Ionizing Radiation: Exposure to the Population of the U.S., Report 93 (N C R P Report)

NCRP REPORT No. 93

IONIZING RADIATION EXPOSURE OF THE POPULATION OF THE UNITED STATES Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS

Issued September 1,1987 First Reprinting April 15,1998 National Council on Radiation Protection and Measurements 7910 W O O D M O N T AVENUE I BETHESDA, MD 20814

LEGAL NOTICE This report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its reporta. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this report, nor any person acting on the behalf of any of these parties (a) makes any warranty or representation, express or implied, with respect 'to the accuracy, completeness or usefulness of the information contained in this report, or that the use of any information, method or process disclosed in this report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or process disclosed in this report, under the Cwil Rights Act of 1964, Sectton 701 et seq. as amended 42 U.S.C. Section 2000e et seq. (Title VII) or any other statutory or common law theory governing linbility.

Library of Congress Catploging-in-PublicationData National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States. (NCRP report ;no. 93) Bibilography: p. Includes index. 1. Ionizing radiation-Dosage. 2. Ionizing radiation-Environmental aspectsUnited States. 3. Health risk assessment-United States. I. Title. II. Series. fDNLM: 1. Environmental Exposure. 2. Radiation Injuries-prevention & control. 3. Radiation Monitoring. WN 650 N279il RA569.N353 1987a 363.1'79 87-22062 ISBN 0-913392-91-X

Copyright O National Council on Radiation Protection and Measurements 1987 All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews. Library of Congress Catalog Card Number International Standard Book Number

Preface The NCRP has recognized for some time the need for a clear assessment of the magnitude of the doses from various sources of radiation to which the population of the U.S. is exposed In anticipation of the need to gather basic data for input into this process five assessment committees, each addressing a different source category, were established in 1971. NCRP reports assessing exposures from natural background and from consumer products were produced (NCRP 1975, 1977), but no attempt was made to develop a comprehensive account of all sources of exposure. In 1985, the NCRP reconsidered its overall effort in this area and, with the further support and stimulation of the Committee on Interagency Radiation Research and Policy Coordination (Office of Science and Technology Policy, Executive Office of the President of the United States), undertook to evaluate the exposure of the U.S. population from all sources. Six organizational groups were constituted or reconstituted to address different phases of the task and the results of their work are summarized in this document, which describes the exposure of the U.S. population from all known sources. The six organizational groups and their members are:

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PREFACE

Scientific Committee 28Radiation Exposure from Comurner Products

D.W. Moeller, Chairman R.J. Guimond J.W.N. Hickey E. Miller G.D. Schmidt

Scientific Committee 43Natural Background Radiation J.H. Harley, Chru'rman RB. Holtzman W.M.Lowder D.P.Meyerhof A.B. Tanner N.A. Wogman Consultants: B.S. Pasternack J.K.Soldat J.A. Young

Scientific Committee 44Radiation Associated with Medical Examinations RD. Moseley, Jr.7. Chairnun (19761987) F. A. Mettler. Jr., Chairman (1987J.S. Acarese W.W. Burr, Jr. R.O. Gorson S. Marks A. Raventos M. Rosenstein E.L. Saenger B. Shleien Advisors: D.L. Abernathy R.E. Bunge L.A. Selke Consultant: J.G. Kereiakes

Scientific Committee 45Radiation Received by Radiation Employees D.E.Barber, Chairman B.G. Brooks L.H.Lanzl R.E.Shore P.S. Stansbury R.A. Wynveen

scientific Committee 64, Task Group 5*Public Exposure from Nuclear Power B. Kahn, ChairM.J.BelJ ILL. Blanchard E.F. Branagan, Jr. K.Cowser K.F.Eckerman J.M. Hardin R.E. Luna E.Y.S. Shum C. W i l l s J.F. Wing

Scientific Committee 64, Special Group*MisceUaneow Environmental Sources W.E. Kreger, Co-Chairman W.A. Mills, Co-Chairman

t Deceased. *Subgroups of Scientific Committee 64 on Radionuclides in the Environment; Chairman, M.W. Carter, M. Eisenbud, J.W. Healy, W.E. Kreger, W.A. Mills, J.N. Stannard, J.E. Till and M.E. Wrenn

PREFACE

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These groups, except for the Special Group on Miscellaneous Environmental Sources, are in the course of producing separate NCRP reports. This summary report was prepared by the NCRP's Scientific Committee 48, Assessment of Exposures of the Population Contributed by Various Sources. Serving on Scientific Committee 48 during the preparation of this report were: W.K. Sinclair, Chairman National Council on Radiation Protection and Measurements Bethesda, Maryland

S.J. Adelstein

J.H. Harley Hoboken, New Jersey

Haward Medical School Boston, Maesachusetta M.W. Carter Georgia Institute of Technology Atlanta, Georgia

D.W. Moeller Haward School of Public Health Boston, Massachusetts

NCRP Secretariat-T.M. Koval

The NCRP is grateful to all who not only contributed their expertise in developing the contents of these reports but also made extraordinary efforts in managing their schedules of work to make this coordinated comprehensive report possible. Dr. F.A. Mettler, Jr. and Dr. D.E. Barber made important contributions to this text. Tom Koval served as the NCRP staff member providing support for all of these organizational groups. The International System of Units (SI) is used in this report followed by conventional units in parentheses in accordance with the procedure set forth in NCRP Report No. 82, SI Units in Radiation Protection and Measurements. Warren K. Sinclair President, NCRP

Bethesda, Maryland 11 June 1987

Contents .

1 Introduction 1.1 Sources of Population Exposure . . . . . . . . . . . . . . . . . . . 1.2 Earlier Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Quantities and Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 The Present Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Public Radiation Exposure from Natural Background 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Sources of Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Naturally Occurring Radionuclides . . . . . . . . . . 2.2.2 External Radiation . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Radionuclides in the Body . . . . . . . . . . . . . . . . . 2.3 Exposure Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Cosmic Radiation . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Terrestrial Gamma Radiation . . . . . . . . . . . . . . . 2.3.3 Cosmogenic Radionuclides . . . . . . . . . . . . . . . . . 2.3.4 Inhaled Radionuclides . . . . . . . . . . . . . . . . . . . . . 2.3.5 Radionuclides in the Body . . . . . . . . . . . . . . . . . 2.3.6 Total Exposure from Natural Background . . . . 2.4 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Occupational Exposure 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Sources of Data and Estimates . . . . . . . . . . . . . . . . . . . . 3.3 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Public Radiation Exposure from Nuclear 'Power Generation 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Sources of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Estimates and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Public Radiation Exposure from Consumer Products 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Sources of Data and Estimates . . . . . . . . . . . . . . . . . . . . 5.3 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .

