UNSCEAR Report 2000: Sources and Effects of Ionizing Radiation

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Abstract
Since its inception in 1955 the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has periodically undertaken a broad review of the sources and the effects of ionising radiation. This latest report is in two concurrently published separate volumes. Volumes I and II provide updated reviews of sources and effects of ionising radiation respectively. A considerable amount of new material supports the review of sources. The review of biological effects takes on board various contemporary issues raised by the `linear-no-threshold' controversy. Particular emphasis has been given to the evaluation of exposures and health consequences of the Chernobyl accident. As usual the report has an extended summary (17 pages) backed up by a number of technical annexes (A-J). The annexes have a wealth of basic data, extensive tables and voluminous reference lists. The two volumes are available separately from the United Nations and cost £60 per volume. They are also available from the Stationery Office, but only as a two volume set. If you are interested only in `sources' then you should be aware that the source-related aspects of the Chernobyl accident are largely covered in volume II on effects. Annex A of volume I provides a description of the methodologies used for the assessment of doses from natural, man-made environmental, medical and occupational radiation exposures, which are presented in subsequent annexes B, C, D and E respectively. The components of natural radiation (cosmic rays, terrestrial gamma rays, inhalation and ingestion) have been evaluated and added to provide an estimate of the global average exposure. Since there are wide distributions of exposures from each source, the consequent effective doses combine in various ways at each location, depending on the specific concentration of radionuclides in the environment and in the body, the latitude and altitude of the location, and many other factors. The total annual global per caput effective dose due to natural radiation sources is 2.4 mSv. A typical range of individual doses is considered to be 1-10 mSv. In any large population about 65% would be expected to have annual effective doses between 1 and 3 mSv, about 25% of the population would have annual effective doses less than 1 mSv and 10% would have annual effective doses greater than 3 mSv. The main man-made contribution to the environmental exposure of the world's population has come from the testing of nuclear weapons in the atmosphere that occurred mainly between 1945-1980. Since the previous UNSCEAR review of this topic in 1982 new information, previously classified, has become available on the numbers and yields of nuclear tests. An updated listing of atmospheric nuclear tests conducted at each of the test sites is included in this report. Although the total explosive yields of each test have been divulged, the fission and fusion yields are still mostly suppressed. Some general assumptions have been made to allow the evaluation of fission and fusion yields of each test in order to estimate the amounts of radionuclides produced in the explosions. The estimated total of fission yields of individual tests is in agreement with the global deposition of the main fission radionuclides 90Sr and 137Cs, as determined by worldwide monitoring networks. It has been calculated that the world average annual effective dose reached a peak of 150 μSv in 1963 and has since decreased to about 5 μSv in 2000, from residual radionuclides in the environment, mainly of 14C, 90Sr and 137Cs. The contribution to man-made exposures from the generation of electrical energy by nuclear power reactors has been estimated using a collective (100 year truncated) dose of 6 man-Sv per GW year. Assuming the present annual generation of 250 GW years continues, the truncated collective dose per year of practice is 1500 man-Sv to the world population, giving an estimated maximum per caput dose of less than 0.2 μSv per year. UNSCEAR has assessed the exposures from medical radiation procedures based on information obtained from questionnaires distributed to all Member States. Four levels of health care have been distinguished based on the number of physicians available to serve the inhabitants of a country. They range from one physician per 1000 population at the highest level (health-care level I), to one physician for more than 10,000 population (health-care level IV). The available data have been averaged to obtain representative frequencies of procedures or exposure within countries at each level. These were then extrapolated to the populations of all countries within each level. The worldwide average annual effective dose of 0.4 mSv from diagnostic medical exposures is somewhat larger than the previous estimate of 0.3 mSv. UNSCEAR's current estimate of the worldwide collective effective dose to workers from man-made sources for the early 1990s (2700 man-Sv) is lower by a factor of about two than that made previously by the Committee for the late 1970s. A significant part of the reduction comes in the nuclear power fuel cycle, particularly in uranium mining. However, reductions are seen in all the main categories: industrial uses, medical uses, defence activities and education. This trend is also reflected in the worldwide average annual effective dose (over more than 6 million monitored workers), which has fallen from about 1.9 mSv to 0.6 mSv. The report also separately estimates the occupational exposure to enhanced natural sources of radiation, which now includes a contribution from elevated levels of radon. The average over a monitored worldwide workforce of 6.5 million is 1.7 mSv per year. A detailed consideration of the population doses arising as a result of the Chernobyl accident is presented in annex J of volume II. UNSCEAR provided a review of exposures from the Chernobyl accident in its 1988 report, based mainly on early measurements of the deposition of dispersed radionuclides. They gave average individual and population doses for the various regions and countries, and for the northern hemisphere as a whole. In