506 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



10,000 



20,000 



DAYS 



Fig. 9 Predicted dose commitments due to ^ ^ ' Pu in different organs of Standard Man 



Commission on Radiological Protection, 1959) are bone, if the plutonium is "soluble," 

 and lung if the plutonium is "insoluble." For relatively short exposure times, the model, 

 which makes no distinction between soluble and insoluble, predicts tliat the cumulative 

 dose to lungs would be considerably higher than that to bone (Fig. 8), but the cumulative 

 doses to 70 yr are about the same. For exposure times longer than 70 yr, the dose to 

 bone would be higher than the dose to lungs because of the relatively short biological 

 half-life of plutonium in lungs (500 days) compared with tliat of bone (36,500 days). 

 Since the estimated dose to lungs is higher tlian that to bone and the exposure periods 

 (<70 yr) and the permissible dose to lungs are lower (see below), the lung (i.e., the 

 pulmonary region of the respiratory tract) is the critical organ. 



Permissible Dose Criteria 



Current ICRP recommendations (International Commission on Radiological Protection, 

 1966) concerning "dose hmits for individual members of the public" indicate that the 

 dose to lungs should not exceed "1.5 rems in a year." Annual dose rates to a given organ 

 can be estimated on the basis of predicted organ burdens as a function of exposure time. 

 For present purposes we shall consider only the equilibrium lung burden, which, for 

 practical purposes, is constant for chronic exposure times in excess of about 10 yr. 