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CONTENTS 5.4 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Public Radiation Exposure from Miscellaneous Environmental Sources 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 SourcesofData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Estimates and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Fallout from Nuclear Weapons Testing . . . . . . . . . . . . . 6.6 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Public Radiation Exposure from Medical Diagnosis and Therapy 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Sources of Data for Diagnosis . . . . . . . . . . . . . . . . . . . . . 7.3 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Estimates and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Radiation Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Summary and Conclusions 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 The Exposure of the U.S . Population to All Sources . . 8.3 The Most Significant Exposures . . . . . . . . . . . . . . . . . . . 8.4 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Recommendations for Dose Reduction . . . . . . . 8.5.2 Recommendations for Improved Data in the Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2.1 Natural Background . . . . . . . . . . . . . . 8.5.2.2 Occupational . . . . . . . . . . . . . . . . . . . . 8.5.2.3 Nuclear Fuel Cycle . . . . . . . . . . . . . . . 8.5.2.4 Consumer Products . . . . . . . . . . . . . . . 8.5.2.5 Miscellaneous Environmental Sources . . . . . . . . . . . . . . . . . . . . . . . 8.5.2.6 Medical Sources . . . . . . . . . . . . . . . . . . 8.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A . Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The NCRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NCRP Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction 1.1 Sources of Population Exposure All members of the public in the United States are inevitably exposed to sources of ionizing radiation, some to a wide variety of such sources, others to only a few. The sources involved are of three general types, those of natural origin, unperturbed by human activities, those of natural origin but affected by human activities (termed enhanced natural sources), and man-made sources. Natural sources include cosmic radiation from outer space, terrestrial radiation from natural radioactive sources in the ground, radiations from radionuclides naturally present in the body and inhaled and ingested radionuclides of natural origin. Each of these natural sources has certain characteristics which lead to varying human exposures depending on locality and other special circumstances. When these exposures are substantially above the average they are referred to as "elevated." Enhanced natural sources include those for which human exposures have been increased as a result of man's actions, deliberate or otherwise. For example, air travel, especially at very high altitudes, increases the exposure to cosmic radiation, whereas movement of radionuclides on the ground, as in phosphate mining, can increase the terrestrial component to persons living in houses built on phosphate and other waste landfills. Radon exposures indoors might be considered, in some instances at least, to be due to elevated natural levels and also to be "enhanced natural" since the exposure can be increased by the characteristics of the home. In a sense also, all the operations of the nuclear fuel cycle, starting with mining, could be considered to be enhanced exposures from natural materials but these are generally included with "man-maden exposures. A variety of exposures results from man-made materials and devices, e.g., radiopharmaceuticals and x rays in medicine, and consumer products containing radioactive materials such as some smoke detectors or static eliminators. Exposures may also result from episodic events due to man's activities, such as atmospheric nuclear weapons testing, accidents in nuclear power plants, etc. Although some previous attempts have been made to determine the dose from all sources (as noted below), none of these has been fully 1

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1. INTRODUCTION

comprehensive for the United States. It is therefore timely to make as accurate an assessment as possible of the overall exposure of the U.S. population from all sources of ionizing radiation. It is convenient to categorize these sources according to the origin of the exposure, namely: natural radiation, occupational, the nuclear fuel cycle, consumer products, miscellaneous environmental sources connected with human activities, and medical diagnosis and therapy. Unfortunately, there are limitations in the accuracy of the data available in each of these categories, which will become clear in the discussion below. Nevertheless, this Report provides a comprehensive account of the exposure of members of the U.S. public to all sources of ionizing radiation.

1.2 Earlier Surveys Selected surveys of one or more sources of radiation, especially medical, have been made from time to time but few of these have attempted to be comprehensive. Furthermore, assessment activities at the international level, as exemplified by the earlier reports of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (UNSCEAR, 1958), and by joint groups of the International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) (ICRP-ICRU, 1957, 1961) on medical exposure, have tended to emphasize global considerations rather than assessments of population exposure in individual countries. However, the UNSCEAR reports, which continue to provide global assessments of population exposures from a variety of sources, inevitably rely, to a substantial degree, on well-documented critical assessments of exposures at the national level (UNSCEAR, 1982). One early assessment of the exposure of the population in the U.S. was made by Moeller, Terrill, and Ingraham (1953). This pioneering report drew attention to natural background radiation and to medical diagnostic radiation. The former was estimated to result in an exposure of about nine roentgens in a 70-year lifetime and for the latter, the average annual dose to a limited region of the body was estimated to be about two roentgens to a "large portion" of the U.S. population. Other less important sources treated in that report included medical therapy, dental x rays, x rays in industry and research, radioisotopes in medicine, radium in luminous paints, static eliminators, the shipping of radioactive materials, nuclear reactors, and particle accelerators. It was noted that it was not possible to reach definitive conclu-

1.2 EARLIER SURVEYS

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sions on the magnitude of the radiological health problem occasioned by these sources of ionizing radiation because of the extremely fragmentary nature of much of the data In 1960 the Federal Radiation Council published a document which contained a section on sources of radiation exposure (FRC, 1960). This report concluded that the exposure of human beings from natural sources led to average annual dose equivalents to bone marrow, gonads and soft tissue of between 0.8 mSv (80 mrem) and 1.7 mSv (170 mrem), while from medical sources the annual genetically significant dose (GSD) was from 0.8 mSv (80 mrem) to 2.8 mSv (280 mrem) and the mean bone marrow dose was of the same order. Weapons testing fallout was identified as an important contributor, resulting in an average genetically significant dose of 0.53 mSv (63 mrem) over the following 30 years if atmospheric nuclear testing were not resumed after the cessation in 1958, but eight times this dose if atmospheric testing were resumed and continued at the same rate as in the previous five years. The mean bone marrow dose over 70 years would be 3.3 mSv (330 mrem) and 26.5 mSv (2,650 mrem), respectively, for these two circumstances. The nuclear fuel cycle was not believed to release radioactivity sufficient to cause a significant cnntribution to the population dose. A new effort was initiated by the Federal Radiation Council in 1970, and resulted in a report being produced by the Environmental Protection Agency (EPA, 1972) on the exposure of the U.S. population for the years 1960-1970 with predictions to the year 2000. Only environmental sources were considered, and the average annual natural background exposure was found to be about 1.3 mSv (130 mrem) in 1960, while fallout from earlier atmospheric tests of nuclear devices contributed an additional ten percent. All other sources were minor. The annual dose from fallout was expected to decline to about three percent of the natural background by 1970 and to stay at about that level until the year 2000. A few years later the EPA again reviewed the sources of ionizing radiation exposure to the population (EPA, 1977). The conclusions of this report were that the four major source categories contributing to the collective exposure of the United States population to ionizing radiation were environmental radiation, medical and dental radiation, the application of radiopharmaceuticals in medicine, and technologically enhanced natural radiation. However, the largest doses on an individual basis were identified as those from technologically enhanced natural radiation, medical radiation, environmental radiation, consumer products, occupational and industrial operations, and federal nuclear facilities. A particular source responsible for high individual doses in the category of technologically enhanced natural radiation

