1. International Atomic Energy Agency ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKE OF RADIONUCLIDES Biokinetic Models

2. International Atomic Energy Agency Biokinetic Models – Unit Objectives The objective of this unit is to provide an overview of principles for development and use of biokinetic and dosimetric models for internal dose assessment. The unit describes intake, transfer and excretion, and outlines the features of the respiratory tract and gastrointestinal tract models. At the completion of this unit, the student should understand the principles involved in development and use of biokinetic models, as well as the need for individual specific models when intakes approach relevant limits.

3. International Atomic Energy Agency Biokinetic Models - Unit Outline l Introduction l Inhalation l Ingestion l Entry through Wounds and Skin l Systemic Activity l Excretion

4. International Atomic Energy Agency Introduction

5. International Atomic Energy Agency Metabolic vs. Dosimetric models l Modeling - Mathematical descriptions used to describe the processes involved in physical movement of radionuclides in the body following intake, and the deposition of energy that constitutes exposure l Biokinetic modeling includes two types of models u Metabolic models u Dosimetric models

6. International Atomic Energy Agency Metabolic models l Describe deposition and movement of radioactive material through the body l Depend on the intake mode, element, chemical form and physical form, and particle size (inhalation) l Tissues (including fluids) and organs, termed “Compartments” l Transfer routes l Transfer rates, l Excretion routes AB Intake C Urine Faeces a b

7. International Atomic Energy Agency Dosimetric models l Address the micro and macro distribution of the radionuclide within the tissues or organs where significant deposition may occur l Take into account the radiosensitivity of the deposition site tissues or organs - w T l Include consideration of w R, especially for alpha emitting radionuclides l Depend on the decay properties of the radionuclide - particle type and energy l Address contribution to other target organs

8. International Atomic Energy Agency ICRP recommendations on biokinetics l ICRP Recommendations on: u Assessing radionuclide intake, and u Resulting doses, u From monitoring data. l For occupationally workers, a suite of models to represent radionuclide behaviour after entry by: u Inhalation or u Ingestion

9. International Atomic Energy Agency Extrinsic removal Inhalation Exhalation Ingestion Respiratory tract model Lymph nodes Liver Gastro- intestinal tract model Kidney Urinary bladder Urine Faeces Other organs Transfer compartment Subcutaneous tissue Skin Sweat Wound Direct absorption Routes of intake, transfers and excretion

10. International Atomic Energy Agency Other routes of intake l For other routes of exposure, intakes are only likely to occur as a result of accidents l Almost no internationally accepted models for: u Entry through intact skin or u Wounds l Exception - 3 H 2 O u Readily absorbed through intact skin. u Assumed to give additional tritium intake u Equal to 50% of the inhaled tritium

11. International Atomic Energy Agency Tissue weighting factors, w T l w T introduced to calculate committed effective dose equivalent from individual tissue dose equivalents l Provided a common way of expressing external and internal doses l ICRP used w T in biokinetic models for dose equivalents to organs and tissues from: u Inhalation and u Ingestion l Earlier models didn’t fully decribe biokinetics

12. International Atomic Energy Agency ICRP defined tissue weighting factors

13. International Atomic Energy Agency Inhalation Respiratory tract model Gastrointestinal tract model Ingestion Faecal excretion Transfer compartment Tissue compartment 1 Tissue compartment 2 Tissue compartment 3 Tissue compartment i a1a1 a2a2 a3a3 aiai Excretion Urinary bladder Urinary excretion Systemic faecal excretion Gastrointestinal tract model fufu f General model for radionuclides kinetics

14. International Atomic Energy Agency ICRP Biokinetic models l ICRP biokinetic models are to be used in normal situations, e.g. doses from routine monitoring measurements. l Evaluation of accident doses needs specific information: u Time and pattern of intake, u Physicochemical form of the radionuclides, u Individual characteristics (e.g. body mass).

