Sources and Effects of Ionizing Radiation



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UNITED NATIONS SCIENTIFIC COMMITTEE

ON THE

EFFECTS OF ATOMIC RADIATION

Sources and Effects of Ionizing Radiation


The UNSCEAR 2000 Report to the General Assembly
(without Scientific Annexes)

6 June 2000
6 June 2000

UNSCEAR 2000 Report to the General Assembly



CONTENTS
Page
INTRODUCTION 4
I. HIGHLIGHTS 5

A. THE EFFECTS OF RADIATION EXPOSURE 5

B. LEVELS OF RADIATION EXPOSURE 6

C. THE RADIOLOGICAL CONSEQUENCES OF THE CHERNOBYL ACCIDENT 7


II. SOURCES OF RADIATION EXPOSURE 7

A. NATURAL RADIATION EXPOSURES 8

B. MAN-MADE ENVIRONMENTAL EXPOSURES 9

C. MEDICAL RADIATION EXPOSURES 10

D. OCCUPATIONAL RADIATION EXPOSURES 11

E. COMPARISON OF EXPOSURES 13


III. RADIATION-ASSOCIATED CANCER 14

A. RADIOBIOLOGICAL EFFECTS AFTER LOW DOSES OF RADIATION 14

B. COMBINED EFFECTS 16

C. CANCER EPIDEMIOLOGY 17


III. THE CHERNOBYL ACCIDENT 20

A. RELEASE OF RADIONUCLIDES 20

B. EXPOSURE OF INDIVIDUALS 21

C. HEALTH EFFECTS 21



Appendix I MEMBERS OF NATIONAL DELEGATIONS ATTENDING 44th to 49th SESSIONS 23



Appendix II SCIENTIFIC STAFF AND CONSULTANTS COOPERATING WITH THE COMMITTEE

IN THE PREPARATION OF THIS REPORT 24

INTRODUCTION
1. During the last few years, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)1 has undertaken a broad review of the sources and effects of ionizing radiation. In the present Report2, the Committee, drawing on the main conclusions of its scientific assessments, summarizes the developments in radiation science in the years leading up to the new millennium.
2. The present report and its scientific annexes were prepared between the forty-fourth and the forty-ninth sessions of the Committee. The following members of the Committee served as Chairman, Vice chairman, and Rapporteur, respectively, at the sessions: forty-fourth and forty-fifth sessions: L. Pinillos-Ashton (Peru), A. Kaul (Germany), and G. Bengtson (Sweden); forty-sixth and forty-seventh sessions: A. Kaul (Germany), L.-E. Holm (Sweden), and J. Lipsztein (Brazil); forty-eighth and forty-ninth sessions: L.-E. Holm (Sweden), J. Lipzstein (Brazil), and Y. Sasaki (Japan). The names of members of national delegations who attended the forty-fourth to the forty ninth sessions of the Committee as members of national delegations are listed in Appendix I.
3. The Committee wishes to acknowledge the help and advice of a group of consultants and contributors who helped in the preparation of the scientific annexes. Their names are given in Appendix II. The sessions of the Committee were attended by representatives of the World Health Organization (WHO), the International Atomic Energy Agency (IAEA), the International Commission on Radiological Protection (ICRP), and the International

Commission on Radiation Units and Measurements (ICRU). The Committee wishes to acknowledge their contributions to the discussions.