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1. INTRODUCTION

was radon from uranium mill tailings that had been used in the construction of residences. I t was noted that' the dose received by the patient from the limited use of radionuclide-powered cardiac pacemakers was significant to the individual but the report considered that this should be weighed against the benefit derived from these devices. The report also noted that there were many gaps in the dose data discussed in the report and the resulting observations and comments were necessarily restricted by this fact. The report emphasized that there was a need for major improvements in the data base for dose assessment in the United States. 1.3 Quantities and Units At the low doses likely to be received by members of the public, the effects of concern are assumed to be the small probabilities of stochastic effects such as the induction of cancer and/or severe genetic defects. In general, nonstochastic effects are presumed not to occur because the expected doses will be below the threshold for any such effects. This is true in all cases with the exception of some nonstochastic side effects in patients receiving radiation therapy. These side effects, however, are beyond the scope of this Report. The dosimetric quantities used in this Report include the absorbed dose, the dose equivalent, the effective dose equivalent, the collective effective dose equivalent, and the genetically significant dose equivalent. The absorbed dose, D, is the energy imparted to matter by ionizing radiation per unit mass of irradiated material at the place of interest. The dose equivalent, H, is a quantity used for radiation protection purposes that expresses on a common scale for all radiations, the irradiation incurred by exposed persons. It is defined as the product of the absorbed dose, D, and the quality factor, Q, which accounts for the variation in biological effectiveness of different types of radiation. For the purpose of relating exposure to risk, a convenient quantity is the effective dose equivalent, HE, which is either Hwb, the dose equivalent when the whole body is irradiated uniformly, or the weighted sum of the dose equivalents, HT, to each of the tissues (T) of the body, i.e., HE = C HTwT = H w b . By such weighting, one obtains a value of HEwhich is estimated to be proportional to the radiationinduced risk (somatic and genetic) even though the body is not uniformly irradiated. In this Report, the effective dose equivalent and the weighting factors (w,) defined by the International Commission on Radiological Protection (ICRP) (ICRP, 1977, 1978) will be used (see Glossary). Both the dose equivalent and the effective dose equivalent are expressed in sieverts (hundreds of rem). (It should be noted that

1.3 QUANTITIES

AND UNITS

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the uncertainties in risk estimates are considerable and information on both somatic and genetic risks continues to develop. Consequently, both the total magnitude of the risk and the weighting factors are subject to modification.) Among the exposures to be described in this Report are some that result in the irradiation of specific organs or tissues only (such as the lung in the case of radon), others that result in partial irradiation of the body (as in many medical procedures), and some that irradiate the entire body more or less uniformly (such as cosmic radiation). To account for these differing circumstances the effective dose equivalent is appropriate. For the irradiation of selected organs, the dose equivalent to the organ will be specified in this text and the appropriate weighting factors will be used to obtain HE.For "whole-body irradiation," the dose equivalent, Hwb,will be the same as the effective dose equivalent for that circumstance. Measurements of the dose equivalent to the whole body are usually only approximations to H,b which are, however, considered acceptable for the purposes of this Report. These approximations vary with the circumstances and, in the case of photons, are more accurate for high energies than for low energies (ICRU, 1985). For each source and source category, the number of people involved and the average effective dose equivalent to those exposed will be presented. The collective effectivedose equivalent is obtained by multiplying the average effective dose equivalent to the exposed population by the number of people exposed. This collective effective dose equivalent is then divided by the entire U.S. population (230,000,000 in 1980) to obtain the average effective dose equivalent for a member of the U.S. population. This quantity is, in some circumstances, such as occupational exposure, a highly artificial number but it is nevertheless useful for comparison purposes. It will be the quantity used in this report to describe population exposure from the various sources. For purposes of expressing the genetic risk, a convenient quantity is the genetically significant dose equivalent (GSD). This is the dose equivalent to the gonads weighted for the age and sex distribution of the irradiated population, i.e., to take into account the expected number of future children for each sex and age category (see Glossary). The GSD is expressed in sieverts (hundreds of rem). The gonadal dose equivalent is an upper limit to the GSD and when the dose equivalent is small, the gonadal dose equivalent itself may be used for the GSD. Additionally, when the exposure is to the entire U.S. population, the average gonadal dose equivalent and the GSD are equal. In some instances, the dose equivalent to some specific organ will be the quantity of interest for deriving the HE or the GSD. These organ dose equivalents are listed in the tables where appropriate.

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1. INTRODUCTION

Most of the exposures to be discussed below arise from radiation having low linear energy transfer (LET) (see Glossary); where highLET alpha-ray sources are involved, a quality factor, &, of 20 will be applied to absorbed doses to estimate the dose equivalents. For neutrons, the quality factor will be specified where appropriate. Throughout this Report, SI units will be shown first followed by the value in present conventional units in parentheses [see NCRP Report No. 82 (NCRP, 1985)l. Conventional prefixes in the SI system (pico, femto, atto, etc., see Glossary) will also be used. The terms "dose" and "exposure" will also be used throughout the text in their general sense.

1.4

The Present Report

This Report provides, from the source material in the six categories described earlier, a comprehensive data base and dose assessment for the population of the United States. These six categories are natural sources, occupational exposure, the nuclear fuel cycle, consumer products, miscellaneous environmental sources, and medical diagnosis and therapy. Each of these categories is treated in a separate Section of this Report and each Section represents a summary of a separate detailed NCRP Report (except for that on miscellaneous environmental sources). One of these Reports is NCRP Report No. 92, Public Radiation Exposure from Nuclear Power Generation in the United States ( N C R P , 1987a). The others are in the course of publication. In the Sections to follow, the present estimates of the effective dose equivalent and the gonadal dose equivalent (and/or the GSD) for each source category are described. The contributions of each source to the average effective dose equivalent and genetically significant dose to the U.S. population are discussed in the summary. Some indication of the range of uncertainties in the estimates is also given. The limitations in the accuracy of the data are substantial, and it is to be hoped that more accurate data will become available in the future. Specific suggestions for improvement are made in this document. Some of the sources turn out to be minor in significance and certainly sources producing an annual dose equivalent of less than 0.01 mSv (1 mrem) to an individual can be dismissed from further consideration (NCRP, 1987b). In applying this criterion it is, of course, necessary to examine all sources before determining which of them can subsequently be neglected. It is also possible that some individuals

1.4 THE PRESENT REPORT

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may be exposed to more than one of these sources, but in considering the problem the NCRP concluded that it is most unlikely that many persons would be exposed to more than ten such sources (NCRP, 198%). Other sources result in appreciable exposure for which efforts a t dose reduction are considered warranted. Obviously, the higher exposures justify more effort. The Report addresses methods of dose reduction in those instances. It also identifies existing gaps in our information on exposure and the research or other steps needed to obtain better information.

2.

