Chapter VII 



CONSIDERATION OF PARTICULAR RADIONUCLIDES '^^ 



A. The Alkaline Earths (Calcium, Strontium, Barium, Radium) 



The alkaline earths are metabolized in a manner qualitatively similar to calcium. 

 Consequently, in the animal body they are rapidly and almost exclusively deposited in 

 the skeleton. Unless the physical half-life of the isotope in question is sufficiently 

 short, some of the radionuclide deposited in the skeleton will remain there through- 

 out life. Unlike the rare earths, the alkaline earths may be readily absorbed from 

 the intestine. A single exposure, or multiple exposures of reasonably short duration, 

 produce a very heterogeneous pattern of deposition in bone. Continued exposure re- 

 sults in a much more uniform pattern of deposition. The dependence of distribution 

 on duration of exposure further complicates definition of the effectiveness of given 

 doses (see Sect. IV. A). 



1. Radium-226 



Radium-226 is of especial importance; its toxicity in man has long been estab- 

 lished and has, therefore, been used as the basis for estimating the potential toxicity 

 of other bone-seeking radioelements. These estimates are generally made on the 

 basis of experimental observations using the expression: 



Effect of Ra in human ^ Effect of X in human 



Effect of Ra in animal Effect of X in animal 



There has been no direct evidence regarding the applicability of this expression. As 

 the experimental animal approaches the size and life span of man, the difficulties in 

 applying the equation are presumably diminished. 



In 1941 the maximum permissible body burden of Ra226 in man was set^Z at 

 0. 1 ^}g. This decision was based on the observation that no definite manifestations of 

 injury from radium had at that time been seen in any individual bearing less than 1 fxg. 

 Since some effects attributed to Ra226 have now been noted at slightly lower levels, 

 it has been suggested that the 0. l-^g value for maximum permissible burden be 

 decreased^S, 



It must be emphasized that the permissible burden was established at 0. 1 ^xg 

 without reference to the time or the magnitude of the initial exposure. The manifesta- 

 tions of radium injury have been found in individuals some 20 to 30 years after ex- 

 posure to radium. During the period immediately following intake, the amount of 

 radium in the body was certainly considerably higher than that finally measured. The 

 best estimates44 are that an individual retaining 0. 1 ^g Ra226 30 years after intake 

 must have initially absorbed about 10 |jig. 



Much of our information on Ra226 toxicity is derived from studies on persons 

 exposed in the dial-painting industry and from patients given radium therapeutically. 



10 