1 The United Nations Scientific Committee on the Effects of Atomic Radiation was established by the General Assembly at its tenth session, in 1955. Its terms of reference are set out in resolution 913 (X) of 3 December 1955. The Committee was originally composed of the following Member States: Argentina, Australia, Belgium, Brazil, Canada, Czechoslovakia, Egypt, France, India, Japan, Mexico, Sweden, Union of Soviet Socialist Republics, United Kingdom of Great Britain and Northern Ireland, and the United States of America. The membership was subsequently enlarged by the General Assembly in its resolution 3154 C (XXVIII) of 14 December 1973 to include the Federal Republic of Germany, Indonesia, Peru, Poland, and the Sudan. By resolution 41/62 B of 3 December 1986, the General Assembly increased the membership of the Committee to a maximum of 21 members and invited China to become a member.
2 For the previous substantive Reports of UNSCEAR to the General Assembly, see Official Records of the General Assembly, Thirteenth Session, Supplement No. 17 (A/3838); ibid., Seventeenth Session, Supplement No. 16 (A/5216); ibid., Nineteenth Session, Supplement No. 14 (A/5814); ibid., Twenty-first Session, Supplement No. 14 (A/6314 and Corr.1); ibid., Twenty-fourth Session, Supplement No. 13 (A/7613 and Corr.1); ibid., Twenty-seventh Session, Supplement No. 25 (A/8725 and Corr.1); ibid., Thirty-second Session, Supplement No. 40 (A/32/40); ibid., Thirty-seventh Session, Supplement No. 45 (A/37/45); ibid., Forty-first Session, Supplement No. 16 (A/41/16); ibid., Forty-third Session, Supplement No. 45 (A/43/45), ibid., Forty-eighth Session, Supplement No. 46 (A/48/46); ibid., Forty-ninth Session, Supplement No. 46 (A/49/46); ibid. Fifty-first Session, Supplement No. 46 (A/51/46). These documents are referred to as the 1958, 1962, 1964, 1966, 1969, 1972, 1977, 1982, 1986, 1988, 1993, 1994, and 1996 Reports, respectively. The 1972 Report with scientific annexes was published as Ionizing Radiation: Levels and Effects, Volume I: Levels and Volume II: Effects (United Nations publication, Sales No. E.72.IX.17 and 18). The 1977 Report with scientific annexes was published as Sources and Effects of Ionizing Radiation (United Nations publication, Sales No. E.77.IX.1). The 1982 Report with scientific annexes was published as Ionizing Radiation: Sources and Biological Effects (United Nations publication, Sales No. E.82.IX.8). The 1986 Report with scientific annexes was published as Genetic and Somatic Effects of Ionizing Radiation (United Nations publication, Sales No. E.86.IX.9). The 1988 Report with annexes was published as Sources, Effects and Risks of Ionizing Radiation (United Nations publication, Sales No. E.88.IX.7). The 1993, 1994, and 1996 Reports, with annexes, were published as Sources, Effects and Risks of Ionizing Radiation (United Nations publication, Sales Nos. E.94.IX.2, No. E.94.IX.11 and E.96.IX.3), respectively.

4. In carrying out its work, the Committee applied its scientific judgment to the material it reviewed and took care to assume an independent and neutral position in reaching its conclusions. The results of its work are presented for the general reader in the this report to the General Assembly. The supporting scientific annexes are aimed at the general scientific community.


5. UNSCEAR, a scientific Committee of the General Assembly, is the body in the United Nations system with a mandate to assess and report levels and effects of exposure to ionizing radiation. The fact that UNSCEAR holds this specific mandate from such an authoritative body greatly enhances its ability to provide an effective and independent service to the world. The United Nations through its General Assembly, can take credit for providing this service. The information provided by the Committee supports the General Assembly when considering and making recommendations particularly relevant to international collaboration in the health field, in issues related to sustainable development and, to some extent, to the maintenance of international peace and security.
6. New challenges on the global levels of radiation exposure continue to arise and new biological information on the effects of radiation exposure is becoming available. For example, large amounts of radioactive waste have built up as a result of both peaceful uses of nuclear energy and military nuclear operations, and radiation sources used in military and peaceful operations have been abandoned, creating a situation that is prone to illicit trafficking and other criminal activities. Moreover, the potential risks from low-level radiation exposure, i.e. exposure to radiation comparable with natural background radiation, are the cause of lively debate and controversy. UNSCEAR will respond to those challenges with new initiatives to be included in its future assessments of radiation sources, levels, and effects.
7. Governments and organizations throughout the world rely on the UNSCEAR evaluations of the sources and effects as the scientific basis for estimating radiation risk, establishing radiation protection and safety standards, and regulating radiation sources. Within the United Nations system, these estimates are used by the International Atomic Energy Agency in discharging its statutory functions of establishing standards for the radiation protection of health and providing for their application. The Committee is proposing a renewed programme of work to fulfill its obligations to the General Assembly.

