HUMAN EFFECTS AND RISK ESTIMATES 693 



measurement of gastrointestinal absorption in man, one must be cautious in applying the 

 animal data to populations ingesting very low levels of transuranics in unknown chemical 

 forms. Certain complexed forms of plutonium and hexavalent plutonium compounds are 

 known to be more readily absorbed than other plutonium compounds (Durbin, 1973). 



Under present conditions of recent fallout deposition, most ingested transuranics are 

 likely to be either swallowed following inhalation or consumed as external contaminants 

 on food (Bennett, 1976). With the passage of time, however, biologically incorporated 

 transuranics may become a more important factor relative to other forms of ingested 

 transuranics and relative also to inhaled transuranics. One might expect biologically 

 incorporated transuranics to be more readily absorbed than inorganic forms, and there are 

 limited animal data on milk (Finkel and Kisieleski, 1976), meat (Sullivan and Crosby, 

 1976), and alfalfa (Sullivan and Garland, 1977) which suggest that this may indeed be 

 true for plutonium. There are also data that suggest the opposite conclusion for 

 neptunium (Sullivan and Crosby, 1976). The extent to which transuranics may become 

 biologically incorporated in foods and the gastrointestinal absorbability of such material 

 are uncertain factors in any evaluation of the impact of environmental transuranics on 

 man. 



Another uncertainty relating to ingestion is the question of enhanced absorption in 

 the very young. There is now an abundance of data attesting to the fact that neonatal rats 

 (SuUivan and Crosby, 1975; 1976; Ballou, 1958; Sikov and Mahlum, 1972a; Sullivan, 

 1978), cats (Finkel and Kisieleski, 1976), and swine (Sullivan, 1978; Buldakov et al., 

 1969) absorb a very much larger fraction of ingested plutonium than do adult animals. 

 Such anomalous absorption by the infant has been reported for many other normally 

 nonabsorbed substances in many species, including man (Koldovsky, 1969; Sikov and 

 Malilum, 1972b). How one should extrapolate these animal data on transuranics to man is 

 not clear either with regard to the magnitude of increased absorption or to the duration 

 of this effect. In rats absorption drops to near-adult levels by the age of weaning 

 (3 weeks) (Ballou, 1958). The hfe-style of the infant may protect it from plutonium 

 ingestion, as compared with the adult, and plutonium deposited at an early age will be 

 subsequently diluted as a consequence of growth. It is possible, however, that the infant 

 is more sensitive to the production of deleterious effects from deposited plutonium, 

 although preliminary reports of studies in rats (Mahlum and Sikov, 1974) and dogs 

 (Stevens et al., 1978) suggest that this is not the case. 



Distribution and Retention of Transuranics in Man 



Although few health consequences and no fatalities have been observed to result from 

 transuranic deposition in man, the distribution and retention of these elements in man 

 can be measured. Such distribution and retention data permit the calculation of radiation 

 doses in human tissues which can be compared with tissue doses from other forms of 

 radiation known to produce effects in man. The similarity of human distribution and 

 retention data to that measured in experimental animals also lends confidence to the 

 extrapolation of health -effects data obtained in experimental animals. 



Most directly relevant to environmental transuranics are the deposition data for 

 fallout plutonium in man. An example of such data is shown in Table 1, which hsts 50th 

 percentile values (50% of individual values are lower) for tissues from more than 170 U.S. 

 autopsies (Mclnroy et al., 1977). Also shown in Table 1 are computed estimates of 

 plutonium concentration in tissues (Bennett, 1976), which are based on measured 



