CRITICAL ORGAN ——— 109 "|mse 10-1 LyeNO eee a:cn 1 | | ; Lungs —— LE—< According to the Task Group model (Figure 4 and Appendix A), thoracic lymph nodes receive the highest dose (Figures 8 and 9) but the critical organs ; recognized by ICRP (1959) are bone, if the Pu is "soluble," and lung if the Pu is "insoluble." For relatively short exposure times, the model, which makes no distinction between "soluble" and “insoluble,” predicts that the cumulative dose to lungs would be considerably higher than the cumulative dose to bone (Figure 8), but the cumulative doses to 70 years are about the same. For @xposure times longer than 70 years, the dose to bone would be higher than the dose to lungs. This is due to the relatively short biological half-Life of plutonium in lungs (500 days) compared to that of bone (36,500 days). As the estimated dose to lungs is higher than the estimated dose to bone, the exposure periods < 70 years, and the permissible dose to lungs is lower (see below), the lung (i.e., the pulmonary region of the respiratory tract) is the critical organ. PERMISSIBLE DOSE CRITERIA 10-31 Current ICRP (1966) recommendations concerning "dose limits for individual members of the public" indicate that the dose to lungs should not exceed 10-4 |---|--------] "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 years. ESTIMATION OF ASC 106° | 10,000 er--7 Dose Commitment, rems 10-2 des The acceptable soil concentration (ASC) is defined as the average concentration of 239Pu in the soil of an area occupied by the hypothetical Standard Man which would result in a dose to the lungs equal to or less than the permissible dose. This value is estimated as follows: 20, 00 Time, Days S DUE TO 239 Pu IN FIGURE 9, PREDICTED DOSE COMMITMENT DIFFERENT ORGANS OF STANDARD MAN ASC = (1.5/365)/(E ¥og/™ = 2817 pci(??’ pu) /p(soil) where (1.5/365) is the "permissible dose rate" in rem/day, £ for 739pu is 51.2159 x 53 = 27144 (rem/day) /pCt or 0.0027144 (rem/day)/pCi, 681 680