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NCRP REPORT No. 106 LIMIT FOR EXPOSURE TO "HOT PARTICLES" ON TH.E SKIN Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS Issued December 31, 1989 National Council on Radiation Protection and Measurements 7910 WOODMONT AVENUE / 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 usefulnees of the information contained in this report, or that the use of any information, method or procees 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 Civil Rights Act of 1964, Section 701 et seq. as amended 42 U.S.C. Section 2000e et seq. (Title VII) or any other statutory or common law theory governing liability. Library of Congress Cataloging-in-PublicationData National Council on Radiation Protection and Measurements. Limits for exposure to "hot particles" on the skin: recommendations of the National Council on Radiation Protection and Measurements. p. cm.-(NCRP report ; no. 106) "Issued January 15, 1990." Includes bibliographical references. ISBN 0-929600-11-8 : $12.00 (est.) 1. Ionizing radiation-Safety measures. 2. Ionizing radiationDosageStandards. 3. Beta rays-Health aspects. 4. Skin-Effect of radiation on. I. Title. 11. Series. [DNLM: 1. Environmental Exposure. 2. Radiation Injuries. 3. Skin-radiation effects. WR 100 N277Ll RA569.N353 1990 612'.01448-dc20 DNLMDLC for Library of Congress 89-71259 CIP Copyright 8 National Council on Radiation Protection and Measurements 1989 All rights resewed. 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. Preface This report has been prepared as a result of a request to the National Council on Radiation Protedion and Measurements (NCRP) from the Nuclear Regulatory Commission (NRC). In recent years, nuclear utilities have identified the potential for a limited number of employees to come in contact with microscopic particles that are radioactive. These particles have been given the generic name, independent of their source and particular chemical and radioactive content, of "hot particles." This report addresses the potential biological effects of hot particles on the skin and reviews the presently available information on the subject. This information is presently less complete than one would wish, and more information is to be expected in the near future. In the meantime, the report makes recommendations on limits of exposure from hot particles in the work place based on avoidance of severe deterministic effects. The support of the NRC for this particular facet of the NCRP program on radiation effects on the skin is gratefully acknowledged. Serving on Scientific Committee 80-1, that prepared this report were: Thomas F. Gesell, Chairman U.S. Department of Energy Idaho Falls, Idaho Members P. Donald Forbes Temple University Philadelphia, Pennsylvania William C.Roesch Richland, Washington Charles B. Meinhold Brookhaven National Laboratory Upton, New York H. Rodney Withers University of California at Los Angeles Los Angeles, California Consultants R. J. Michael Fry Oak Ridge National Laboratory Oak Ridge, Tennessee Roy E. Shore New York University Medical Center New York, New York NCRP Secretariat- William M. Beckner The Council wishes to express its gratitude to the members and consultants of the Committee for the time and effort devoted to the preparation of this report. Bethesda, Maryland October 25, 1989 Warren K.Sinclair President, NCRP Contents Preface ............................................................. 1. Introduction ................................................... 2. Scope of the Report ........................................... 3. Biological Effects of Irradiation of the Skin ............. 3.1 Introduction ................................................ 3.2 Nonstochastic Effects (Deterministic Effects) ........... 3.2.1 Acute Nonstochastic Effects ....................... 3.2.2 Late Nonstochastic Effects ......................... 3.2.3 Nonstochastic Effects Versus Dose for Large Area Irradiation ..................................... 3.2.4 Nonstochastic Effects From Hot Particles ........ 3.2.5 Review of Biological Effects of Hot Particle Irradiations .......................................... 3.2.5.1 Monkey Experiments ...................... 3.2.5.2 Human Experiment ........................ 3.2.5.3 Swine Experiments ........................ 3.2.5.4 Comparison of Studies ..................... 3.3 Stochastic Risk of Irradiation ............................. 3.4 Comparison of Nonstochastic Effects (Deterministic effects) and Stochastic Risk of Hot Particle Irradiation 4 Approach to Establishing a Practical Limit ............. 5. Interpretation of Experiments With Hot Particles Using the Approach of the Total Number of Beta Particles Emitted ............................................. 5.1 Monkey Studies ............................................ 5.2 Human Study ............................................... 5.3 Swine Studies with Microspheres ........................ 5.4 Swine Studies with Other Sources ....................... 5.5 Estimation of a Threshold ................................. 6 Observations on Humans Exposed Inadvertently in the Work Place ................................................ 7 Derivation of an Exposure Limit for a Hot Particle on the Skin ......................................................... 8. Recommendations on Radiation Exposure Limits for the Special Case of a Hot Particle on the Skin .......... Appendix A: Comparison of Point Dose with Dose Measured with an Extrapolation Chamber . . . ~i 1 CONTENTS Appendix B: Number of Beta Particles from Irradiated a96UC.Microspheres ............................. References ......................................................... The NCRP .......................................................... NCRP Publications ............................................. Index ................................................................ 