The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) was established in 1955, with a mandate from the United Nations General Assembly to assess and report levels and health effects of exposure to ionizing radiation. During the last few years, UNSCEAR has undertaken a broad review of the sources and effects of ionizing radiation. The Committee has just adopted its 2000 report to the General Assembly, summarizing the developments in radiation science in the years leading up to the new millennium. The report has 10 scientific annexes which are extensive reviews and assessments on: exposures from natural background, man-made, medical and occupational radiation; radiation-associated cancer risks; DNA repair; effects at low doses; and effects of radiation in combination with other agents. One annex is specifically dedicated to an assessment of the radiological consequences of the Chernobyl accident.

Mikael Jensen/SRPI

The accident at the Chernobyl nuclear power plant in 1986 is the most serious accident involving radiation exposure. It resulted in widespread radioactive contamination in areas of Belarus, the Russian Federation and Ukraine. The accident caused long-term changes in the lives of the people living in the contaminated districts, since measures intended to limit radiation exposure included resettlement, changes in food supplies, and restrictions on the activities of individuals and families. These changes were later accompanied by major economic, social and political changes that took place when the former Soviet Union was dismantled.

Of the 600 workers present on the site during the early morning of 26 April 1986, 134 received high doses and suffered from radiation sickness. Of these, 28 died in the first three months and 2 soon afterwards. In addition, about 200,000 recovery operation workers received doses between 0.01 and 0.5 Gray. This group is at potential risk of late consequences, such as cancer, and their health will be followed closely.

The radionuclides that caused radiation exposure were mainly iodine-131, caesium-134 and caesium-137.

Iodine-131 can be transferred to humans from the air and through milk and leafy vegetables. For reasons related to the intake of such foods by children, as well as the size of their thyroid glands and their metabolism, the radiation doses are usually higher for this age-group than for adults. The caesium isotopes have relatively longer half-lives and cause longer-term exposures through ingestion and external exposure from their deposition on the ground. Many other radionuclides were associated with the accident, and these have been considered as well in the exposure assessments.

A majority of the epidemiological studies completed to date are descriptive in nature. Individual dosimetry is generally not available, thus it is difficult to determine whether the effects are radiation-related and it is impossible to reliably estimate cancer risks. The reconstruction of individual doses is therefore crucial for future research. Average doses to those most affected by the accident were about 100 mSv for 240,000 recovery operation workers, 30 mSv (milliSieverts) for 116,000 evacuated persons and 10 mSv during the first decade after the accident for those who continued to reside in contaminated areas in Belarus, the Russian Federation and Ukraine. Maximum values of the dose may be an order of magnitude higher. In other European countries, doses were, at most, 1 mSv in the first year after the accident, with progressively decreasing doses in subsequent years. Those are comparable to an annual dose from natural background radiation and of little health significance.

A total of 1,800 thyroid cancers have been reported in individuals exposed in childhood, particularly in the severely contaminated areas of the three affected countries. If the current trend continues, many more cases may occur in future decades, especially in those who were exposed at a young age. Notwithstanding problems associated with screening, these cancers were most likely caused by radiation exposures received at the time of the accident. Apart from this increase in thyroid cancer, no increase in overall cancer incidence or mortality has been observed that could be attributed to radiation, not even among the recovery operation workers. The accident caused a serious social and psychological disruption in the lives of those affected, but there has been no increase of non-malignant disorders that can be directly related to radiation. However, there were widespread psychological reactions to the accident — due to fear of the radiation, not due to the radiation doses as such.

There is a tendency to attribute increases in the rates of all cancers over time to the Chernobyl accident. However, increases were also observed in the affected areas before the accident. Moreover, a general increase in mortality has been reported in recent years in most areas of the former Soviet Union, and this must be taken into account when interpreting the results of the Chernobyl-related studies.

The Chernobyl accident might shed light on the late effects of protracted exposure, but given the low doses received by the majority of exposed individuals, any increase in cancer incidence or mortality will be difficult to detect in epidemiological studies. Many health problems other than cancer have been noted in the populations, but they are less likely to be related to radiation exposure. From a scientific point of view, there is a need to evaluate and understand the technical causes and health effects of the accident. From a human point of view, there is also an obligation to provide an objective analysis of the health consequences of the accident for the people involved. One future challenge will be to develop individual dose estimates, including estimates of uncertainty, and determine the effects of doses accumulated in the contaminated areas over a long period of time.

Fact Sheet: Rehabilitating the Semipalatinsk Region After 40 years of hosting nuclear tests, Semipalatinsk hostes United Nations efforts in human, ecological and economic revitalization. Semipalatinsk, which lies along the Irtysh River in East Kazakhstan, was the major nuclear-testing site for the Soviet Union since the first explosion of a plutonium bomb in 1949. For 40 years, the equivalent of 17,400,000 tons of trinitrotoluene were exploded in the 18,500 square kilometre Polygon in which it became known. Between 1949 and 1963, all 116 tests were above ground. In 1963, they went underground when the Soviet Union signed the Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and under Water. The impact of those tests includes the release of radioactive gases into the atmosphere, contaminated ecological relationships, and even the creation of Lake Balapan — the "atomic lake" — and, not the least, the health of the population exposed to radiation. A presidential decree closed the site in August 1991, but the damage had been done. In 1992, Kazakhstan recognized that 1.6 million persons received radiation as a result of the testings. The figure was reduced to 1.2 million in 1998 through birth and death rate differentials and emigration. In the 1992 Law on Social Protection of the Citizens and Victims of the Semipalatinsk Nuclear Test Site, the Government identified approximately 67,000 persons living in 19 villages as having received the heaviest radiation, with a dose of more than 1 Sievert, with the maximum dose at 2.8 Sievert. About 27,000 of this group survived. Recognizing the seriousness of the situation the General Assembly adopted resolution 52/169 M in 1997 calling on the international community to assist the Government of Kazakhstan in its efforts to meet the needs of those affected by the history of the Polygon. In mid-1998, a joint mission to assess the consequences of the nuclear testing was conducted. Participants included the Kazakhstan Government and United Nations agencies, including the United Nations Development Programme (UNDP), the International Atomic Energy Agency (IAEA), the United Nations Educational, Scientific and Cultural Organization (UNESCO), the United Nations Children's Fund (UNICEF), the World Health Organization (WHO), the United Nations Population Fund (UNFPA), the United Nations Volunteers (UNV), the United Nations Industrial Development Organization (UNIDO), the Office for the Coordination of Humanitarian Affairs (OCHA), the International Labour Organization (ILO), and other participants from the international community. The assessment covered four areas, namely, the environment, health, economic recovery and humanitarian assistance. The product was a special report of the Secretary-General, submitted to the General Assembly in 1998. In 1999, a National Action Plan based on the mission's findings was formulated by the Government with substantive inputs from UNDP. An International Conference in Tokyo in September 1999, jointly organized and sponsored by UNDP and the Government of Japan, became the first follow-up action to the Secretary-General's special report. Some $20 million out of the $43 million prioritized package of the Semipalatinsk Relief and Rehabilitation Programme has been pledged from Japan, the General Board of Global Ministries, the North Atlantic Treaty Organization, the World Bank and UN agencies.