Health effects of the Chernobyl accident: an overview

April 2006
Background
On 26 April 1986, explosions at reactor number four of the nuclear power plant at Chernobyl in Ukraine, a Republic of the former Soviet Union at that time, led to huge releases of radioactive materials into the atmosphere. These materials were deposited mainly over countries in Europe, but especially over large areas of Belarus, the Russian Federation and Ukraine.
An estimated 350 000 clean-up workers or "liquidators" from the army, power plant staff, local police and fire services were initially involved in containing and cleaning up the radioactive debris during 1986-1987. About 240 000 liquidators received the highest radiation doses while conducting major mitigation activities within the 30 km zone around the reactor. Later, the number of registered liquidators rose to 600 000, although only a small fraction of these were exposed to high levels of radiation.
In the spring and summer of 1986, 116 000 people were evacuated from the area surrounding the Chernobyl reactor to non-contaminated areas. Another 230 000 people were relocated in subsequent years.
Currently about five million people live in areas of Belarus, the Russian Federation and Ukraine with levels of radioactive caesium deposition more than 37 kBq/m2 1 . Among them, about 270 000 people continue to live in areas classified by Soviet authorities as strictly controlled zones (SCZs), where radioactive caesium contamination exceeds 555 kBq/m2.
Evacuation and relocation proved a deeply traumatic experience to many people because of the disruption to social networks and having no possibility to return to their homes. For many there was a social stigma associated with being an "exposed person".
In addition to the lack of reliable information provided to people affected in the first few years after the accident, there was widespread mistrust of official information and the false attribution of most health problems to radiation exposure from Chernobyl.
This fact sheet gives an overview of the health effects of the Chernobyl accident that can be established from high quality scientific studies. For people most affected by the accident, provision of sound, accurate information should assist with their healing process.
WHO health effects review
Within the UN Chernobyl Forum initiative the World Health Organization (WHO) conducted a series of expert meetings from 2003 to 2005 to review all scientific evidence on health effects associated with the accident. The WHO Expert Group used as a basis the 2000 Report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), updated with critical reviews of published literature and information provided by the governments of the three affected countries. The Expert Group was composed of many scientists who had conducted studies in the three affected countries as well as experts world wide. Special health care programmes, established to treat people in the three countries which were most affected by the accident, were also considered. This resulted in a WHO report on "Health Effects of the Chernobyl Accident and Special Health Care Programmes" (see www.who.int/ionizing_radiation).
The WHO Expert Group placed particular emphasis on scientific quality, using information mainly in peer-reviewed journals, so that valid conclusions could be drawn. In addition, comparisons were made with the results from studies of people involved in previous high radiation-exposure situations, such as the atomic bomb survivors in Japan.
Radiation exposure
Ionizing radiation exposure is measured as "absorbed dose" in gray (Gy). The "effective dose" measured in sievert (Sv) takes account of the amount of ionizing radiation energy absorbed, the type of radiation and the susceptibility of various organs and tissues to radiation damage. For most exposures from the Chernobyl accident, absorbed doses are similar to effective doses (i.e. 1Gy is approximately equal to 1 Sv).
As human beings we are continually exposed to ionizing radiation from many natural sources, such as cosmic rays, and naturally occurring radioactive materials in all the foods we eat, fluids we drink and air we breath. This is called natural background radiation.
UNSCEAR reports that the average natural background radiation dose to human beings worldwide is about 2.4 mSv2 each year, but this varies typically over the range 1-10 mSv. However, for a limited number of people living in known high background radiation areas of the world, doses can exceed 20 mSv per year. There is no evidence to indicate this poses a health risk.
For most people more than half of their natural background radiation dose comes from radon, a radioactive gas that can accumulate in homes, schools and workplaces. When inhaled, the radiation exposure from radon may lead to lung cancer. Radiation doses to humans may be characterized as low-level if they are comparable to natural background levels.
Doses received from the Chernobyl accident
Below are the total average effective doses accumulated over 20 years by the highest Chernobyl exposed populations. These can be compared with the average doses people normally receive from natural background over 20 years. Doses from typical medical procedures are also given for comparison purposes.