734 Book Reviews

SIMPEDS and BCIRA cyclones. The chapter finishes with a discussion of denuder technology, and seven questions (one of which, set in 1996, is already out-dated) and 130 references (23 of them to articles in this journal, but some subject to over- enthusiastic editing with Ann. replaced by Am.).

Throughout the book, Dr Perkins provides the tools for the professional hygienist, and demonstrates how they should be used-with follow-up questions to give the student the opportunity to test his/her learning. As an example, question 13.3: You have just been hired as the first hygienist at a 5000 employee petroleum refinery that has 32 process units. Generally, exposures are to hydrocarbons, but other contaminants, process additives, catalysts, and so forth, are used, such as methyl ethyl ketone, ammonia, nickel, and phenol. Examine literature resources on petroleum refining and hydrocarbons. Where should you place most monitoring emphasis in your first year? Do you need a .formal scheme such as the VHI for making this decision? Why? (Answers are to be found on page 810 of the book, s’o contact the U.K. agent-Chapman & Hall on 0171 865 0066but, be warned, the cost is s37.50.)

R. J. Sherwood Honey Bottom

Honey Bottom Lane Dry Sandford

Abingdon OX13 6BX, U.K.

Deposition, Retention and Dosimetry of Inhaled Radioactive Substances NCR]? Report No. 125. National Council on Radiation Protection and Measurements, Bethesda, MD, 1997.

As the opening words of its Preface states, ‘The development of a respiratory tract model which accurately reflects reality is a difficult and complicated effort’. Many factors contribute to this. For radiation protectlon purposes, the respiratory tract has to be considered as both a target organ for damage, and as a route of entry to the systemic circulation from which activity may deposit in and irradiate other organs. However, both the radiosensitivity of the tissues, and the clearance characteristics of deposited material, vary greatly between the different regions of the respiratory tract. Furthermore, the pattern of deposition of inhaled activity between the respiratory tract regions depends on the size of the inhaled particles, on the dimensions of the airways and on the airflow patterns within them. Because o:f the importance of the inhalation route of intake not only for radioactive substances, but for toxic materials in the workplace and the environment in general, deposition and clearance of inhaled particles and vapours have been, and continue to be, studied widely.

Although the subject is complex, this excellent report demonstrates that the problems are not intractable. The authors, members of an NCRP Task Group chaired by Dr Richard G. Cuddihy, should be congratulated on their effort in

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producing an up-to-date model. The report starts by describing the anatomy, morphometry and physiology of a ‘normal’ respiratory tract and moves on to discuss how these are affected by factors such as disease, tobacco smoke and other irritants. This description is developed into a formal mathematical model of clearance, which is supplemented by a deposition model to allow the complete biokinetic behaviour of aerosols and vapours to be modelled. By identifying cells at risk from radiation, a methodology similar to that of the ICRP for calculating absorbed doses is applied, and doses from a selection of inhaled radionuclides are presented.

Given that the International Commission on Radiollogical Protection (ICRP) has recently published (ICRP, 1994) its own state-of-the-art respiratory tract model, the reader may well ponder the intended use for another model. Anticipating the potential for confusion, the NCRP makes clear from thle outset that the model is not recommended as an alternative to the ICRP rnodel for calculating dose coefficients for radiological protection purposes, but rather it provides an independent scientifically developed respiratory tract model. In a sense, since the NCRP model was developed independently, it supplements the ICRP model by enhancing confidence in the results of calculating doses from inhaled radionuclides. Furthermore, since the model has a different structure, it can be applied in some situations where the ICRP model cannot. For example, in the NCRP model:

-all 16 generations of the conducting airways are treated individually. -dissolution rates are represented by time-dependent functions.

It is clearly beyond the scope of this review to give a detailed comparison and explanation of the differences in calculated doses between the two models, yet now the NCRP model has been published, such a project would make an interesting challenge. It would also highlight areas of difference and help to focus further scientific research on areas of uncertainty.

On a less ambitious scale, the rest of this review will concentrate on some of the similarities and differences between the NCRP and ICRP models themselves.

Overview

Both models have broadly similar scope, and bothL effectively replace the model of the Task Group on Lung Dynamics (TGLD, 1966) which was used in ICRP Publication 30 (ICRP, 1979), and which was developed 30 years ago. However, the remits of the task groups that developed the two new models were somewhat different: the NCRP’s ended at absorbed doses to regions of the respiratory tract, whereas the ICRP’s went further to consider different regional radiosensitivities in order to provide an equivalent dose to the lungs. This in turn influenced the development of the model and it is thus hardly surprising that two groups of experts, working to similar but not identical guidelines, and over similar, but not identical periods, would come up with models that are broadly similar, but differ in a number of details.