Public Radiation Exposure from Natural Background 2.1 Introduction

Natural radiation and naturally occurring radioactive materials in the environment provide the major source of human radiation exposure. For this reason, natural radiation is frequently used as a basis of comparison for various man-made sources of ionizing radiation. In addition, numerous epidemiological studies have attempted to relate health effects to exposures from elevated natural radiation. None has provided definitive results but such epidemiological uses of the data make it highly desirable to quantify the average exposure of the population and to determine the distribution of natural exposures expected under various conditions. In 1975, the National Council on Radiation Protection and Measurements (NCRP) issued Report No. 45, Natural Background Radiation in the United States (NCRP, 1975). That Report presented a comprehensive picture of exposure to natural background radiation in the United States. The recognition more recently of the high exposures from indoor radon decay products (NCRP, 1984a) and additional data on exposures from other sources led t o an updating of the Report (NCRP, 1987~). Useful summary reports on exposures to natural radiation are available from a number of sources. The most comprehensive are those prepared by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1966, 1972, 1977, 1982). Oakley (1972) authored a report, Natural Radi~tionExposure in the United States, which dealt with external natural radiation. This was published by the Environmental Protection Agency and was used extensively in the preparation of the 1975 NCRP Report. Also, the Committee on the Biological Effects of Ionizing Radiation of the National Academy of Sciences included data on natural background radiation in its 1972 and 1980 reports (NAS-NRC, 1972,1980). In deriving the effective dose equivalent, HE, and the GSD, it is useful first to express the results in terms of the annual dose equivalent, in mSv (or mrem), in specific tissues. HE is the sum of the dose 8

2.1 INTRODUCTION

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equivalents to these tissues, each weighted by the appropriate ICRP factors. For whole body irradiation (external) HE is taken to be the same as the dose equivalent to whole body. The GSD is equal to the gonadal dose equivalent. Absorbed doses are converted to dose equivalents using quality factors (Q) of 20 for alpha radiation, a mean of 5 for neutrons from cosmic rays1 and 1 for photons, electrons and muons (NCRP, 1987~).All of the radiation doses discussed here are low and most are delivered a t low dose rates.

2.2 Sources of Exposure Exposures to natural background radiation may be classified in several ways, but in this Report they will be divided into those from external sources and those from radionuclides in the body. The dose estimates for external source irradiation used in this Report are based on measurements, while those for internal sources must be calculated from limited analyses of radionuclide concentrations in individual organs, often from autopsy specimens. 2.2.1

Naturally Occurring Radionuclides

All the exposures treated in this section, except those due to direct cosmic radiation, are produced by radiation coming from the natural radionuclides in the environment. These radionuclides are of two general classes, primordial and cosmogenic. Most primordial radionuclides are isotopes of the heavy elements of the three radioactive series headed by uranium-238, thorium-232 and uranium-235. The radiation from these are significant contributors to the average effective dose equivalent to members of the population. The major primordial radionuclides which decay directly to stable nuclides are potassium-40 and rubidium-87. The relatively constant isotopic abundance of 40Kin potassium is only 0.0118 percent, but potassium is so widespread that 40Kcontributes about one-third of both the external terrestrial and the internal whole-body dose arising from natural sources. The isotopic abundance of "Rb is considerably higher, but rubidium's abundance in the earth's crust is two orders of magnitude less than potassium, and thus its contribution to the HE from natural background is much lower than that of 40K.

' Thii is a calculated value for the known energy spectrum of cosmic riiy neutrons (NCRP,1975).

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2. RADIATION EXPOSURE FROM NATURAL BACKGROUND

Most cosmogenic radionuclides are beta, gamma or x-ray emitters with low to intermediate atomic numbers, and are produced by interactions of cosmic nucleons with target atoms in the atmosphere or in the earth. The four radionuclides of dosimetric interest are 'H,'J3e, "C, and 22Na.Three of these, 3H, l4C, and nNa, are isotopes of major elements in the body. A number of other radionuclides are of scientific interest in tracing atmospheric processes but are not of significance with respect to population doses.

2.2.2

External Radiation

External radiation comes from two sources of approximately equal magnitude, the cosmic radiation from outer space and terrestrial gamma radiation from radionuclides in the environment, mainly the earth. The external radiation field consists of energetic penetrating radiations and may be considered, as a first approximation, to irradiate the whole body uniformly. In the case of cosmic radiation, the charged particles, primarily protons from extra-terrestrial sources that are incident on the earth's atmosphere, have sufficiently high energies to generate secondary particles. These are mostly high-energy muons and electrons, commonly referred to as the ionizing component, and a smaller number of neutrons. These neutrons are strongly absorbed in the atmosphere, so they are not important a t sea level, but are significant at higher altitudes. There is some shielding by housing and, to account for this, indoor exposures from cosmic radiation are assumed in this Report to be 80 percent of outdoor exposures. The three contributors to the terrestrial gamma radiation field are 40 K, and the members of the thorium and the uranium series. Most of the gamma radiation comes from the top 20 cm of soil, with a small contribution from airborne radon decay products. The absorbed dose in air is converted to dose equivalent in the whole body using a factor of 0.7 to allow for self-shielding (UNSCEAR, 1982). It appears that the amount of indoor exposure from radionuclides in the environment is close to that outdoors, due to a balance between shielding by housing materials and the geometry of exposure from radionuclides in the walls when the individual is within a structure (NCRP, 1987~).

2.2.3 Radionuclides in the Body For the case of irradiation by radionuclides within the body, where measurement of dose equivalent is difficult, it is necessary to calculate

2.2 SOURCES OF EXPOSURE

/

11

the annual dose equivalents. For all cases except radon decay products in the lung, this calculation is based on measured concentrations of radionuclides, e.g., ?K, in the specific organs of interest. For radon decay products, it is necessary to use atmospheric characteristics and a lung deposition model to convert concentrations in air to tissue doses. The primordial radionuclides considered include the series isotopes and 232Thseries, plus *K, of U, Th, Ra, Rn, Po, Bi, and Pb of the 238U "Rb, and the only cosmogenic nuclide of importance, 14C.These enter the body by ingestion of food, milk, and water or by inhalation. The isotopes follow the normal chemical metabolism of the element and the long-lived radionuclides are usually maintained a t an equilibrium concentration or increase slowly with age. The shorter-lived radionuclides disappear by decay, but concentrations in the body are continually renewed by fresh intake.

