HEALTH RISKS FROM EXPOSURE TO LOW LEVELS OF IONIZING RADIATION

Source:  HEALTH RISKS FROM EXPOSURE TO LOW LEVELS OF IONIZING RADIATION    Tag:  national radiation protection board
http://www.nap.edu/openbook.php?record_id=11340&page=R1

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BEIR VII PHASE 2

Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation

Board on Radiation Effects Research

Division on Earth and Life Studies

NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES

THE NATIONAL ACADEMIES PRESS
Washington , D.C.

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THE NATIONAL ACADEMIES PRESS
500 Fifth Street, N.W. Washington , DC 20001

NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine . The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by Environmental Protection Agency Grant #X-826842-01, Nuclear Regulatory Commission Grant #NRC-04-98-061, and U.S. Department of Commerce, National Institute of Standards and Technology Grant #60NANB5D1003. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project.

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RICHARD R. MONSON (chairman),

Harvard School of Public Health , Boston , MA

JAMES E. CLEAVER (vice chairman),

University of California , San Francisco , CA

HERBERT L. ABRAMS,

Stanford University , Stanford , CA

EULA BINGHAM,

University of Cincinnati , Cincinnati , OH

PATRICIA A. BUFFLER,

University of California , Berkeley , CA

ELISABETH CARDIS,

International Agency for Research on Cancer, Lyon , France

ROGER COX,

National Radiological Protection Board, Chilton, Didcot, Oxon , United Kingdom

SCOTT DAVIS,

University of Washington and Fred Hutchinson Cancer Research Center , Seattle , WA

WILLIAM C. DEWEY,

University of California , San Francisco , CA

ETHEL S. GILBERT,

National Cancer Institute, Rockville , MD

ALBRECHT M. KELLERER,

Ludwig-Maximilians-Universität, München , Germany

DANIEL KREWSKI,

University of Ottawa , Ottawa , Ontario , Canada

TOMAS R. LINDAHL,

Cancer Research UK London Research Institute, United Kingdom

KATHERINE E. ROWAN,

George Mason University , Fairfax , VA

K. SANKARANARAYANAN,

Leiden University Medical Centre, Leiden , The Netherlands

DANIEL W. SCHAFER,

Oregon State University , Corvallis , OR (from May 2002)

LEONARD A. STEFANSKI,

North Carolina State University , Raleigh , NC (through May 2002)

ROBERT L. ULLRICH,

Colorado State University, Fort Collins, CO

CONSULTANTS


JOHN D. BOICE, JR.,

International Epidemiology Institute, Rockville , MD

KIYOHIKO MABUCHI,

National Cancer Institute, Rockville , MD

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This is the seventh in a series of reports from the National Research Council (NRC) prepared to advise the U.S. government on the relationship between exposure to ionizing radiation and human health. In 1996 the National Academy of Sciences (NAS) was requested by the U.S. Environmental Protection Agency to initiate a scoping study preparatory to a new review of the health risks from exposure to low levels of ionizing radiations. The main purpose of the new review would be to update the Biological Effects of Ionizing Radiation V (BEIR V) report (NRC 1990), using new information from epidemiologic and experimental research that has accumulated during the 14 years since the 1990 review. Analysis of those data would help to determine how regulatory bodies should best characterize risks at the doses and dose rates experienced by radiation workers and members of the general public



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With respect to modeling, the committee will (1) develop appropriate risk models for all cancer sites and other outcomes for which there are adequate data to support a quantitative estimate of risk, including benign disease and genetic effects; (2) provide examples of specific risk calculations based on the models and explain the appropriate use of the risk models; (3) describe and define the limitations and uncertainties of the risk models and their results; (4) discuss the role and effect of modifying factors, including host (such as individual susceptibility and variability, age, and sex), environment (such as altitude and ultraviolet radiation), and life-style (such as smoking history and alcohol consumption) factors; and (5) identify critical gaps in knowledge that should be filled by future research.



