Introduction

This is the first analysis of the global health effects due to radiation exposure after the Fukushima accident done with the aim to estimate its potential public health impact so that future possible health needs can be anticipated.

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.

What happened exactly on 11 March 2011 in Japan?

On 11 March 2011, a magnitude 9 earthquake and a series of large tsunami waves hit the east coast of Japan, causing widespread damage to infrastructures. At the Fukushima nuclear power plant, the power supply and the means to control and cool the reactors, were knocked out. In the days that followed, three reactors experienced meltdown, and a series of hydrogen gas explosions released radionuclides into the environment. A 3-km evacuation zone was established and then quickly increased to a 20-km evacuation zone. Other protective actions were implemented to reduce 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 people actually used it.

What are the potential consequences of a radioactive material release?

Adverse health effects of ionizing radiation may 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.

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.

What were the exposure levels of the various populations and main associated risks identified?

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.

No acute effects of radiation exposure such as acute radiation syndrome or skin injuries have been observed among the general population.

Apart for the workers who were carrying dosimeters, it is very difficult to evaluate the amount of radiation that people were exposed to. Estimates are deliberately made to limit the possibility of underestimating eventual health risks. In general, females show a greater increase in risk of cancer compared to males, and people exposed as infants show a greater increase in risk than people exposed as children or adults.

• Among Fukushima Daiichi nuclear power plant emergency workers:
  • The risk that they might develop leukaemia, thyroid cancer or another solid cancer is increased compared to the general population.
  • Twelve of the emergency workers were exposed to larger amounts of radioactive iodine and may develop non-cancer thyroid disorders.
• In the most affected area of Fukushima prefecture, the estimated radiation doses for the first year ranged from 12 to 25 mSv. In this situation, the largest estimated lifetime risks increase over baseline cancer rates are:
  • all solid cancers - around 4% for females exposed as infants, 3% for males;
  • breast cancer - around 6% for females exposed as infants;
  • leukaemia - around 6% for females exposed as infants, 7% for males;
  • thyroid cancer - around 70% for females exposed as infants, around 60% for males.

These are exposure increases over the baseline level which means, for instance, that if the expected risk of thyroid cancer in females over their lifetime is 0.77%, and that the additional lifetime risk assessed for females exposed as infants is 70%, that is 0.52%; then the total expected risk is 1.29%.

• 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.
• Outside Japan this health risk assessment concludes that no discernible increase in health risks from the Fukushima event is expected.
• The radiation doses in Fukushima prefecture were well below certain radiation dose levels which, when exceeded, may produce a direct impact on health, and therefore such effects are not expected to occur in the general population.

Health effects other than cancer may include thyroid diseases (nodules, dysfunction), visual impairment (lens opacities, cataracts), acute skin reactions, hematopoietic, gastrointestinal and neurovascular disturbances, depending on the dose range.

What are the uncertainties around these risk estimates and priorities set for the coming years?

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.

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.

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

GreenFacts highlights of the WHO report: "Health risk assessment from the nuclear accident after the 2011 Great East Japan earthquake and tsunami, based on a preliminary dose estimation (2013)" http://www.who.int/ionizing_radiation/pub_meet/fukushima_risk_assessment_2013/en/