Both the ICRP and NCRP models: -aim to incorporate recent scientific findings, and overcome problems identified

with the Publication 30 lung model, particularly limitations in scope; -provide supporting scientific background information;

136 Book Reviews

-are designed to calculate doses to the general population, not only to healthy workers;

-are suitable for interpreting bioassay measurements as well as for prospective dose calculations;

-divide the respiratory tract into regions similar to those of the Publication 30 model, but calculate doses to each region separately;

-address the morphometry of the respiratory tract; -consider the physiology of breathing; -consider the regional deposition of particles of all sizes of practical interest,

and also gases and vapours; -consider retention in and clearance from each region; -identify cells at risk in each region; -address modifying factors that could influence individual doses, such as

smoking; -could in principle be used for non-radioactive materials. This is simply noted

in the ICRP report, but the NCRP report includes a chapter on considerations which might apply to toxic chemicals.

Deposition

In evaluating deposition, both models take account of ‘inspirability’, the reduction in intake of particles larger than a few micrometres diameter, due to inertial effects. However, although based on much the same data, somewhat different functions are used. Similarly, both base deposition in the extra-thoraci,c airways on empirical functions, which mainly rely on in vivo data for particles larger than about 0.2 pm, and measurements in hollow casts of the head airways for smaller particles. Since experimental data on thoracic (lung) deposition is mainly limited to healthy adult males, both use theoretical models to extrapolate to other subject types, and to subdivide the deposit in t;he lungs, but have taken different approaches. The deposition model used by ICRP was chosen because it was considered to be particularly well suited to scaling to children. However, it is so complex that calculations required a ‘supercomputer’. A dataset of results was generated to which algebraic functions were fitted which relate regional deposition to particle size and breathing parameters.. The NCRP uses a simpler model, enabling regional deposition under specified conditions to be calculated directly on a personal computer (PC). Moreover it gives more detail: deposition in each of sixteen airway generations of the trachea-bronchial tree (T-B), rather than in the two ICRP model regions: bronchial and bronchiolar. The results of the two deposition models have been compared recently (Yeh et al., 1996). For palrticles >0.2 pm results are similar, but for smaller particles the NCRP model gives significantly higher deposition in the T-B and correspondingly lower alveolar deposition than the ICRP model. Other factors being equal, this would lead to higher doses from radon progeny using the NCRP model (Yeh et al., 1996). Both reports recognise that for water-soluble particles hygroscopic growth could significantly alter the deposition pattern. Currently, however, neither readily enables a user to predict the regional deposition of a hygroscopic aerosol under specified conditions.

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Clearance

The two models take the same basic approach to the treatment of retention and clearance, but differ in detail. Both assume that:

-material is removed from the anterior nasal passage by extrinsic means (nose- blowing);

-in all other regions clearance results from competition between ‘mechanical’ transport of particles to the GI tract and lymph nodes, and absorption of material into the blood;

-mechanical transport rates are the same for all materials; -absorption rates are the same in all respiratory ‘tract regions; -both mechanical transport and absorption rates are time-dependent.

However, the ICRP model represents time-dependent rates by combinations of compartments that clear at constant rates, whereas the NCRP model uses rates that are functions of time. There are also some important differences in the estimated mechanical transport rates, particularly in the T-B. The ICRP model includes a slow phase of clearance and particle retention in the airway wall, but the NCRP model does not. Whether to include a slow phase of T-B clearance was a difficult issue in the development of the ICRP model. It remains unresolved and is being investigated at several laboratories. Its effect is that the mean residence time for small particles deposited in the T-B is much longer in the ICRP modlel than in the NCRP model. This can significantly affect the dose from medium and1 long-lived a-emitters (Bailey et al., 1995). As for T-B deposition, the NCRP model treats the rapid phase of mucociliary clearance in much greater detail than the ICRP model, assigning a different mucus velocity to each of the 16 generations, rather than average rates to the bronchi and bronchioles. Both models take the rate of clearance from the alveolar (pulmonary) region to the GI tract. to decrease from about 0.5% dd’ initially to 0.1% dd ’ at 6 months, but the NCRP model assumes that it remains at this level, whereas the ICRP model assumes that it con.tinues to decrease, ultimately to 0.01% d-‘. The ICRP model also has less transfer from the lungs to lymph nodes.

Both models emphasise that rates of absorption of radionuclides from the respiratory tract to the blood should be based wherever possible on experimental data, and ideally in vivo data. The NCRP report (Appendix A) gives example absorption functions for compounds of 14 elements, for which suitable data were available. Its Introduction advises that this can be revised and expanded as data become available, and that in the absence of such dalta information on the ICRP Publication 30 categories (inhalation Classes D, W and Y) should be used. However, quite how this should be done is not specified: recommended default absorption functions for the three Classes are not provided. How such functions would be derived is not obvious because there is a conceptual difference between the treatment of clearance in the Publication 30 model and that in the new NCRP (and ICRP) model. The Publication 30 Classes describe overall clearance: particle transport, as well as absorption differ for Classes D, W and Y. ICRP Publication 66 discusses absorption mechanisms, but does not recommend absorption rates for specific compounds. This has been left to the planned revision of Publication 30, although a Technical Document is in preparation that will give guidance on the derivation of

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material specific absorption parameters for use with the new ICRP model. Publication 66 does however give parameter values for its three default absorption Types: F (fast), M (moderate) and S (slow), which broadly correspond to Classes D, W and Y.