15. International Atomic Energy Agency Individual specific data Individual specific data may be obtained through special monitoring, i.e. repeated direct measurements of: l Whole body, l Specific sites and/or l Excretion measurements

16. International Atomic Energy Agency Inhalation

17. International Atomic Energy Agency Definitions l Aerodynamic diameter The diameter of the unit density sphere that has the same terminal settling velocity in air as the particle of interest l AMAD - Activity median aerodynamic diameter 50% of the activity (aerodynamically classified) in the aerosol is associated with particles of aerodynamic diameter (d ae ) greater that the AMAD. A log-normal distribution is usually assumed

18. International Atomic Energy Agency Definitions l Thermodynamic diameter The diameter of a spherical particle that has the same diffusion coefficient in air as the particle of interest (geometric diameter) l AMTD - Activity median thermodynamic diameter 50% of the activity (thermodynamically classified) in the aerosol is associated with particles of thermodynamic diameter (d th ) greater that the AMTD

19. International Atomic Energy Agency Definitions Biological half-life, T b The time taken for a biological system, such as a tissue compartment or the whole body, to eliminate, by natural processes other than by radioactive decay, 50% of the amount of a radionuclide that has entered it Effective half-life, T e The time taken for the amount of a radionuclide deposited in a living organism to be reduced by 50% as a result of the combined action of radioactive decay and biological elimination

20. International Atomic Energy Agency Respiratory tract model Extrathoracic (ET) l ET 1, anterior nasal passage, l ET 2, posterior nasal and oral passages, the pharynx and larynx Thoracic l Bronchial (BB: trachea, and main bronchi), l Bronchiolar (bb: bronchioles) l Alveolar-interstitial (AI: the gas exchange region). Lymphatic tissue Extrathoracic Thoracic BB Bronchial ET 2 ET 1 Larynx Trachea Oral part Nasal part Main bronchi Bronchi Bronchioles bb Al Anterior nasal passage Posterior nasal passage Pharynx bb Al Alveolar duct + alveoli Respiratory bronchioles Terminal bronchioles Bronchioles Bronchiolar Alveolar - interstitial

21. International Atomic Energy Agency Masses of respiratory tract target tissues Target tissue masses have been specified for 6 age classes. Adult tissue masses are:

22. International Atomic Energy Agency Physiological parameters For Reference Man - 176 cm, 73 kg

23. International Atomic Energy Agency Physiological parameters

24. International Atomic Energy Agency What has changed? l ICRP Pub. 30 treats only average lung dose l ICRP Publication 66: u Calculates doses to specific RT tissues, and u Includes differences in radiosensitivity u RT is represented by five regions n Extrathoracic (ET) airways are divided into ET 1, and ET 2 n Thoracic regions are bronchial (BB), bronchiolar (bb) and alveolar–interstitial (AI), the gas exchange region. n Lymphatic tissue is associated with the extrathoracic and thoracic respectively (LN ET and LN TH ).

25. International Atomic Energy Agency Respiratory tract model features Deposition of inhaled particulates: l Calculated for each RT region l Both inhalation and exhalation are considered, as a function of: u Particle size, u Breathing parameters and/or u Work load, u Assumed independent of chemical form

26. International Atomic Energy Agency Respiratory tract model features Default deposition parameters: l Age dependent l Range of particle sizes: u 0.6 nm activity median thermodynamic diameter (AMTD) to u 100  m activity median aerodynamic diameter (AMAD). l For occupationally exposed individuals, based on average daily patterns of activity

27. International Atomic Energy Agency Respiratory tract model features Inhalation dose coefficients: l AMAD of 5  m - Now considered most likely for the workplace l AMAD of 1  m - Previous workplace default value l AMAD of 1  m - Default for the public

28. International Atomic Energy Agency Inhalation - Deposition model l Evaluates fractional deposition in each region l Aerosol sizes of practical interest - 0.6 nm to 100 μm l ET regions u Measured deposition efficiencies related to: n Particle size n Airflow u Scaled by anatomical dimensions

29. International Atomic Energy Agency Inhalation - Deposition model l Thoracic airways - theoretical model for gas transport and particle deposition is used l Calculates particle deposition in BB, bb, and AI regions l Quantifies effects of lung size & breathing rate l Regions treated as a series of filters l Efficiency is evaluated considering: l Aerodynamic processes (gravitational settling, inertial impaction) l Thermodynamic processes (diffusion)

30. International Atomic Energy Agency Inhalation - Deposition model l Regional deposition fractions calculated for lognormal particle size distributions l Geometric standard deviations (  g ) - a function of the median particle diameter l From 1.0 at 0.6 nm to 2.5 above ~ 1 μm l Deposition parameters are given for three reference levels of exertion for workers u Sitting u Light exercise u Heavy exercise