29 32 36 43 53 1. Introduction Small alpha-emitting radioactive particles in the lung have been previously recognized as a radiation protection issue (NCRP, 19751.' Recently, irradiation of the skin by small beta or beta-gamma emitting particles has become of concern in the nuclear reactor industry. This concern has occurred, a t least in part, as a result of the employment of more sensitive personnel monitoring equipment than was previously available. This increased sensitivity has resulted in an increase in the number of incidents involving identification of radioactive particles on the skin. These particles are known as "hot particles," 'Yeas," or "specks." The term "hot particles" will be used in this report. They most commonly contain T o or fission products. The likely source of the particles containing *Co is particles of wear-resistant alloy from valve seats, etc., containing a high percentage of stable cobalt, that enter the primary coolant and become activated in the core via the reaction 6 9 C(n, ~ Y) '%o. The source of the particles which contain fission products is fuel elements which have defects in their cladding. Hot particles usually cannot be detected by the unaided eye because they range in size from approximately one pm to several hundreds of pm. Hot particles apparently become electrically charged as a result of radioactive decay and, therefore, tend to be fairly mobile, "hopping" from one surface to another. The radioactivity of particles containing fission products ranges from 40 Bq to 400 kBq (1nCi to 10 pCi) with most particles being in the range of 400 Bq to 40 kBq (10 nCi to 1 pCi). The radioactivity of the O ' Co particles ranges from 40 Bq to 20 MBq (1nCi to 500 pCi), with most in the range from 400 Bq to 200 kBq (10 nCi to 5 pCi) (Warnock, et al., 1987). The particles are not water soluble and if embedded in clothing are difficult to remove, even by laundering. "Clean" laundry has NCRP,1975 concluded that particulate plutonium in the lung causes no greater risk of lung cancer than the same amount of plutonium more uniformly distributed throughout the lung. However, for hot particles on the skin the effect to be protected against is the nonstochastic risk of acute ulceration of the skin (see Section 3.41, for which the concept used in NCRP, 1975 may not be applicable and therefore it is not considered further in this report. 2 1 1. INTRODUCTION been implicated a s the source of hot particles involved in some skin contamination events (INPO, 1987). A unique aspect of hot particles in contact with the skin is that very small amounts of tissue can be exposed to very large, highly non-uniform doses. Average doses can be calculated if the particle can be characterized by nuclide and by activity, but the result will depend strongly upon the amount of tissue included in the averaging process and the depth or depths a t which the averaging is performed. The interpretation of the resultant dose, when various forms of averaging are used, is not straightforward. Existing methods for assessing exposure of the skin are appropriate when large areas of skin, greater than a few tens of square centimeters, are irradiated. For skin irradiation of a few square centimeters, the existing limits provide more than adequate protection, and for very small areas of skin irradiation, such a s occurs with hot particles, the existing limits (NCRP, 1987) are overly restrictive. Minimizing the production and release of hot particles and prevention of skin contamination are clearly the preferable control methods, but the possibility of contamination events cannot be ignored. A consistent method for assessing the biological effect of these events is required so that reasonable radiation protection criteria can be applied to this unique situation. 2. Scope of the Report This report reviews the radiobiological effects of hot particles on the skin and recommends a limit on the product of their beta-particle emission rate and duration of e ~ p o s u r eFor . ~ the end points addressed by this report, beta particles are the radiation of concern. Relative to the beta particle dose, the gamma radiation associated with a beta-emitting hot particle on the skin does not contribute significantly to the tissue dose in the vicinity of the particle. This report does not deal with general skin contamination, skin exposure from distant sources, or inhalation or ingestion of hotparticles, and it does not deal with the special cases that might arise, for example, as a result of hot particles in the eyes or on the eardrums. For the purpose of this report, a hot particle is arbitrarily considered to be a discrete radioactive fragment that is insoluble in water and is no larger than approximately 1mm in any dimension. In addition, only hot particles directly on the skin are considered in this report. Beta emission rate is the rate of beta particles emitted from the radionuclide(s1 making up the hot particle and not the rate of beta particles emitted from the surface of the hot particle. 3. Biological Effects of Irradiation of the Skin 3.1 Introduction The fimdamental philosophy of radiation protection includes: (1) prevention, to the extent practicable, of the occurrence of severe nonstochastic diseases (deterministic effects), (2) limitation of stochastic risks, fatal cancer and genetic effeds, to a reasonable level in comparison with non-radiation risks, and (3)maintenance of radiation exposure a t levels as low as reasonably achievable (ALARA) economic and social factors being taken into account (NCRP, 1987). The biological effects of irradiation of the skin that are of interest are acute and chronic nonstochastic effects and the stochastic risk of non-melanoma skin cancer. 3.2 3.2.1 Nonstochastic Effects ( D e t e r m h b t i c Effects) Acute Nonstocbtic Effects Acute, nonstochastic effects on the skin (those occurring in a few hours to a few weeks) after irradiation of 1cm2 or greater are, with increasing dose, transient erythema, more prolonged erythema, dry and then moist desquamation (after a latency of three to six weeks) and finally secondary ulceration. The latency period of these effects is not strongly dependent on dose. Dry and moist desquamation are related to the reduced reproductive capacity of target cells in the basal layer of the epithelium a t a depth of about 20 to 120 pm. If moist desquamation persists, secondary ulceration may develop with loss of dermal tissue; such ulceration heals by invasive fibrosis. 3.2.2 Late Nonstochclstic Effects Late or chronic nonstochastic effects developing after protracted irradiation of large areas of skin, with increasing dose, and possibly