Dosimetry

Both models take the epithelial basal and/or secretory cells to be the target cells in the airways. This has necessitated extensive calculations of the fractions of CI- and b-energy absorbed (as functions of energy). For the T-B, both models represent sources and targets by co-axial cylinders. The ICRP model uses two geometries: one representative of the bronchi, the other of the bronchioles, with the dimensions of the source in each dependent on whether the activity is being cleared by muc:us (rapidly or slowly), or retained in the airway ,wall. ICRP Publication 66 provides specific absorbed fractions (SAF: the fraction of energy emitted by a source which is absorbed per unit mass of target) for each source-target combination, for a range of m-particle and electron energies. The NCRP Report provides more basic information: tables of SAFs, again for a range of a-particle and electron energies, but also for ranges of airway diameters and depths. This enables users to calculate doses to specific tissues in each airway generation, and even apply it to animals for which the appropriate dimensions might be quite different (if known). The average dose to the T-B region can be obtained either from a weighted average of the doses to each generation, or, as in the ICRP model, by taking the dose to a representative airway.

For the extra-thoracic airways, the ICRP model also uses a cylindrical geometry (representing the pharynx and larynx), whereas the NCRP model uses a pair of plane sources (representing the posterior nasal passage). Similar approaches are taken to the provision of SAFs for the extra-thoracic airways in the two reports a.s for the T-B airways.

Implementation and software

Finally, in view of the wide scope of the models and the need to calculate doses to specific tissues, rather than simply the average dose to the lungs, they are inevitably more complex than the ICRP Publication 30 model they replace, and implementation on a computer is essential for most practical purposes. The ICRP model has been implemented at several institutes and PC software is generally available for those wishing to use it themselves in specific situations (Jarvis et al., 1996). PC software has also been developed to implement the NCRP model, and a contact address is given in the NCRP Report for information on its availability. Since the extent to which the NCRP model is applied in practice will inevitably depend on the availability of user-friendly software to run it, it is hoped that such software will soon be forthcoming. In the meantime, the NCRP report provides those with an interest in the inhalation of toxic substances in general and of radionuclides in particular with a comprehensive overview of the subject of respiratory tract modelling.

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This review wasjrst published in the NRPB Radiological Protection Bulletin, and is reproduced here by kind permission of the Editor.

Michael Bailey and Alan Birchall National Radiological Protection Board

Chilton Didcot

Oxfordshire OX11 ORQ, U.K.

REFERENCES

Bailey, M. R., Dorrian, M.-D. and Birchall, A. (1995) Implications of airway retention for radiation doses from inhaled radionuclides. Journal of Aerosol Medicine 8, 373-,390.

ICRP (1979) Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part I. Annuls ofthe ICRP 2(3/4). Pergamon Press, Oxford.

ICRP (1994) Human Respiratory Tract Model for Radiological Protection, ICRP Publication 66. Annuls of fhe ICRP 24(1-3). Elsevier Science, Oxford.

Jar&, N. S., Birchall, A., James, A. C., Bailey, M. R. and Dorrian, M.-D. (1996) LUDEP 2.0. Personal computer program for calculating internal doses using the ICRP Publication 66 respiratory tract model. Chilton, NRPBSR287.

TGLD (Task Group on Lung Dynamics) (1996) Deposition and retention models for internal dosimetry of the human respiratory tract. Health Physics 12, 173-207.

Yeh, H. C., Cuddihy, R. G., Phalen, R. F. and Chang, I. Y. (1996) Comparisons of calculated respiratory tract deposition of particles based on the proposed NCRP model and the new ICRP66 model. Aerosol Science and Technology 25, 134-140.

Non-Ionizing Radiation: Proceedings, Third International Non-Ionizing Radiation Workshop, Baden, Austria, 1996. Edited by R. Matfhes. ICNIRP, 1996. 388 pp. (ISBN 3-9804789-l-2).

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) was formed in 1992 as an independent scientific organization to provide advice and guidance on exposure to all forms of non-ionising radiation (NIR) from static fields to ultraviolet radiation, and to ultrasound. This book provides a record of a workshop held near Vienna last year in which members of ICNIRP and other invited experts from around the world presented their views and opinions on the nature of the health risks posed by exposure to NIR ;and ultrasound.

Following the logical approach adopted by the workshop, the book is organised into different sections dealing with discrete parts of the spectrum, viz.: ultraviolet radiation, visible and infrared radiation, radiofrequency fields, and static and extremely low frequency electric and magnetic fields. Different regions of the spectrum interact with biological materials in different ways and so cause different biological effects. There are also sections covering ultrasound and laser radiation. Each of these sections is complete within itself and all contain a vast amount of data and information. The chapters comprising each section contain similar themes and deal with such matters as sources and measurements, dosimetry, biology and health effects, and standards and protective measures. There: is also a very useful section