31. International Atomic Energy Agency Respiratory tract - Deposition

32. International Atomic Energy Agency Respiratory tract - Clearance

33. International Atomic Energy Agency Clearance from the respiratory tract Clearance from the respiratory tract is treated as two competing processes: l Particle transport (by mucociliary clearance or translocation to lymph nodes), and l Absorption to blood

34. International Atomic Energy Agency Particle transport l Treated as a function of deposition site l Independent of particle size and material l Modeled using several regional compartments with different clearance half-times, e.g. u AI region given 3 compartments, u Clearing to bb with biological half-lives of about 35, 700 and 7000 days.

35. International Atomic Energy Agency Particle transport

36. International Atomic Energy Agency Clearance Particles in initial state Particles in transformed state Body fluids Deposition s pt spsp stst

37. International Atomic Energy Agency Particle transport l Similarly, bb and BB have fast and slow clearance compartments l Clearance from the AI region also involves transfer to lymphatic tissue l For bb, BB and ET; u Compartments to represent material sequestered in tissue and transported to lymphatic tissue

38. International Atomic Energy Agency Absorption into blood l Depends on the physicochemical form of the radionuclide l Independent of deposition site - Except ET 1 (no absorption is assumed). l Changes in dissolution and absorption with time are allowed

39. International Atomic Energy Agency Absorption into blood l Material specific dissolution rates preferred l Use default absorption parameters if no specific information is available: F (fast) M (moderate) S (slow). l Broadly correspond to lung classes D (days), W (weeks) and Y (years), but lung classes referred to overall lung clearance rates

40. International Atomic Energy Agency Absorption rates l Expressed as: u Approximate biological half-lives, and u Corresponding amounts of material deposited in each region that reach body fluids l All the material deposited in ET 1 is removed by extrinsic means, such as nose blows

41. International Atomic Energy Agency Absorption rates l In other regions, most material not absorbed is cleared to the gastrointestinal tract by particle transport. l Small amounts transferred to lymph nodes are absorbed into body fluids at the same rate as in the respiratory tract.

42. International Atomic Energy Agency Absorption rates - Default values

43. International Atomic Energy Agency Absorption types

44. International Atomic Energy Agency Deposition of gases and vapours l Respiratory tract deposition is material specific l Inhaled gas molecules contact airway surfaces l Return to the air unless they dissolve in, or react with, the surface lining l Fraction of an inhaled gas or vapour deposited depends on its solubility and reactivity l Regional deposition of a gas or vapour obtained from in-vivo experimental studies

45. International Atomic Energy Agency Solubility/reactivity classes

46. International Atomic Energy Agency Deposition of gases and vapours l Guidance on the deposition and clearance of gases and vapours similar to particulates l Default SR classes and absorption types l Type F l Type V, very rapid absorption recommended for elements for which inhalation of gas or vapour form is important l Only low mass concentrations of gases and vapours is considered.

47. International Atomic Energy Agency Ingestion

48. International Atomic Energy Agency Ingestion - Gastrointestinal tract model l Current model has 4 compartments, u Stomach, u Small intestine, u Upper large intestine and u Lower large intestine l Uptake to blood in the small intestine - specified by fractional uptake (f 1 ) values

49. International Atomic Energy Agency Gastrointestinal tract model Ingestion Stomach (ST) Small intestine (SI) Upper large intestine (ULI) Lower large intestine (LLI) Faecal Excretion Body fluids ST B LLI ULI SI Transfer constant,, = 1/residence time

50. International Atomic Energy Agency New Human Alimentary Tract model planned ICRP 60 introduced specific risk estimates and w T for radiation-induced cancer of the oesophagus, stomach and colon Dose estimates needed for each region ICRP 30 model u Did not include the oral cavity or the oesophagus u Treated the colon as two regions - upper and lower large intestine.