I. HIGHLIGHTS
A. THE EFFECTS OF RADIATION EXPOSURE
8. Radiation exposures can damage living cells, causing death in some of them and modifying others. Most organs and tissues of the body are not affected by the loss of even considerable numbers of cells. However, if the number lost is large enough, there will be observable harm to organs and may lead to death. This type of harm occurs in individuals who are exposed to radiation in excess of a threshold level. Other radiation damage may also occur in cells that are not killed but modified This damage is usually repaired. If the repair is not perfect, the resulting modification will be transmitted to further cells and may eventually lead to cancer. If the cells modified are those transmitting hereditary information to the descendants of the exposed individual, hereditary disorders may arise.
9. Radiation exposure has been associated with most forms of leukaemia and with cancers of many organs, such as lung, breast and thyroid, but not with some other organs such as prostate. However, a small addition of radiation exposure (e.g. about the global average level of natural radiation exposure) would produce an exceedingly small increase in the chances of developing an attributable cancer. Moreover, radiation-induced cancer may manifest itself decades after the exposure and does not differ from cancers that arise spontaneously or are attributable to other factors. The major long-term evaluation of populations exposed to radiation is the study of the approximately 86 500 survivors of the atomic bombings of Hiroshima and Nagasaki. It has revealed an excess of a few hundred cancer deaths in the studied population. Since approximately half of this population is still alive, additional study is necessary in order to obtain the complete cancer experience in this group.
10. Radiation exposure has also the potential for causing hereditary effects in the offspring of persons exposed to radiation. Such effects were once thought to threaten the future of the human race by increasing the rate of natural mutation to an inappropriate degree. However, radiation-induced hereditary effects have yet to be detected in human populations exposed to radiation, although they are known to occur in other species. UNSCEAR is preparing a comprehensive report on hereditary effects of radiation exposures to be submitted to the General Assembly to its fifty-sixth session.

B. LEVELS OF RADIATION EXPOSURE
11. All persons are exposed to natural radiation. The natural sources of radiation are cosmic rays and naturally occurring radioactive substances existing in the earth itself and inside the human body. A significant contribution to natural exposure of humans is due to radon gas, which emanates from the soil and may concentrate in dwellings. The level of natural exposure varies around the globe, usually by a factor of about 3. At many locations, however, typical levels of natural radiation exposure exceed the average levels by a factor of 10, and sometimes even by a factor of 100.
12. Human activities involving the use of radiation and radioactive substances cause radiation exposure in addition to the natural exposure. Some of these activities simply enhance the exposure from natural radiation sources. Examples are the mining and use of ores containing naturally radioactive substances and the production of energy by burning coal that contains these substances. Environmental contamination with radioactive residues from nuclear weapons testing continues to be a global source of human radiation exposure. The production of nuclear materials for military purposes has left a legacy of large amounts of radioactive residues in some parts of the world. Nuclear power plants and other nuclear installations release radioactive materials into the environment and produce radioactive waste during operation and on their decommissioning. The use of radioactive materials in industry, agriculture and research is expanding around the globe, and people have been harmed by mishandled radiation sources.
13. These human activities generally give rise to radiation exposures that are only a small fraction of the global average level of natural exposure. However, specific individuals residing near installations releasing radioactive material into the environment may be subject to higher exposures. The exposure of members of the public to regulated releases is restricted by internationally recognized limits, which are set somewhat less than the global average level of natural exposure. It is to be noted that, should some of the sites with high levels of radioactive residues be inhabited or re-inhabited, the settlers would incur radiation exposures that would be higher than the global average level of natural exposures.
14. The medical use of radiation is the largest, and a growing, man-made source of radiation exposure. It includes diagnostic radiology, radiotherapy, nuclear medicine, and interventional radiology. Large numbers of people (particularly in developing countries) cannot yet take advantage of many of these medical procedures, which are not available worldwide. For the time being, therefore, these people receive less radiation exposure from medical diagnosis and treatment than those in countries benefiting from advanced medical procedures, a situation that is expected to change in the future and will need to be followed by the Committee.
15. The average levels of radiation exposure due to the medical uses of radiation in developed countries is approximately equivalent to 50% of the global average level of natural exposure. In these countries, computed tomography accounts for only a few per cent of the procedures but for almost half of the exposure involved in medical diagnosis. Severe radiation-related injuries have occurred as a result of poor practice of some interventional techniques (such as radiological procedures to monitor the dilating of coronary arteries) and radiotherapy.

