Chernobyl accident. Exposures and effects

The Chernobyl accident that occurred in Ukraine in April 1986 happened during an experimental test of the electrical control system as the reactor was being shut down for routine maintenance. The operators, in violation of safety regulations, had switched off important control systems and allowed the reactor to reach unstable, low-power conditions. A sudden power surge caused a steam explosion that ruptured the reactor vessel and allowed further violent fuel-steam interactions that destroyed the reactor and the reactor building. The Chernobyl accident was the most serious to have ever occurred in the nuclear power industry. The accident caused the early death of 30 power plant employees and fire fighters and resulted in widespread radioactive contamination in areas of Belarus, the Russian Federation, and Ukraine inhabited by several million people. Radionuclides released from the reactor that caused exposure of individuals were mainly iodine-131, caesium-134 and caesium-137. Iodine-131 has a short radioactive half-life (8 days), but it can be transferred relatively rapidly through milk and leafy vegetables to humans. Iodine becomes localized in the thyroid gland. For reasons of intake of these foods, size of thyroid gland and metabolism, the thyroid doses are usually greater to infants and children than to adults. The isotopes of caesium have relatively long half-lives (caesium-134: 2 years; caesium-137: 30 years). These radionuclides cause long-term exposures through the ingestion pathway and from external exposure to these radionuclides deposited on the ground. In addition to radiation exposure, the accident caused long-term changes in the lives of people living in the contaminated regions, since measures intended to limit radiation doses included resettlements, changes in food supplies, and restrictions in activities of individuals and families. These changes were accompanied by major economic, social and political changes in the affected countries resulting from the disintegration of the former Soviet Union. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has given particular attention to the accident. Estimates of average doses in separate regions of countries and for the population of the northern hemisphere as a whole were presented in Annex D of the UNSCEAR 1988 Report. The experience gained in treating the immediate radiation injuries of workers and fire fighters involved in controlling the accident were also reviewed in the UNSCEAR 1988 Report (Annex G). The UNSCEAR Committee is currently involved in the final phase of preparation of a further assessment of the exposures and effects of the accident. During the last several years, considerable attention has been devoted to investigating possible associations between health effects in the populations and the exposure to radionuclides released and dispersed following the Chernobyl accident. Of particular note has been the occurrence of numerous thyroid cancers in children. The number of thyroid cancers in individuals exposed in childhood, particularly in the severely contaminated areas of Belarus, Russia, and Ukraine is considerably greater than expected based on previous knowledge. The high incidence and the short induction period have not been experienced in other populations, and other factors are most certainly influencing the risk. If the current trend continues, further thyroid cancers can be expected to occur, especially in those exposed at young ages. The most recent findings indicate that the thyroid cancer risk for those older than 10 years of age at the time of the accident is leveling off, while the increase continues for those younger than 4-5 years in 1986. Apart from the dramatic increase in thyroid cancer after childhood exposure, there is no evidence of a major public health impact 14 years after the Chernobyl accident. No increases in overall cancer incidence or mortality have been observed that could be attributed to ionizing radiation. Risk of leukaemia, one of the major concerns after radiation exposure, does not appear to be elevated even among the recovery workers. Neither is there any scientific proof of other non-malignant disorders, somatic or mental, that are related to ionizing radiation. A majority of the epidemiological studies completed to date are of the descriptive type, in which average population exposures are correlated with the average rates of cancer incidence in specific time periods. As long as individual doses with reasonably low uncertainties are not available, the extent to which health effects might be radiation-related remains unclear. The reconstruction of individual doses is a key element in future research on radiation associated cancers related to the Chernobyl accident. Although the Chernobyl accident could shed some light on the knowledge on the late effects of protracted radiation exposures, it must be recognized that because of the relatively low doses received by the majority of exposed individuals, any increase in cancer incidence or mortality will be difficult to detect in epidemiological studies. (author)