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Ionizing radiation can consist of electromagnetic radiation, such as X-rays or gamma rays (γ-rays), or of subatomic particles, such as protons, neutrons, and α-particles. X- and γ-rays are said to be sparsely ionizing, because they produce fast electrons, which cause only a few dozen ionizations when they traverse a cell. Because the rate of energy transfer is called linear energy transfer (LET), they are also termed low-LET radiation; low-LET radiations are the subject of this report. In contrast, the heavier particles are termed high-LET radiations because they transfer more energy per unit length as they traverse the cell.

Since the high-LET radiations are capable of causing more damage per unit absorbed dose, a weighted quantity, equivalent dose, or its average over all organs, effective dose, is used for radiation protection purposes. For low-LET radiation, equivalent dose equals absorbed dose. For high-LET radiation—such as neutrons, α-particles, or heavier ion particles—equivalent dose or effective dose equals the absorbed dose multiplied by a factor, the quality factor or the radiation weighting factor (see Glossary), to account for their increased effectiveness. Since the weighting factor for radiation quality is dimensionless, the unit of equivalent dose is also joules per kilogram. However, to avoid confusion between the two dose quantities, the special name sievert (Sv) has been introduced for use with equivalent dose and effective dose.



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TABLE 1 Units of Dose

Unita
Symbol
Conversion Factors
Becquerel (SI)
Bq
1 disintegration/s = 2.7 × 10−11 Ci
Curie
Ci
3.7 × 1010 disintegrations/s = 3.7 × 1010 Bq
Gray (SI)
Gy
1 J/kg = 100 rads
Rad
rad
0.01 Gy = 100 erg/g
Sievert (SI)
Sv
1 J/kg = 100 rem
Rem
rem
0.01 Sv
NOTE: Equivalent dose equals absorbed dose times Q (quality factor). Gray is the special name of the unit (J/kg) to be used with absorbed dose; sievert is the special name of the unit (J/kg) to be used with equivalent dose.
aInternational Units are designated SI.





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The health effects of low levels of ionizing radiation are important to understand. Ionizing radiation—the sort found in X-rays or gamma rays1—is defined as radiation that has sufficient energy to displace electrons from molecules. Free electrons, in turn, can damage human cells. One challenge to understanding the health effects of radiation is that there is no general property that makes the effects of man-made radiation different from those of naturally occurring radiation. Still another difficulty is that of distinguishing cancers that occur because of radiation exposure from cancers that occur due to other causes. These facts are just some of the many that make it difficult to characterize the effects of ionizing radiation at low levels.






Because ionizing radiation is a threat to health, it has been studied extensively. This report is the seventh in a series of publications from the National Academies concerning radiation health effects, referred to as the Biological Effects of Ionizing Radiation (BEIR) reports



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HOW IONIZING RADIATION WAS DISCOVERED


Low levels of ionizing radiation cannot be seen or felt, so the fact that people are constantly exposed to radiation is not usually apparent. Scientists began to detect the presence of ionizing radiation in the 1890s.

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HOW IONIZING RADIATION IS DETECTED


The detection of ionizing radiation has greatly improved since the days of Roentgen, Becquerel, and the Curies. Ionizations can be detected accurately by Geiger counters and other devices. Because the efficiency of the detector is known, one can determine not only the location of the radiation, but also the amount of radiation present. Other, more sophisticated detectors can evaluate the “signature” energy spectrum of some radiations and thus identify the type of radiation.

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WHAT IS MEANT BY LOW DOSES OF IONIZING RADIATION


For this report, the committee has defined low dose as doses in the range of near zero up to about 100 mSv (0.1 Sv) of low-LET radiation.




EXPOSURE FROM NATURAL BACKGROUND RADIATION


Human beings are exposed to natural background radiation every day from the ground, building materials, air, food, the universe, and even elements in their own bodies. In the United States , the majority of exposure to background ionizing radiation comes from exposure to radon gas and its decay products. Radon is a colorless, odorless gas that emanates from the earth and, along with its decay products, emits a mixture of high- and low-LET radiation. Radon can be hazardous when accumulated in underground areas such as poorly ventilated basements. The National Research Council 1999 report, Health Effects of Exposure to Radon (BEIR VI), reported on the health effects of radon, and therefore those health effects are not discussed in this report. Average annual exposures worldwide to natural radiation sources (both high and low LET) would generally be expected to be in the range of 1–10 mSv, with 2.4 mSv being the present estimate of the central value




After radon, the next highest percentage of natural ionizing radiation exposure comes from cosmic rays, followed by terrestrial sources, and “internal” emissions. Cosmic rays are particles that travel through the universe. The Sun is a source of some of these particles. Other particles come from exploding stars called supernovas.






