, on the basis of scientific knowledge about the relationships between radiation dose and biological effects (dose-response relationships);.
• Estimation of exposure and lifetime organ doses, based on the preliminary dose assessment for the general population within geographical locations ranging from the most affected areas of Fukushima prefecture to the rest of the world. Based on available data on occupational exposure assessed by the operator of the nuclear power plant, first-year organ doses were also estimated for emergency workers (exposure assessment);
• Estimation of the lifetime risks of cancer for all solid cancers combined, and also for individual cancer sites most closely associated with radiation exposure and with a known dependence of the magnitude of risk on age-at-exposure (leukaemia, thyroid cancer and female breast cancer).

Based on this study, what are the next steps?

These estimates provide valuable information for setting priorities in the coming years for population health monitoring, as has already begun with the Fukushima Health Management Survey.

On the basis of these findings, the continued monitoring of foods and the environment remains important. When additional dose estimations become available from studies undertaken by UNSCEAR and others, such data can be used to further refine these risk estimates.