2.3 Exposure Estimaters 2.3.1

Cosmic Radiation

The average cosmic-ray annual dose equivalent is about 0.26 mSv (26 mrem) at sea level. This essentially doubles with each 2,000 m increase in altitude in the lower atmosphere. Latitude, solar cycle variations and other factors modify these exposures by about ten percent. In the United States, cities such as Denver (at 1,600 meters) have an annual external exposure from cosmic radiation of 0.5 mSv (50 mrem) (NCRP, 1987~). The earlier NCRP Report on natural background (NCRP, 1975) noted that air travel at an altitude of 12 km (39,000 feet) gave an enhanced cosmic-ray exposure of 5 p9v/h (0.5 mrem/h). A more recent study (NAS-NRC, 1986), which considered additional sources of information, came to a similar conclusion. Air travel statistics (U.S. Bureau of the Census, 1986) indicate that the average time in the air is about 1.5 hours per trip and that there were 340 million revenue passengers in 1984. This would give an annual collective dose equivalent of about 2,500 person-Sv (250,000 person-rem) or an average annual individual effective dose equivalent of about 0.1 mSv (1 mrem).

2.3.2 Terrestrial Gamma Radiation The average annual gamma-ray effective dose equivalent is about 0.28 mSv (28 mrem) derived from airborne radiation measurements.

12

/

2. RADIATION EXPOSURE FROM NATURAL BACKGROUND

This varies geographically. The annual effective dose equivalent (to the whole body) on the Atlantic and Gulf coastal plain is 0.16 mSv (16 mrem), for a region on the eastern slopes of the Rockies i t is 0.63 mSv (63 mrem), and for the remainder of the country it is about 0.30 mSv (30 mrem). A ground-based survey of four countries, including the United States, showed a normal distribution with 95 percent of the measurements within 50 percent of the mean value and thus the variability seems small. There are a few areas known to have elevated exposures but even aerial surveys do not have broad enough coverage to define the extent of such exposures.

This source gives a n annual effective dose equivalent of about 0.01 mSv (1 mrem) to the whole body, mostly from 14C in tissues. This exposure is uniform on a global basis, since the source is atmospheric carbon dioxide, which is uniformly distributed a t ground level. 2.3.4

Inhaled Radionuclldes

The annual dose equivalents for various inhaled radionuclides are listed in Table 2.1 The p,rimary tissue of concern involving these radionuclides is the bronchial epithelium in the upper airways which is the site of most lung cancers whether or not the lung cancer was induced by radiation (NCRP, 1984b). The major contributors are the short-lived decay products of radon. TABLE 2.1-Annual dose eouivalents to hna tissue from inholed radionuclides Radionuclides

Assumed air concentration

Average annual dose eauivalent (rsv)' Whole lung

w-22BRa "'Rn P1ap0-214p0 210Po ZS2Th-rURa mRn z12pb-Z1Zpo

700 n13q/m3 40 Bq/m3 8 x lo-' J/m3 70 pBq/m3 400 nBq/m3 200 mBq/m3 70 mBq/m3

Bronchial eDithelium

0.1 200 24,000 8 0.2 0.1 400

Note: The conversion coefficients between air concentrations and annual dose equivalents are given in NCRP, 1987c. '1 pSv = 0.1 mrem. 1 J/m3 = 0.5 X lo6 WL (see Glossary).

2.3 EXPOSURE ESTIMATES

/

13

Surveys of homes show an apparent log-normal distribution of concentrations of these decay products in indoor air. The distribution cannot be well defined with the present data but the NCRP (NCRP, 1984a) estimated that more than one percent of the population would be exposed to concentrations greater than five times the average and smaller numbers of people to higher concentrations. Note that inhalation exposures due to a natural radionuclide (21"Po) in tobacco products are discussed in Section 5.

2.3.5 Radionuelidesinthe Body Except for 40K, there are very few data to assess the dose equivalent from these radionuclides, and the annual dose equivalents given in Table 2.2 provide no information on the distribution of exposures. The doses are dominated by 40Kand 'loPo and, while the former is under homeostatic control and is directly proportional to lean body mass, the latter is a function of intake and may vary widely.

2.3.6 Total Exposure from Natural Background The exposures resulting from the various sources described in this Section are summarized in Table 2.3. The dominant annual dose equivalent is that to the bronchial epithelium from the decay products TABLE 2.2-Average annual dose equiualents to various tissues from natural mdionuelides contained in the body (pSu)" Radionuclide

"C 'OK "Rb W-='U ='lh %.a

'9, 210pb-2"Jpo 532Th 228Ra-mRa mRn Rounded total

Soft tissue

10 180 10 5 0.1 3 7 140 0.1 2 1 360

Bone surfaces

8 140 14 3 6 90 14 700 2 120 1,100

Bone marrow

30 270 7 0.4 1 15 14 140 0.4 22 -

500

Note: 1. Individual organs may sometimes receive an annual dose equivalent from individual radionuclides up to twice those shown for "Soft Tissues." 2. These dose equivalents include those estimated in Table 2.1. " 1 pSv = 0.1 mrem.

14

/

2. RADIATION EXPOSURE FROM NATURAL BACKGROUND

TABLE 2.3-Estimated average annual dose equivalents to various tissuea for a member of the population in the United States from vario1t.s sources of natuml background radiation (u.Svlm Bone Other soft Bone Bronchial Source

evithelium

timum

Cosmic Coamogenic

270 10

270 10

Terrestrial Inhaledb h the body'

280

280

24,000

-

360 26.000

360

Rounded total

900

surfaces

marow

270 10 280

270 30 280

1,100 1,700

600 1,100

-

-

'1 pSv = 0.1 mrem. D m s to other tissues from inhaled radionuclides are included under "In the Body." 'This includes all radionuclides in the body (seeTable 2.2) excluding the cosmogenic component shown separately in this Table.

of radon. The differences in the dose equivalent rate reported here and those in the earlier NCRP Report (NCRP, 1975)are quite marked. The major change is in the annual dose estimate to the bronchial epithelium from inhaled radon decay products, which increased from 4.5 to 24 mSv (450 to 2,400 mrem). The increases in the estimated dose equivalent from internal emitters were due to the higher quality factor for a radiation, to data showing higher concentrations of the 210Pb-210Po pair in bone, to a higher estimate for the tissue dose from radon decaying in the body, and to higher radon levels indoors as compared to outdoors. The dose equivalent values for cosmic radiation, cosmogenic radionuclides and terrestrial gamma radiation are very little changed from the previous estimates. The annual dose equivalents have been converted to effective dose equivalent using the weighting factors (WT)of the ICRP (ICRP, 1977, 1981). The individual contributions are shown (in PSV)in Table 2.4, and their sum is a total of HEof 3.0 mSv (300 mrem). This estimate of average effective dose equivalent is considered to apply to both sexes and all ages. In the case of irradiation of the entire public (such as by natural background), the GSD is equal to the gonadal dose equivalent. The value of the gonadal dose equivalent is the same as that for other soft tissues shown in Table 2.3, viz 0.9 mSv/y (90 mrem/y). We assume that the average effective dose equivalents given in Table 2.4 apply to all members of the U.S. population (230,000,000) and therefore the collective effective dose equivalent from natural sources is 69 X lo4person-Sv (69 million person-rem).

