692 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Two routes of exposure, ingestion and inhalation, are of significance tor general 

 environmental dispersal. Of these, inhalation is the better understood because the 

 mechanisms and kinetics of deposition and retention in the lung are primarily determined 

 by physical factors, which have been studied for many substances in man. Such data can 

 be extrapolated with some confidence in predicting the behavior of transuranics in the 

 lung. The absorption of ingested transuranics from the intestinal tract is of more critical 

 uncertainty, however, because such absorption is primarily determined by chemical 

 factors unique to each individual transuranic and because only that fraction which is 

 absorbed is of primary hazard concern. Unabsorbed alpha-emitting transuranics, while 

 passing through the gastrointestinal tract, are conservatively assumed by the International 

 Commission on Radiological Protection (ICRP) to deliver 1% of their decay energy to 

 sensitive cells of the intestinal wall. On this basis the intestine becomes the critical organ 

 for ingestion of insoluble transuranics (International Commission on Radiological 

 Protection, 1960), although it is unlikely that significant damage actually occurs by this 

 mechanism. 



Inhalation 



The model usually employed to describe the kinetics of inhaled transuranics in man is 

 that of the ICRP Task Group on Lung Dynamics (1966) as modified by the ICRP Task 

 Group on Plutonium and Other Actinides (1972). This model adjusts for three classes of 

 parficle solubility and for a wide range of particle sizes. To illustrate the general case of 

 environmental transuranics, we will assume insoluble particles (class Y) oiOA-jim activity 

 median aerodynamic diameter (AM AD), which were assumed by Bennett (1976) to be 

 typical of airborne fallout. The model predicts that 32% of such an inhaled aerosol will be 

 deposited in the pulmonary region of the lung; the remainder will be immediately exhaled 

 or rapidly cleared from the nasopharynx or tracheobronchial region. Of the 32% 

 deposited in the pulmonary region, 40% is cleared with a half-Hfe of 1 day; the remaining 

 60%, which is equivalent to about 20% of the aerosol initially inhaled, is retained in the 

 lung with a half-life of 500 days — this fraction is responsible for essentially the total 

 irradiation of the lung. About 6% of the quantity initially inhaled eventually reaches the 

 bloodstream and is distributed among the systemic organs. Although these fractions will 

 vary with chemical form, particle size, and other exposure variables, a sizable fraction of 

 inhaled transuranic will, under any circumstance, be tenaciously retained by man. This is 

 an efficient route of entry, as compared to ingestion, and atmospheric transport is a 

 correspondingly hazardous environmental pathway. 



Ingestion 



Because of the general insolubility of transuranic oxides and hydroxides and the 

 propensity for more soluble compounds to hydrolyze at physiological pH, one anticipates 

 httle absorption of these elements from the gastrointestinal tract. The ICRP assumes a 

 fraction absorbed of 3 X 10~^ for plutonium and 10""* for americium and curium 

 (International Commission on Radiological Protection, 1960); a value of 10~^ is 

 suggested as appropriate for plutonium oxide (International Commission on Radiological 

 Protection, 1972). These values are based on the results of animal studies. Several recent 

 investigations have indicated somewhat higher absorption than that assumed by the ICRP 

 and a typically large variability from experiment to experiment (Durbin, 1973; Sullivan 

 and Crosby, 1975; 1976). Because of diis variability and because there is no direct 



