Implications of the dosimetric model for the respiratory system on limits for intakes of radionuclides by workers (ICRP Publication 30).

While evaluating respiratory tract kinetic models, we have encountered a number of problems with the recently published ICRP Dosimetric Model for the Respiratory System. Some of those to be discussed are the following. 1. The assumption that the nasopharyngeal (N-P) region can be ignored, despite considerable evidence of significant retention in the N-P region and of pathological effects. 2. Treatment of the tracheobronchial tree (T-B), the pulmonary (P) region and the pulmonary lymph nodes (PLN) as one combined organ for calculating dose. 3. The lack of deposition estimates for very small particles ( <0.1 /im dia.) and the use of a probit scale for percentage deposition. 4. Use of the aerodynamic equivalent diameter of particles beyond the range of applicability of the model and the desirability of defining an appropriate 'equivalent deposition diameter' suitable for the whole range of particle sizes of interest. 5. Treatment of components of inhaled mixtures as if they are independent in regard to both deposition and clearance characteristics. 6. Limitations of the currently used rigid solubility classifications (D, W or Y) of inorganic compounds in estimating retention and disposition of deposited particles. I N T R O D U C T I O N THE U.S NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS (NCRP) Scientific Committee on Internal Emitter Hazards established a number of task groups to evaluate and make recommendations on various aspects of this important problem. The mission of one of these concerns Respiratory Tract Kinetic Models. Some of the problems that this task group raised with the recently published ICRP Dosimetric Model for the Respiratory System (ICRP PUBLICATION 30, 1979) are discussed here, but it should be emphasized that this paper does not represent an official NCRP position. In the Introduction, ICRP PUBLICATION 30 (1979) states that one of the reasons for revising ICRP PUBLICATION 2 (1960) is that there have been some misconceptions about its intent and some misuse of its recommendations. The same statement could be made about the report of the ICRP TASK GROUP ON LUNG DYNAMICS (1966) (TGLM). 163 at U niersity of C alirnia, Iine on D ecem er 0, 2016 http://annhfordjournals.org/ D ow nladed from 164 DOUGLAS K. CRAIG ei al. Even though ICRP PUBLICATION 30 (1979) contains numerous caveats restricting the applicability of its recommendations to the estimation of radiation dose for occupationally exposed adults, it is the tendency of agencies and organizations to take models published by prestigious organizations like the ICRP and apply them to population groups and situations for which they were not intended and for which they may be inappropriate. The most recently updated dosimetric model is similar to the 1966 TGLM recommendations, but, while it incorporates some recent information on the respiratory tract, it still appears to have several shortcomings, some of which could be serious if the model is misused. A number of models have been developed over the last half-century to describe the deposition of inhaled particles in the various regions of the respiratory tract, their retention and subsequent translocation to other body organs, and eventual excretion from the body. Factors considered in these models (which have been adequately reviewed by the 1966 TGLM) include the physiology of the respiratory tract and respiration, the physico-chemical characteristics of the inhaled particles, and interactions between deposited particles and the body systems. Even though it was never explicitly adopted by the ICRP, the TGLM presented comprehensive deposition and retention models which have been widely used since their 1966 publication. Some of the parameters were later updated and officially adopted (ICRP PUBLICATION 19, 1972). ICRP PUBLICATION 30 (1979) as it relates to the new 'Dosimetric Model for the Respiratory System' is summarized in the next section, following which some of the perceived shortcomings of the model are presented and discussed as to their implications. SUMMARY OF A NEW MODEL ICRP PUBLICATION 2 (1960) recommended values of the maximum permissible concentrations in air and water (MPC) and of maximum permissible body burden (MPBB) for a number of radionuclides. These were based upon the concept of limiting the dose equivalent from inhaled or ingested radionuclides for any stated period to ICRP recommended values for occupational exposure. Misconceptions about the intent, and misuse of, these recommendations, together with the accumulation of new and more reliable biological and physical data, and changes in the basic recommendations of the Commission (ICRP PUBLICATION 26, 1977) made preparation of a new report necessary. The new recommendations are designed to avoid radiation-induced effects of early onset and later to minimize the possibility of radiation-induced cancer and hereditary disease, using the hypothesis that the risks of developing fatal cancer in exposed people or of genetic effects in their offspring are linearly related to dose equivalent, that there is no threshold dose-equivalent below which no effects would occur and that effects are independent of dose-equivalent rate and dose-equivalent fractionation. The concepts of MPC and MPBB are not used in the new report, being replaced by the Annual Limit of Intake (ALI) for ingestion or inhalation, and the Derived Air Concentration (DAC) of each radioactive material. The primary standard is the Committed Dose equivalent, H50, denned as the total dose equivalent in any tissue over the 50 yr after the intake of the radionuclide into the body. Values of H50, ALI and DAC for radionuclides are intended only for interpreting the occupational exposure of at U niersity of C alirnia, Iine on D ecem er 0, 2016 http://annhfordjournals.org/ D ow nladed from Limits for radionuclide intake (ICRP Publication 30) 165 adults and the ALI values are based only on radiation, not chemical, toxicity. The assumptions made in deriving the values given should not be regarded as matters of established fact. Chapter 2 of ICRP PUBLICATION 30 (1979) presents the basic limits for the control of internal dose, while Chapter 3 considers secondary and derived limits for the control of internal dose in more detail. Standards are based on the anatomical and physiological values given in ICRP PUBLICATION 23 (1975). In Chapter 3 ALI is defined as the greatest value of / which satisfies both the inequalities WT(HS0T per unit intake) < 0.05 Sv