51. International Atomic Energy Agency New Human Alimentary Tract model planned Considerable data is now available on material transit through the different regions of the gut Data have been obtained using non-invasive techniques New data includes: u Differences between solid and liquid phases u Age and sex related differences u Effect of disease conditions

52. International Atomic Energy Agency New Human Alimentary Tract model planned Data are being used: u to determine default transit rates for the defined regions of the alimentary tract u for the 6 age groups given in ICRP 56 New information: u For morphometrical and physiological parameters, and u On the location of sensitive cells and in different regions of the alimentary tract

53. International Atomic Energy Agency Structure of the new HAT Model

54. International Atomic Energy Agency Entry through Wounds and Skin

55. International Atomic Energy Agency Entry through wounds and skin l Much of the material may be retained at the wound site, however l Soluble material can be transferred to other parts of the body via blood l Insoluble material translocated slowly to regional lymphatic tissue l Gradually dissolves and eventually enters the blood

56. International Atomic Energy Agency Entry through wounds and skin l Some insoluble material can be retained at the wound site or in lymphatic tissue for life l If particulate material enters the blood, it deposits in phagocytic cells in u Liver u Spleen u Bone marrow

57. International Atomic Energy Agency Entry through wounds and skin l Insoluble materials at wound sites  wound site tissues will have most radiation exposure l May need to excise contaminated tissues l Determine the variation of contamination depth at the wound site accurately

58. International Atomic Energy Agency Entry through wounds and skin Absorbed dose at wound site and in regional lymph nodes can be assessed from l Activity of the deposited material, l Characteristics of the radionuclides l Mass of tissue irradiated and l Time since exposure

59. International Atomic Energy Agency Entry through wounds and skin l Soluble materials may translocate from the wound site to the blood l Translocation rate depends on solubility l Distribution of the soluble component similar to material entering blood from lungs or GI, however l Some exceptions for radionuclide chemical forms entering blood directly

60. International Atomic Energy Agency Entry through intact skin l Several materials can penetrate intact skin u Tritium labelled compounds, u Organic carbon compounds and u Compounds of iodine, l A fraction of these activities enter the blood l Specific models need to be developed to assess doses from such intakes, e.g. behaviour of tritiated organic compounds after direct absorption is quite different from that after inhalation or ingestion

61. International Atomic Energy Agency Entry through intact skin l Both the equivalent dose to the contaminated area and the effective dose need to be considered after of skin contamination. l ICRP biokinetic models can only be used for the calculation of the effective dose arising from the soluble component, once the systemic uptake has been determined.

62. International Atomic Energy Agency Systemic Activity

63. International Atomic Energy Agency Intake l The fraction of an intake entering the systemic circulation is referred to as the uptake l ICRP models for radionuclides in systemic circulation are used to calculate dose coefficients l Following review of data behaviour of radionuclides in the body, a number of elemental models have been revised l Revised models were also used to calculate dose coefficients for workers

64. International Atomic Energy Agency LUNG ABSORPTION TYPES AND SOURCES OF BIOKINETIC MODELS FOR SYSTEMIC ACTIVITY USED TO CALCULATE INHALATION DOSE COEFFICIENTS FOR WORKERS

65. International Atomic Energy Agency Revision of systemic models l Models for several elements have been revised, particularly to account for recycling of radionuclides between compartments l Previously, a number of radionuclides (e.g. 239 Pu) were assumed to be retained on bone surfaces - a conservative assumption l Evidence indicates a fraction of plutonium is buried as a result of bone growth and turnover

66. International Atomic Energy Agency Revision of systemic models l Another fraction is desorbed and re-enters the blood l Some may be redeposited in the skeleton and liver or be excreted l In contrast, ‘bone volume seeking’ nuclides, such as 90 Sr and 226 Ra, have been assumed to instantaneously distribute in bone volume

67. International Atomic Energy Agency Revision of systemic models l The process is actually progressive l Generic models for plutonium, other actinides, and for the alkaline earth metals have been developed to: u Allow for the known radionuclide behaviour u Account for knowledge of bone physiology l The model for alkaline earth metals has also been applied, with some modifications, to lead and to uranium.