2.3 EXPOSURE ESTIMATES

/

15

TABLE 2.4-Estimated total werage a n d effectwe dose equivalents to various tissues for a member of the population in the United States from varwua sources of natural backround radiation (LSD)' Bone Bone Other HE Source surfaces

Lung

Cosmicb Cosmogenic Terrestrial Inhaled' In the body Rounded total

30 1 30 ~.'JOO 40 2.100

70 2 70 90 230

8 10 30 50

marrow

tissues

130

30 4 30

140

60 120

170 440

-

3

-

270 10 280 2,000 390d -3.000

-

'1pSv = (1.1mrem.

Includes 1x lod Sv population dose from air travel. 'Derived from calculations of ICRP Publication 32 (ICRP. 1981). The ICRP cites a w~ of 0.12 for whole lung and 0.06 each for bronchial epithelium and pulmonary tissue, respectively. Radon and its daughters irradiate the tracheobronchial region mainly. In ICRP Publication 32, the ICRP uses the James-Birchall model to establish dose equivalents to the bronchial basal cell layer in the range 0.06 to 0.18 Sv per WLM (Table 2) for an average of 0.12 Sv/WLM. This, combined with the ICRPconversionfactor (derived in equation 16)of 0.01 Sv/WLM, yields an effective weighting factor of 0.08. It is clear that weighting factors for use in these circumstances are debatable, as indeed are the conversion factors for effective dose equivalent (in sievert) from the a particle concentration (measured in WLM). Uncertainties of *50% derive from this source alone. dThis is an approximation derived by assuming that the rest of the organs (w = 0.48) had the same annual dose equivalent as soft tissue, namely 360 pSv, thus adding 170 ~ S for V a total of 390 pSv.

2.4 Recommendations The components of natural background include external cosmic and terrestrial radiation, radionuclides &I the body, and inhaled radon and its decay products. External cosmic radiation varies to some degree with altitude but is otherwise essentially constant over the U.S. External terrestrial radiation varies little over the surface of the U.S. in normal (undisturbed) circumstances. Radionuclides in the body, are essentially constant. None of these is amenable to mainly 40K, dose reduction in any obvious and simple way. Radon as a source is not only the largest component of natural background, but also the most variable. It may be responsible for a substantial number of lung cancer deaths annually (NCRP, 1984b), but its actual concentration indoors is still not well known in all parts of the country. Consequently, NCRP Reports No. 77 and No. 78 (NCRP, 1984a, 198413) recommended actions relative to the control of indoor radon sources. This Report draws attention to these actions

16

1

2. RADIATION EXPOSURE

FROM NATURAL BACKGROUND

which included conducting a nationwide survey of radon levels, the recommendation of remedial action levels at concentrations above about 400 mBq/m3 (actually specified was 2 WLM/y) (NCRP, 1984a), and the introduction of mitigation techniques to reduce radon levels indoors (NCRP, 1987g). The radon problem is now receiving significant attention nationally and the Environmental Protection Agency has set guidance at about 150 mBq/mS (actually specified was 4 pCi/ 1) (EPA, 1987). We can hope that as a result of these activities, the true extent of the problem will become better known and the higher indoor radon levels reduced.

3. Occupational Exposure 3.1 Introduction It has long been appreciated that the health risks of ionizing radiation are most likely to be faced by those who use ionizing radiation and/or radioactive materials in their work. Consequently, recommendations and standards to control exposure were first applied to radiation workers (by the NCRP and the ICRP in 1934, for example) and have constantly been re-examined and revised, as the need arose, to provide adequate worker protection. The principal areas which involve radiation workers have varied over the years but now include medical radiology, industrial applications such as radiography, nuclear power, and some research activities. The exposure of workers contributes to the collective effective dose equivalent of the entire population and hence to the somatic and genetic detriment in the population. Thus, the level of exposure of radiation workers, the number exposed and the average exposure have been a matter of considerable general interest. Recently, trends in these levels in given occupations have also been studied (EPA, 1984a). In this Section, the doses presented will be those resulting exclusively from occupational exposures to ionizing radiation. Many accounts of the exposure of selected worker populations have been published over the years, and such reports are cited and discussed in a forthcoming NCRP Report on the subject (NCRP, 1987d). Fewer comprehensive accounts of the exposure of workers in all occupational categories have been published, however. The Federal Radiation Council in 1960, in preparing guidance for occupational exposure, developed the first estimates of the number of radiation workers in the U.S. and their doses (FRC, 1960). The principal occupations considered at that time related to medical applications of x rays, industrial radiography and the activities of the Atomic Energy Commission (AEC). In AEC facilities, the average exposure of the 66,000 radiation workers for the year 1958 was of the order of 2 to 4 mSv (200 to 400 mrem). For the 250,000 medical workers and industrial radiographers, exposures were in the range of 5 to 50 mSv (500 to 5,000 mrem). (A specific year was not stated but presumably these exposures were for the mid-to-late 1950s.) In a 1972 report by the EPA (EPA, 1972), 771,800 workers were identified with a mean annual exposure of 2.1 mSv (210 mrem) for the

18

/

3. OCCUPATIONAL EXPOSURE

year 1970, and the contribution to the average dose equivalent to a member of the U.S. population was estimated to be 8 PSV(0.8 mrem) for that year. The report included workers in the medical and dental field, the armed services, other federal workers, AEC licensees and licensees in agreement states. In 1980, the EPA (EPA, 1980) published a follow-up report on occupational exposures in the U.S. in the year 1975. This report stated that about 1.1 million workers were potentially exposed to ionizing radiation of which 370,000 received measurable doses. The average annual "dose" to all workers was 1.2 mSv (120 mrem) and their annual collective "dosen was 1,300 person-Sv (130,000 person-rem). A comprehensive report was prepared recently by the EPA (EPA, 1984). This report finds 1.32 million people in the U.S. in 1980 engaged in radiation work with an average dose equivalent of 1.1 mSv (110 mrem). Of these persons, only about one half had measurable exposures and for these the average dose equivalent was 2.3 mSv (230 mrem). The collective dose equivalent was 1,500 person-Sv (150,000 person-rem) and the occupational contribution to the average dose equivalent to a member of the 1980 U.S. population of 230 million people was thus about 7 &v (0.7 mrem).

