In spite of differences in translocation pathways and biological half-times, the radiation doses to critical organs are surprisingly similar for a given intake situation. This leads us to use the Task Group model because it is recognized by ICRP (ICRP-19, 1972) and the results using this model are not too different from the more elaborate SDB and Modified models. Although still the official model of ICRP, the ICRP-1E model is generally considered to be outdated. The Task Group model was used to calculate the accumulated doses and dose commitments (to 70 years) due to constant intake rates [A_ = 0.002 C (pCi/day) and H_ = 0.19 C_ (pCi/day)], and the results are shown in Figures 8 and 9. Detailed tables of calculations based on 1 pCi/day ingestion or inhalation are given in Appendix A. 1099 nodes 0 10-1 2 Le Ye So aca PRACTICAL APPLICATIONS 10-2 - ee nn ue Dose, rems Our purpose in this discussion is to show how the results of a transport and dose estimation model may be applied to the practical problem of deciding whether and to what extent environmental decontamination might be required to limit or reduce potential health hazards. The procedure suggested for this purpose and outlined below is analogous to the procedure followed by ICRP in calculating maximum permissible concentrations of radionuclides in air and water. The principal steps involved are: (1) fdentification of the critical exposure pathway, (2) identification of the critical organ or organs, (3) selection of maximum permissible dose criteria, (4) calculation of the corresponding maximum permissible concentration of Pu in soil (MPC) , and (5) comparison of the (MPC) with estimated inventories of Pu in the surface soils of contaminated areas at NTS. — ae! - ea Bott To La a Gi tract 10-45 CRITICAL PATHWAY The estimated plutonium ingestion rate for a hypothetical Standard Man living in a contaminated area at NTS is about 100 times the estimated inhalation rate; but, owing to the very small fraction of plutonium transferred from the gastrointestinal tract to blood (3 x 10°°), che gastrointestinal tract is the only organ that receives a significant dose from ingested plutonium. The preferred dose estimation model based on ICRP recommendations (the Task Group model, Figure 4) shows that inhalation accounts for 100 percent of the plutonium deposited in the Lungs and thoracic lymph nodes; and, for an ingestion/ inhalation ratio of 100, inhalation accounts for about 95 percent of the plutonium in bone, liver, and kidney after 50 years of chronic exposure (Table 6). Clearly, inhalation is the critical pathway. 10-5 19-6 10,000 20,000 Days FIGURE 8. PREDICTED CUMULATIVE DOSES DUE TO 239Pu IN DIFFERENT ORGANS OF STANDARD MAN 679