504 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



140 yr after 10 yr of chronic inhalation. The lung burden could thus reach very high 

 levels. A fraction may indeed be retained with a long half-time, but we find it difficult to 

 believe that this fraction could be so liigh (15%). We therefore examined the long-term 

 behavior of the burden in this portion of the lung resulting from a single inhalation. 

 Equations 63 and 64 can be solved to yield 



yLv = 0.15 yp(0)exp(-XAt)exp {-7.3[1 - exp (-0.001 899t)] } (81) 



where yp(0) is the initial amount deposited in the pulmonary lung. 



If we neglect radioactive decay (X^ = 0)> the initial fraction (15%) is reduced to 0.4% 

 after 1 yr, 0.013% after 5 yr, and asymptotically approaches a constant value of about 

 0.01%. On the basis of these results, we assume that this fraction (0.01%) is retained 

 indefinitely and the remaining 14.99% is removed with a 3-yr biological half-time. 



As shown in Tables 10 and 1 1 , inhalation is the critical pathway for plutonium to all 

 organs except GIT. The organ burdens and radiation doses from inhalation are generally 

 1 ,000 to 10,000 times as great as the corresponding burdens and doses from ingesting the 

 same amount of plutonium. Tliis is due to the relatively large fraction (0.2 to 25%) that 

 reaches the blood directly from inhalation vs. the relatively small fraction (0.003%-) from 

 ingestion. For ingestion bone is the critical organ for the ICRP II and Task Group models, 

 whereas liver is the critical organ for the SDB and modified models. This difference is 

 explained by the fraction transferred from blood to the organ, which is 71% to the liver 

 and 25% to bone for the SDB and modified models, whereas the corresponding values for 

 the ICRP II model are 15 and 80%, respectively, and those for the Task Group model are 

 45 and 45%, respectively. Where the fractions are equal, the bone has the larger burden 

 and dose because it has the larger biological half-time. 



For inhalation the lung is the critical organ for all models except ICRP II. The dose to 

 lymph nodes is actually liigher, but ICRP (International Commission for Radiological 

 Protection, 1959) does not recognize lymph nodes as critical organs. For the ICRP II 

 model the bone is the critical organ because this model has the highest fraction of inhaled 

 material that reaches the blood immediately (25%) and the shortest biological half-time in 

 lung (365 days). 



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 Publication 

 19 (International Commission on Radiological Protection, 1972)] , and the results using 

 this model are not too different from the more elaborate SDB and modified models. 

 Althougli still the official model of ICRP, the ICRP II model is generally considered to be 

 outdated. The Task Group model was used to calculate the accumulated doses and dose 

 commitments (to 70 yr) due to constant intake rates [Am - 0.002 Cs (pCi/day) and 

 Hjn = 0.19 Cs (pCi/day)] , and the results are shown in Figs. 8 and 9. 



Practical Applications 



Our purpose in this discussion is to show how the results of a transport- and 

 dose-estimation model can be applied to the practical problem of deciding whether and to 

 what extent environmental decontamination might be required to Umit 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 



