less than the rate of excretion of Ra226 ^t the same time after exposure. However, 

 experiments with rats and dogs have given somewhat different results. These dif- 

 ferences may be related to the fact that in rats and dogs radium and strontium are 

 excreted in roughly equal quantities in the urine and feces^'^. This is also true of 

 strontium in the human; however, radium is excreted from man almost exclusively 

 by way of the gut44. 



B. The Lanthanide and Actinide Rare Earths (Including Yttrium ) 



The lanthanide rare earths are produced in high yields in fission reactions; the 

 parent materials in such reactions are members of the actinide rare earth series. 

 These elements behave similarly in their predominant chemical and biochenaical re- 

 actions. However, there is a progression (with increasing atomic number) in chemi- 

 cal behavior within these groups (particularly the lanthanides)^^, which is reflected 

 in systematic changes in their biologic behavior'^". 



Although radioisotopes of these elements are distributed over the earth as a re- 

 sult of the use of nuclear devices, they have not as yet been identified in appreciable 

 quantity in mammals and man. This is undoubtedly a consequence of their extremely 

 low solubility under biological conditions and correspondingly low absorption from 

 the intestine. In experimental animals, less than 0. 01 percent of an ingested dose is 

 absorbed. There may be an exception in the case of very young animals, since suck- 

 ling mice absorbed 2 to 3 percent of plutonium given orally in milk or as the citrate^ '. 



When these materials are introduced directly into the blood stream, they behave 

 like colloids and become fixed in the reticulo-endothelial system and in the more ac- 

 cessible surfaces of the skeleton. Their rate of loss from the body is very slow^^. 



Locally injected solutions of the uncomplexed ions tend to remain at the site of 

 administration. The complexed ions are removed from the site with fair rapidity and 

 follow the pattern of the intravenously injected material^ ^. 



C. Cesium-137 



This element has been found in mammalian and other vertebrate species. In 

 man the concentration of stable cesium is about 10-10 g/g wet tissue. Cesium-137 

 from nuclear debris has now been measured in foodstuffs and man^". 



The amount of Csl37 in the body reflects the quantity of radionuclide in the 

 diet, which, in turn, is determined by the degree of radioactive contamination. Be- 

 cause of the relatively short residence time of Csl37 in man (biological half-life about 

 140 days)^^, the average content of this nuclide in humans is a good reflection of 

 recent fallout rates. 



The major portion of the Csl37 burden of the United States population is derived 

 from milk; meat products are the second most important source^l. During 1959, the 

 mean Csl37 burden of a U.S. resident reached a maximum, which was estimated62 at 

 0. 01 fic. This burden contributes an irradiation dose of about 2 mr/yr or about 2 

 percent of the natural radiation background. 



Because of the chemical similarities of cesium, potassium, and rubidium, their 

 metabolism is similar. Cesium, like potassium, occurs chiefly within cells, although 



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