An important complication is that many of the former group were also exposed simul- 

 taneously to unknown proportions of other «-emitting elements, including Ra^^S ^^^ 

 Th228_ Consequently, they received a greater radiation dose than that estimated 

 from Ra226 burdens alone. For these reasons, it would appear that the maximum 

 permissible burden assigned to Ra^^^ for man is conservative. These conservative 

 factors are inherent in the estimate of maximum permissible burdens of other radio- 

 active bone-seekers when they are determined by comparisons with Ra226^ 



Recently, new studies on the distribution of radium in human bones have extended 

 the basis for comparison among the radioactive bone seekers. While the radium hot 

 spots were found to deliver very high local doses, the relatively uniform diffuse dis- 

 tribution was observed to contain almost half of the skeletally-bound radium and thus 

 to deliver a dose to all the cells in bone. This may be of considerable significance in 

 long-term damage^^"'^'^. The difficulties in assessing hazards from various bone- 

 seeking radionuclides on the basis of dosage comparison between alpha and beta 

 emitters have been discussed35,48, 49_ 



Finally, there have been no clinical observations indicating the extent to which 

 the effects of Ra^^B j^^ children might be different from those observed in adults, and 

 there has been little experimental work in this area. 



2. Radio strontium 



Both Sr^9 and Sr^*^ are beta- ray emitters, as is the Y^^ daughter of the latter. 

 Their half-lives are 50. 5 days (Sr^S) and 27. 7 years (Sr^O), so that the latter becomes 

 increasingly important with the passage of time after fission. Strontium-85 is a gamma- 

 ray emitter and is used in clinical studies, since it can be measured in vivo in very 

 small amounts by whole-body counting techniques. 



Strontium- 90 has been found universally in the biosphere. In the western hemis- 

 phere, its primary source for man is calcium-rich food, especially milk. While the 

 metabolic course of Sr^O is qualitatively similar to that of calcium, there are some 

 quantitative differences. For example, there is greater absorption of calcium than 

 strontium from the gastrointestinal tract and greater renal excretion of strontium 

 than of calcium ^^. Growing animals have been shown to retain strontium more ef- 

 ficiently than adults, a reflection of the fact that calcium deposition proceeds more 

 actively in younger animals"'-^. 



Recent measurements of Sr^^ retention in normal, adult humans have shown 

 results similar to those found in animals^^. The retention of the alkaline earths, 

 including Sr^^, can be described by a power function of the form 



R^.Atb 



where R^ is retention at time ;^in days after injection, ^is equal to R^t ^t one day, and 

 b is the ¥lope of the log-log line^S, 



The slope b for strontium in man is about one-half that estimated for Ra226 in 

 man. In other words, the rate of excretion of radiostrontium at time t is considerably 



11 