3.2 Sources of Data and Estimates The NCRP Report on occupational exposure is intended to be comprehensive (NCRP, 1987d). Not only does it take into account the data in the EPA report, but it also considers individual studies of selected occupational groups that supplement and elaborate on those data. The EPA report is not specific -about how workers are defined. In addition to the nominal 1.32 million workers referred to as "all U.S. workers," the EPA report discusses some additional workers, such as uranium and other miners and personnel in aircraft, totalling 115,000 persons who were exposed on the average to about 1.7 mSv (170 mrem) in 1980. Yet another group, 155,000people who were not workers but visitors to facilities and potentially exposed in occupational circumstances, were exposed on the average to about 0.3 mSv (30 mrem) in 1980. Another source of exposure among miners noted in the EPA report is due to radon decay products. The miners totalled 18,000 persons with a mean annual exposure of 0.4 WLM (see Glossary) or about 4 mSv (400 mrem) annual effective dose equivalent (ICRP, 1981). Thus, the total number of persons exposed occupationally, as identified by the EPA in 1980, was actually 1.59 million, the total collective dose was about 1,740 person-Sv (174,000 person-rem), and

3.2

/

SOURCES OF DATA AND ESTIMATES

19

TABLE 3.1-Exposures of radiation workers to ~OWTLET radiation for year I980 Occupational category

Number of workers (thousands) All

Medicine Industry Nuclear fuel cycle Government Miscellaneous Other workers Others (e.g., visitors) Rounded subtotal Additional Industrial' Uranium minin$ Well loggers DOE contractors USPHS Rounded total

Exposed

Average annual effective dose equivalent (mSv). All

Exposed

Collective effective dose equivalent

[per~on-sv)~

584 305 151 204 76 1b5 155

277 156 91 105 31 107 42

0.7 1.2 3.6 0.6 0.7 1.7 0.25

1.5 2.4 6.0 1.2 1.6 1.8 0.9

410 380 540 120

1,590

810

1.1

2.1

1,700

1.56 3.50 0.07

5.2 1.15 4.2 1.8 0.47

1.24

2.2

6.9 8.7 4.6 1,610+

2.1 10 7.3 81 0.7 911

60 200 40

11' 12' 3V l6oh 0.3b 2,000

" 1mSv = 100 mrem. 1 person-Sv = 100 person-rem. Ten states only. External effective dose equivalent based on sample of 47 open pit miners. Population exposed based on underground mining population. " 1970-75. '1975-76. '1979. 1983.

the average effective dose equivalent for those actually exposed was 2.1 mSv (210 mrem). In this Section, the measurements of the occupational exposure of individuals will be taken to represent the dose equivalent to the whole body of those individuals. The effective dose equ~valent,HE,will be taken to be the same as the dose equivalent. Inaccuracies resulting from the use of these assumptions will be discussed later. The main categories of exposure to low-LET radiation are listed in Table 3.1, which separately identifies those nominally exposed (all) and those actually exposed. The Table includes not only the EPA data but additional data identified in the NCRP Report on occupational exposure (NCRP, 1987d). There may be some small unavoidable overlap in these summations which only a separately undertaken al.djr could be expected to resolve. These data yield an overall total (see Table 3.1) of 1.61 x 10' persons nominally exposed, an average effective dose equivalent to these persons of 1.24 mSv (124 mrem), a

20

/

3. OCCUPATIONAL EXPOSURE

collective dose equivalent of 2.0 x 10"erson-Sv (200 x lo3 personrem), and an annual effective dose equivalent per exposed worker of 2.2 mSv (220 mrem). As indicated in Table 3.2, workers exposed to high-LET radiation include miners exposed to airborne radon decay products, certain workers employed by U.S. Department of Energy contractors, and other workers exposed to neutrons. In the nuclear power industry, the dose equivalents from occupational exposure to high-LET radiations are probably not more than about 10 percent of those from gammaray exposures. The total number of workers involved in all of these occupations is 60,000, the annual collective dose equivalent is 140 person-Sv (14,000 person-rem) and the annual effective dose equivalent for all these workers is 2.3 mSv (230 mrem). Presumably, the Q used for neutrons in these situations was 10, since the data are for 1984 or earlier. For exposures to both low and high-LET radiation, a total of about 1.7 x lo6 persons are nominally exposed occupationally and of these about 930,000 actually receive measurable doses. (There could be some further overlap because some of the workers exposed to high-LET radiation also have low-LET exposures.) The total collective effective dose equivalent is about 2,100 person-Sv (210,000 person-rem) and the average annual effective dose equivalent for those exposed for the year 1980 is about 2.3 mSv (230 mrem).

TABLE 3.2-Exposures of radiation workers to high-LETradiation for 1980 Number of workers (thousands)

Type of radiation and occupational category

All

Exposed

Average annual effective dose equivalent (mSvP All

Exposed

Collective effective dose equivalent (person-SV)~

a Particles

Underground mining'

18

Neutrons (using Q = 10, presumably) DOE contractorsd Nuclear powef Nawl

25

Rounded total

-

10

1.1

1.5

12

60+

23+

'1 mSv = 100 mrem. 1 person-Sv = 100 person-rem. 1977,1980. 1979. " 1984.

2.3

-5.0

140

3.3 SPECIAL CONSIDERATIONS

/

21

3.3 Special Considerations The NCRP Report on occupational exposures (NCRP, 1987d), has the aim of determining to what extent reasonably reliable values of average dose for each occupational category can be established at the present time. The Report notes the problems inherent in data of these types, especially those concerned with dosimeter location, dosimeter accuracy and the relationship of the dosimeter reading to the effective dose equivalent to the person. However, while it is concluded that personnel monitoring can be used for dose determination and risk assessment, a t the present time, occupational exposure values can only be considered approximate. Some improvements in personnel dosimetry are proposed in the Report which, if undertaken, will eventually improve the quality of occupational data. The Report notes the value of using the collective dose equivalent ratio (i.e., the ratio of annual collective dose equivalent resulting from annual dose equivalents above 15 mSv (1.5 rem) to the total annual collective dose equivalent) as an indicator of the probable spread in the distribution of dose equivalents to individuals in different groups. It also notes that dose equivalents to some individuals can be quite high even when averages are low. However, efforts at reducing the exposure to the more highly exposed personnel both in nuclear power plants (INPO, 1986) and in DOE facilities have resulted with time in the virtual elimination of annual dose equivalents above 50 mSv (5 rem) and a decrease in the number of those above 10 mSv (1 rem). The EPA report and the NCRP evaluation of those data clearly show average occupational dose equivalent values declining by a factor of about two over the period 1960 to 1985. The systematic errors in personnel dosimetry include a t least the error associated with the dosimeter measurements, the location of the dosimeter on the body, and the direction, nature and energy of the radiations involved. These are all intrinsically amenable to evaluation, but in most protection services, approximations which tend to overestimate the true values are accepted Consequently, these systematic uncertainties usually far outweigh the statistical errors for both internal and external exposures. For that reason, uncertainties in the numbers are difficult to assess. As noted, the uncertainties depend on the energy of the radiation because the measured dose on the surface of the body will be a better approximation to the effective dose equivalent to the individual a t high gamma ray energies than at low energies (ICRU, 1985). When the dosimeter is calibrated for a specified depth in the body, this energy dependence is reduced. At higher photon energies (>0.5 MeV), the systematic errors in the estimate of effective dose equivalents probably should not exceed a

