HUMAN EFFECTS AND RISK ESTIMATES 695 



The retention of plutonium in the various organs of man must be known if radiation 

 doses are to be calculated. We have already noted the assumptions regarding retention in 

 the lung as postulated in the ICRP lung model. Retention in the systemic organs is known 

 to be prolonged, as deduced from human plutonium excretion data following intentional 

 or accidental administration and from much data showing long-term retention in a variety 

 of animal species (Durbin, 1972). The ICRP has assumed a biological half-Hfe of 

 100 yr for transuranics in bone and 40 yr in liver; 90% of the plutonium deposited in 

 lymph nodes is assumed to be retained with a biological half-Ufe of 1000 days, and the 

 remaining 10% is assumed to be retained without loss (International Commission on 

 Radiological Protection, 1972). On the basis of a thorough review of the pertinent data, 

 Durbin (1972) concluded that human bone plutonium might exhibit a shorter retention 

 half-hfe than Uver plutonium, and, on the basis of recently acquired nonhuman primate 

 data, Durbin and Jeung (1976) have suggested shorter half-lives for both bone and liver 

 plutonium. 



In summary, it would seem fair to conclude that our knowledge of plutonium 

 distribution and retention within the human, although uncertain in many details, is 

 considerably more precise than our knowledge of the environmental and exposure 

 pathways that lead to this deposition and more precise than our knowledge of the health 

 consequences that may result from this deposition. 



Effects of Transuranics in Man 



An unevaiuatable uncertainty attaches to any prediction of specific health effects from 

 the exposure of humans to transuranic elements at levels contemplated for environmental 

 dispersal. This uncertainty is due to the absence of any positive information on the 

 effects of these elements in either man or experimental animals at the exposure levels of 

 concern. Data are available at much higher exposure levels on the effects of transuranics 

 in experimental animals and on the effects of certain other forms of radiation in man. 

 The extrapolation of these data is made difficult by our lack of understanding of the 

 mechanisms by which these effects occur. In the absence of such understanding, it has 

 been common practice to extrapolate from the high-dose data by assuming a linear 

 relationship between radiation dose and biological effect. Such a practice is endorsed by 

 the Advisory Committee on the Biological Effects of Ionizing Radiations (BEIR) of the 

 National Academy of Sciences-National Research Council (1972) as "warrant[ing] use 

 in determining public policy on radiation protection"; in the same sentence they caution 

 that "explicit explanation and qualification of the assumptions and procedures involved 

 in such risk estimates are called for to prevent their acceptance as scientific dogma." 

 Although in this chapter we have used the Unear dose— effect assumption in estimates of 

 the consequences of human exposure to environmental transuranics, we must emphasize 

 that these estimated effects, if they occur at all, will be difficult to detect over the 

 background of indistinguishable effects from other causes. 



Experience with Transuranics in Animals 



Direct information on the toxicity of transuranic elements is available only from studies 

 in experimental animals. The radiobiological literature suggests that the effects observed 

 in such animal experiments will at least qualitatively approximate those which would 

 occur in man if he were exposed under the same conditions. On the basis of extensive 

 data from several animal species, it is concluded that the most probable serious effects of 