68. International Atomic Energy Agency Revision of systemic models

69. International Atomic Energy Agency Model for Sr, Ra and U

70. International Atomic Energy Agency Model for Sr, Ra and U l Skeletal activity first deposits on bone surfaces l Return to plasma or migrates to exchangeable bone volume within a few days l Some activity leaves exchangeable bone volume is assumed to return to bone surfaces l Remainder is assigned to non-exchangeable bone volume l Gradually moves to plasma by bone resorption

71. International Atomic Energy Agency Model for Sr, Ra and U l Soft tissue is represented by 3 compartments, ST0, ST1, and ST2 l For radium and uranium, the liver is kinetically distinct from other soft tissues l Excretion of U through the kidney and exchange of activity between plasma and kidney tissues is also considered

72. International Atomic Energy Agency Model for Th, Np, Pu, Am and Cm

73. International Atomic Energy Agency Model for Th, Np, Pu, Am and Cm l Th and Pu retention and excretion data are most easily reproduced using a 2 compartment model l Liver 2 represents retention t ½ > 1 year l Liver 1 loses a portion of its activity to the GI tract over a relatively short period (1 year) l Liver 2 shows greater retention of Pu Blood Liver 1 Liver 2 GI tract contents Faeces

74. International Atomic Energy Agency Model for Th, Np, Pu, Am and Cm l Am and Cm  Liver can be treated as a uniformly mixed pool, losing activity to blood and the GI tract with a T b of 1 year l Radioactive material enters Liver 1 l Part of the material is removed to the GI tract via biliary secretion l Rest goes to blood (Am) or Liver 2 (Pu, Np) l Material leaving Liver 2 is assigned to blood Blood Liver 1 Liver 2 GI tract contents Faeces

75. International Atomic Energy Agency Model for Th, Np, Pu, Am and Cm l Kidneys are assigned two compartments, u One loses activity to urine u Another that returns activity to blood l “Urinary bladder contents” is treated as a separate pool that receives all material destined for urinary excretion Urine Uinary bladder contents Kidneys Other kidney tissue Urinary path Blood

76. International Atomic Energy Agency Radioactive progeny l A number of radionuclides decay to nuclides that are themselves radioactive l It has been assumed that the decay products would follow the biokinetics of their parents l A few exceptions made for decay products which are isotopes of noble gases or iodine l The revised biokinetic models apply separate systemic biokinetics to the parent and its decay products for intakes of radioisotopes of lead, radium, thorium and uranium.

77. International Atomic Energy Agency Excretion

78. International Atomic Energy Agency Excretion l Urinary bladder and colon are given w T values l Specific information is given on excretion pathways in the urine and faeces in revised biokinetic models for workers l GI tract model is used to assess doses from systemic activity lost into the faeces l Secretion of radionuclides from the blood into the upper large intestine is assumed l A urinary bladder model has been adapted for calculating doses to the bladder wall

79. International Atomic Energy Agency Dose coefficients l Dose coefficients - committed effective doses per unit intake - for ingestion and inhalation l The values of committed effective dose are: u For specific routes of intake u Cannot be used directly to assess doses from; n Injection into the blood or n Transfer to the blood from wounds or skin absorption

80. International Atomic Energy Agency Dose coefficients l For many radionuclides, dose coefficients are given for different lung absorption types and/or for different f 1 values l Best choice of dose coefficient should be based on physical and chemical properties of the materials in the workplace l If there is little information on intake characteristics  use the most restrictive value

81. International Atomic Energy Agency Workplace specific assessments l After accidental exposures, use individual and situation specific parameters to calculate equivalent doses and effective dose l In routine situations specific circumstances of exposure rather than using default parameters. l Respiratory tract model uses an AMAD of 5  m as a default particle size

82. International Atomic Energy Agency Workplace specific assessments l Deposition of airborne particles is subject to: u Sedimentation u Impaction u Diffusion l Deposition and inhalation dose coefficients depend on aerosol parameters, e.g. AMAD l Ingestion dose coefficients depend use of an appropriate f 1 value

83. International Atomic Energy Agency Effect of particle size on aerosol deposition l Aerosol deposition of occupational concern is highest in the AI region of the thorax l AI deposition decreases with increasing particle size l Extent of deposition in each region, as well as the chemical form inhaled, has an appreciable influence on the effective dose

84. International Atomic Energy Agency BB bb Al ET 2 0.1110100 AMAD (  m) 100 10 1 0.1 0.01 Regional deposition (%) ET 1 Influence of particle size on deposition in various regions of the respiratory tract