22

/

3. OCCUPATIONAL EXPOSURE

factor of 2 or 3 overall (provided that the personal dosimeter is placed on the same side of the body as that most frequently struck by the incident radiation). Furthermore, these e m r s will usually be on the side of overestimation of the effective dose equivalent. Comparisons with Canadian data (Fujimoto et al., 1984) indicate average occupational exposures similar to those in the U.S. Exposures are somewhat better documented in Canada and the existence of a central registry makes them more accessible. The highest -effectivedose equivalents are recorded by underground uranium miners followed by certain workers in the nuclear fuel cycle (for example, commercial nuclear power plant operators and maintenance workers), industrial radiographers and well-loggers. Workers in hospitals and in government contractor facilities have the lowest exposures of the larger groups. Relatively few individuals are exposed to high doses and consequently the distribution of doses among worker groups is often approximately log normal. 3.4

Discussion

Workers in radiation-related occupations are increasing in number but the time trends continue to show decreasing average values of dose equivalent. With the average dose for those exposed being about 2.3 mSv (230 mrem) for the year 1980, a figure comparable with exposures from natural background and other sources, progress in the control of occupational exposures can be considered satisfactory. Nevertheless, efforts must continue to be made to reduce doses to the more highly exposed individuals and indeed that continues to be an aim of the NCRP. In the radiation occupations, the application of radiation protection principles, including ALARA (see Glossary), and the presence of knowledgeable radiation protection personnel tend to ensure good practice and decreasing exposures. More care in measurement specification and recording, and development of a better understanding of the relationship between dosimeter reading and dose to the individual, would improve our ability to interpret dose information, and thus, to use the information for risk assessment. The contribution of the average effective dose equivalent from occupational exposure to the total average effective dose equivalent for the U.S. population is estimated to be less than 10 PSV (1 mrem) [actually -9 PSV (0.9 mrem)] annually. This somewhat artificial distribution of occupational exposure across the whole population constitutes only a small source of detriment to society as a whole. The GSD will be less than the average effective dose equivalent, perhaps of the order of 6 pSv (0.6 mrem). However, occupational exposures are unquestionably important to the individuals involved and to the fields of endeaver in which these exposures occur.

4. Public Radiation Exposure from Nuclear Power Generation 4.1 Introduction

Operation of nuclear power plants results in the irradiation of some members of the public from releases of radionuclides to the atmosphere and to bodies of water, and from direct gamma radiation emitted from those facilities. Various regulations promulgated by the Environmental Protection Agency and the Nuclear Regulatory Commission require that releases be controlled to very low levels. The air- and water-borne radionuclides deliver radiation doses by various pathways such as external exposure, intake from breathing or drinking, and intake of foods contaminated by these radionuclides. The most exposed persons are usually those in the immediate vicinity of a facility, and in the following discussion, the dose to the regional population within 80 km (50 miles) will be specified. The entire fuel cycle must be considered in evaluating the radiation dose due to nuclear power production. In the United States, the nuclear fuel cycle consists of uranium mining, uranium milling, uranium hexduoride production, uranium-235 enrichment, uranium oxide fuel fabrication, power production, fuel reprocessing (currently limited to military operations), and low- and high-level radioactive waste management. Some radiation exposure to the public occurs at facilities for each of these fuel cycle components, and also during transportation of radioactive materials between the facilities. In the United States, the fuel cycle is focused almost entirely on the operation of light-water moderated reactors, specifically Boiling Water Reactors (BWRs) and Pressurized Water Reactors (PWRs). The fuel cycle for power generation in the United States is not complete, however; there is no commercial fuel reprocessing, and no permanent high-level waste disposal (spent fuel-elements are now stored in special facilities at nuclear power plant sites). Uranium is also mined, milled, and enriched for the purposes of the military nuclear fuel cycle. Fuel reprocessing and high-level waste storage facilities exist to serve the purposes of the military fuel cycle in the United States. 23

24

/

4. EXPOSURE FROM NUCLEAR POWER GENERATION

Low-level radioactive wastes from the nuclear industry and other sources (e.g., medical applications) are shipped to designated sites for shallow land burial.

4.2 Sources of Data Non-occupational radiation exposures from the nuclear fuel cycle are calculated for the most exposed persons and for the population within 80 km (50 miles) of a facility. Beyond this distance, exposures are too small to warrant consideration. Data to calculate public exposures are obtained by facility operators in response to requirements by the regulatory agencies-the U.S. Nuclear Regulatory Commission (NRC)and the U.S. Environmental Protection Agency (EPA)-or by the U.S. Department of Energy (DOE),which is responsible for some of these facilities. A typical approach is to monitor or estimate annual radionuclide releases to air and to water, consider the various pathways by which persons may be exposed, and calculate the doses using selected computational models. Environmental radiological monitoring is performed, among other reasons, to assure that regulatory limits are not exceeded.

4.3 Special Considerations

In this Section, the effective dose equivalent, HE,is determined from the dose equivalent rates to various body organs resulting from the release of radionuclides to air, water and via the food chain (NCRP, 1987a). These permit comparison of maximum exposures and the addition of collective doses, but it must be realized that the dose values were not derived by uniform methods for each phase of the fuel cycle. Differences exist in the amount of effort expended in determining radionuclide release rates and radiation exposure rates, in the calculational models used to estimate individual and collective effective dose equivalents, and in computation at the point of exposure of doses from calculated radionuclide concentrations. As mentioned previously, collective effective dose equivalents from airborne (and waterborne) radionuclides were calculated only to the population within 80 km (50 miles) of a facility, and only during the life of the persons currently exposed.

4.4 ESTIMATES AND DISCUSSION

4.4

/

25

Estimates and Discussion

The Tables in this Section summarize the annual radiation dose estimates presented in the NCRP Report on public exposures from nuclear power generation (NCRP, 1987a). Sources of information were NRC, EPA, and DOE compilations and selections of available effluent and dose reports by facility operators. In some instances, a range of values was given and a typical value selected. In other instances, a model facility was assumed, based on the range of operating characteristics and a model transportation network. Table 4.1 indicates the annual doses to maximally exposed persons due to airborne effluents; doses due to liquid effluents were generally low. Subsequent Tables (4.2 and 4.3) give annual collective doses and the annual collective dose per nuclear power plant (normalized to one gigawatt electric (1 GWe) power production operating during 80 percent of the year). The essence of these data is that both maximum and collective effective dose equivalents from the nuclear fuel cycle are relatively low. Among the highest annual effective dose equivalents to the maximally exposed individual are (1)values up to about 2.6 mSv (260 mrem) for milling but much smaller values apply to some mills, (2) TAELE4.1-Summary of average annual effective dose equivalents to the mawinally exposed individual member of the public due to airborne radioactive effluents from fuel cycle facilities Average annual Facility effective dose Basis of estimate equivalent Mining Open pit Underground Milling Conversion Wet Dry Enrichment Fabrication Nuclear power plants PWR BWR Low-level waste storage Transportation

" 1 mSv = 100 mrem.

0.26 0.61 0.004-2.6

Model mine Model mine 8 Typical mills

0.008 0.032