Working near Ionizing Radiation


People who work at medical facilities, in mining or milling, or with nuclear weapons are required to take steps to protect themselves from occupational exposures to radiation. The maximum amount of radiation that workers are allowed to receive in connection with their occupations is regulated. In general these limits are 50 mSv per year to the whole body, with larger amounts allowed to the extremities. The exposure limits for a pregnant worker, once pregnancy is declared, are more stringent. In practice the guidelines call for limiting exposures to as low as is reasonably achievable.

[the current levels in japan are 100 mSv for all all people] CP



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Excess cancers represent the number of cancers above the levels expected in the population. In the case of in utero exposure (exposure of the fetus during pregnancy), excess cancers can be detected at doses as low as 10 mSv










 






Health Effects Other Than Cancer


In addition to cancer, radiation exposure has been demonstrated to increase the risk of other diseases, particularly cardiovascular disease




Estimating Risks to Children of Parents Exposed to Ionizing Radiation


Naturally occurring genetic (i.e., hereditary) diseases contribute substantially to illness and death in human populations. These diseases arise as a result of alterations (mutations) occurring in the genetic material (DNA) contained in the germ cells (sperm and ova) and are heritable (i.e., can be transmitted to offspring and subsequent generations). Among the diseases are those that show simple predictable patterns of inheritance (which are rare), such as cystic fibrosis, and those with complex patterns (which are common), such as diabetes mellitus. Diseases in the latter group originate from interactions among multiple genetic and environmental factors.




This report, prepared by the National Research Council’s Committee on the Biological Effects of Ionizing Radiation (BEIR), is the seventh in a series that addresses the health effects of exposure of human populations to low-dose, low-LET (linear energy transfer) ionizing radiation. The current report focuses on new information available since the 1990 BEIR V report on low-dose, low-LET health effects.

Ionizing radiation arises from both natural and man-made sources and at very high doses can produce damaging effects in tissues that can be evident within days after exposure. At the low-dose exposures that are the focus of this report, so-called late effects, such as cancer, are produced many years after the initial exposure.




EVIDENCE FROM BIOLOGY


There is an intimate relationship between responses to DNA damage, the appearance of gene or chromosomal mutations, and multistage cancer development. Molecular and cytogenetic studies of radiation-associated animal cancers and more limited human data are consistent with the induction of a multistage process of cancer development. This process does not appear to differ from that which applies to spontaneous cancer or to cancers associated with exposure to other carcinogens.




A cytogenetic-molecular data set is available on papillary thyroid cancer (PTC) (Bongarzone and others 1997) arising in excess in 131I-exposed children in areas contaminated by the Chernobyl accident (UNSCEAR 2000a). These mechanistic studies were guided by the knowledge that chromosomally mediated rearrangement and activation of the ret proto-oncogene is a frequently early arising feature of PTC (Richter and others 1999).






Mouse Lymphoma and Leukemia

Early studies with radiation-induced thymic lymphoma provided evidence of recurrent RAS gene activation and some indication that the RAS gene mutational spectra differs between X-ray and neutron-induced lymphoma (Sloan and others 1990). Other molecular studies include the finding of recurrent chromosome (chr) 4 deletions in thymic and nonthymic lymphomas (Melendez and others 1999; Kominami and others 2002) and T-cell receptor (Tcr) gene rearrangements and chromosomal events in thymic lymphoma. However, the above and other somatic mutations in mouse lymphoma have yet to be specifically associated with initial radiation damage.

The situation in mouse acute myeloid leukemia (AML; Silver and others 1999) is clearer. AML-associated, region-specific deletion of chr2 has been shown by cytogenetic analysis of in vivo irradiated bone marrow cell populations to be a direct consequence of radiation damage

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To sum…. TEPCO propose there is no-evidence that low dose radiation will harm you, and no studies have been done….



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