16. Radiation exposure also occurs as a result of occupational activities. It is incurred by workers in industry, medicine and research using radiation or radioactive substances, as well as by passengers and crew during air travel. It is very significant for astronauts.


17. The average level of occupational exposures is generally similar to the global average level of natural radiation exposure. However, a few percent of workers receive exposures several times higher than the average exposure to natural radiation. The exposure of workers is restricted by internationally recognized limits, which are set around 10 times the average exposure to natural radiation.

C. THE RADIOLOGICAL CONSEQUENCES OF THE CHERNOBYL ACCIDENT
18. The accident at the Chernobyl nuclear power plant was the most serious accident involving radiation exposure. It caused the deaths, within a few days or weeks, of 30 workers and radiation injuries to hundred others. It also brought about the immediate evacuation, in 1986, of about 116,000 people from areas surrounding the reactor and the permanent relocation, after 1986, of about 220,000 people from Belarus, the Russian Federation, and the Ukraine. It caused serious social and psychological disruption in the lives of those affected and vast economic penalties over the entire region. Large territories of those three countries were contaminated, and deposition of released radionuclides was measurable in all countries of the northern hemisphere.
19. There have been about 1,800 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 great majority of the population are not likely to experience serious health consequences from radiation from the Chernobyl accident.

II. SOURCES OF RADIATION EXPOSURE
20. Ionizing radiation represents electromagnetic waves and particles that can ionize, i.e. remove an electron from an atom or molecule of the medium through which they propagate. Ionizing radiation may be emitted in the process of natural decay of some unstable nuclei or following excitation of atoms and their nuclei in nuclear reactors, cyclotrons, x-ray machines or other instruments. For historical reasons, the photon (electromagnetic) component of ionizing radiation emitted by the excited nucleus is termed gamma rays and that emitted from machines is termed x rays. The charged particles emitted from the nucleus are referred to as alpha particles (helium nuclei) and beta particles (electrons).
21. The process of ionization in living matter necessarily changes atoms and molecules, at least transiently, and may thus damage cells. If cellular damage does occur and is not adequately repaired, it may prevent the cell from surviving or reproducing or performing its normal functions. Alternatively, it may result in a viable but modified cell.
22. The basic quantity used to express the exposure of material such as the human body is the absorbed dose, for which the unit is the gray (Gy). However, the biological effects per unit absorbed dose varies with the type of radiation and the part of the body exposed. To take account of these variations, a weighted quantity called the effective dose is used, for which the unit is the sievert (Sv). For reporting levels of human exposure, the Committee usually uses the effective dose. In this report, both absorbed dose and effective dose are usually simply called dose, for which the units provide the necessary differentiation. A radioactive source is described by its activity, which is the number of nuclear disintegrations per unit time. The unit of activity is the becquerel (Bq). One becquerel is one disintegration per second.
23. To evaluate the effects of exposing a defined population group, the sum of all doses acquired by the members of the group, termed the collective dose (in units of man Sv), may be used. The value of collective dose divided by the number of individuals in the exposed population group is the per caput dose, in Sv. The general procedures used by the Committee to evaluate radiation doses are presented in Annex A of this report, “Dose assessment methodologies”.

A. NATURAL RADIATION EXPOSURES
24. All living organisms are continually exposed to ionizing radiation which has always existed naturally. The sources of this exposure are cosmic rays that come from outer space and from the surface of the sun, terrestrial radionuclides that occur in the earth’s crust, in building materials, and in air, water, foods, and in the human body itself. Some of these exposures are fairly constant and uniform for all individuals everywhere, for example the dose from ingestion of potassium 40 in foods. Other exposures vary widely depending on location: for example, cosmic rays are more intense at higher altitudes, and concentrations of uranium and thorium in soils are elevated in localized areas. Exposures can also vary as a result of human activities and practices. In particular, the building materials of houses and the design and ventilation systems strongly influence indoor levels of the radioactive gas radon and its decay products, which contribute significantly to doses through inhalation.



Table 1

Average radiation dose from natural sources





Source


Worldwide average annual effective dose (mSv)


Typical range (mSv)


External dose

ab

Cosmic rays

Terrestrial gamma rays

0.4


0.5

0.3-1.0

0.3-0.6



Internal dose

cd

Inhalation (mainly radon)

Ingestion

1.2


0.3

0.2-10


0.2-0.8

Total

2.4

1-10

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