85. International Atomic Energy Agency Effect of particle size on aerosol deposition l Committed effective dose for Type M and S 239 Pu compounds decreases with increasing AMAD l Reflects decreasing deposition in the AI region and BB and bb with increasing AMAD l In this case, the assumption of Type M characteristics is more restrictive than Type S for the calculation of effective dose l Other aerosol characteristics have slight influence on the committed effective dose

86. International Atomic Energy Agency Adult male Light work (5.5 h) + sitting (2.5 h) Type M Type S 0.1110100 AMAD (  m) 10 -3 10 -4 10 -5 10 -6 Committed effective dose per unit intake (Sv/Bq) Influence of AMAD on the committed effective dose 239 Pu

87. International Atomic Energy Agency References FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR ORGANISATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, WORLD HEALTH ORGANIZATION, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, Safety Series No. 115, IAEA, Vienna (1996). INTERNATIONAL ATOMIC ENERGY AGENCY, Occupational Radiation Protection, Safety Guide No. RS-G-1.1, ISBN 92-0-102299-9 (1999). INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of Occupational Exposure Due to Intakes of Radionuclides, Safety Guide No. RS-G-1.2, ISBN 92-0-101999-8 (1999). INTERNATIONAL ATOMIC ENERGY AGENCY, Indirect Methods for Assessing Intakes of Radionuclides Causing Occupational Exposure, Safety Guide, Safety Reports Series No. 18, ISBN 92- 0-100600-4 (2002). INTERNATIONAL ATOMIC ENERGY AGENCY, Intercomparison and Biokinetic Model Validation of Radionuclide Intake Assessment, Results of a Co-ordinated Research Programme, 1996-1998, TECDOC 1071, IAEA, Vienna (1999). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Report of the Task Group on Reference Man, ICRP Publication 23, Pergamon Press, Oxford (1975).

88. International Atomic Energy Agency References INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part 1, Annals of the ICRP 2(3/4), Pergamon Press, Oxford (1979). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part 2, Annals of the ICRP 4(3/4), Pergamon Press, Oxford (1980). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part 3 (including addendum to Parts 1 and 2), Annals of the ICRP 6(2/3), Pergamon Press, Oxford (1981). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Individual Monitoring for Intakes of Radionuclides by Workers: Design and Interpretation, ICRP Publication 54, Annals of the ICRP 19(1-3), Pergamon Press, Oxford (1988). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 1, ICRP Publication 56, Annals of the ICRP, 20(2), Pergamon Press, Oxford (1989). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Human Respiratory Tract Model for Radiological Protection, ICRP Publication 66, Annals of the ICRP 24(1-3), Elsevier Science Ltd., Oxford (1994).

89. International Atomic Energy Agency References INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 2, Ingestion Dose Coefficients, ICRP Publication 67, Annals of the ICRP 23(3/4), Elsevier Science Ltd., Oxford (1993). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Dose Coefficients for Intakes of Radionuclides by Workers, ICRP Publication 68. Annals of the ICRP 24(4), Elsevier Science Ltd., Oxford (1994). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 3, Ingestion Dose Coefficients, ICRP Publication 69, Annals of the ICRP 25(1), Elsevier Science Ltd., Oxford (1995). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 4, Inhalation Dose Coefficients, ICRP Publication 71, Annals of the ICRP 25(34), Elsevier Science Ltd., Oxford (1995). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 5, Compilation of Ingestion and Inhalation Dose Coefficients, ICRP Publication 72, Annals of the ICRP 26 (1), Pergammon Press, Oxford (1996). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Individual Monitoring for Internal Exposure of Workers: Replacement of ICRP Publication 54, ICRP Publication 78, Annals of the ICRP 27(3-4), Pergamon Press, Oxford (1997).

90. International Atomic Energy Agency References INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Guide for the Practical Application of the ICRP Human Respiratory Tract Model, ICRP Supporting Guidance 3 (in press). NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Deposition, Retention and Dosimetry of Inhaled Radioactive Substances, NCRP Report No.125, NCRP, Bethesda (1997). NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Evaluating the Reliability of Biokinetic and Dosimetric Models and Parameters Used to Assess Individual Doses for Risk Assessment Purposes, NCRP Commentary No.15, NCRP, Bethesda (1998).