recent years, the continued and intensified collection and evaluation of environmental levels of radioactivity and habit surveys have enabled UNSCEAR to provide a more definitive evaluation. Average doses to those persons most affected by the accident were about 100 mSv for 240,000 recovery operation workers, 30 mSv for 116,000 evacuated persons and 10 mSv during the first decade after the accident for those who continued to reside in contaminated areas. Maximum values of the dose may be an order of magnitude higher. Outside Belarus, the Russian Federation, and the Ukraine, other European countries were subjected to doses that were at most 1 mSv in the first year after the accident, with progressively decreasing doses in subsequent years. The dose over a lifetime was estimated to be 2-5 times the first-year dose. These doses are comparable to an annual dose from natural background radiation and are, therefore, considered by UNSCEAR to be of little radiological significance. The exposures were much higher for those involved in mitigating the accident and those who resided nearby. These exposures are reviewed in great detail. A compendium of some of the natural and man-made worldwide average annual effective doses taken from the various annexes is given in table 1. Table 1. Worldwide average annual effective doses at year 2000 from natural and man-made sources of radiation (mSv). Source of radiation Dose (mSv) Natural background (total, all sources) 2.4 Inhalation (mainly radon) 1.2 Terrestrial gamma rays 0.5 Cosmic rays 0.4 Ingestion 0.3 Diagnostic medical examinations 0.4 Atmospheric nuclear testing 0.005 Chernobyl accident 0.002 Nuclear power production 0.002 Volume II presents a review of the biological effects of radiation exposure. Annexes F and G consider DNA damage and mutagenesis, and biological effects at low radiation doses, respectively. The report follows a conventional view that damage to DNA in the nucleus is the main initiating event by which radiation causes long-term harm to organs and tissues of the body. Double-strand breaks in DNA are regarded as the most likely candidate for causing critical damage. Single radiation tracks have the potential to cause double-strand breaks and, in the absence of 100% efficient repair, could result in long-term damage, even at the lowest doses. The repair of sometimes complex DNA double-strand lesions is largely error-prone and is an important determinant of dose, dose rate and radiation quality effects in cells. The significance to tumorigenesis of adaptive responses to DNA damage is said to remain uncertain. The report comes down against the existence of a threshold dose and concludes that, in general, tumorigenic response does not appear to be a complex function of increasing dose. The simplest representation is a linear relationship, which is consistent with most of the available mechanistic and quantitative data. Annex H considers the combined effects of radiation and other agents. With the exception of radiation and smoking, there is little indication from epidemiological data for a need to adjust for strong antagonistic or synergistic combined effects. The report concludes that, although both synergistic and antagonistic combined effects are common at high exposures, there is no firm evidence for large deviations from additivity at controlled occupational or environmental exposures. Appendix I provides an assessment of the cancer risks from radiation exposures based on reviews of epidemiological studies and results from fundamental radiobiological research. The primary source of information remains the Life Span Study of the survivors of the atomic bombings of Hiroshima and Nagasaki. It includes about 86,500 individuals of all ages and both genders with good dosimetric data over a wide range of doses. About 5% of the 7800 deaths to date from cancer or leukaemia in this group of exposed people is due to radiation. For a population of all ages and both genders, the lifetime risk of dying from cancer is about 9% for men and 13% for women after an acute dose of 1 Sv. Appendix J reviews the exposures and effects of the Chernobyl accident. According to the Committee's scientific assessments there have been about 1800 cases of thyroid cancer in children who were exposed at the time of the accident, and, if the current trend continues, there may be more cases during the next decades. Apart from this increase, there is no evidence of a major public health impact attributable to radiation exposure fourteen years after the accident. There is no scientific evidence of increases in overall cancer incidence or mortality or, in non-malignant disorders that could be related to radiation exposure. The risk of leukaemia, one of the main concerns owing to its short latency time, does not appear to be elevated, not even among the recovery operation workers. Although those most highly exposed individuals are at an increased risk of radiation-associated effects, the report concludes that the great majority of the population is not likely to experience serious health consequences from radiation from the Chernobyl accident. Lars-Erik Holm (Chairman of UNSCEAR) has recently found it necessary to express his concern in a letter to Mr Kofi Annan (Secretary-General United Nations) regarding the publication by the United Nations Office for the Coordination of Humanitarian Affairs of a report entitled `Chernobyl: a continuing catastrophe' (OCHA/99/20, New York and Geneva, 2000). According to Holm the report contains a number of statements regarding radiation-induced health effects which do not appear to have any substance and have no support in scientific assessments. There appears to be a problem of communication within the UN on issues of radiological protection, which reflects the wider problems of communication in this area which readers may be familiar with. The latest UNSCEAR report should provide the factual information that will inform the ongoing debate about radiation levels and risks. There is a need, however, to distill the authoritative and extensive findings of the report into a more digestible form for general consumption.

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