taken as controlling the final choice. This procedure is admittedly cumbersome and 

 sometimes may be unnecessarily restrictive. 



However, the basis for the maximum permissible body burden of those elements 

 that deposit selectively in bone is quite different from that used for other elements. 

 Here experience with the effects of radium in man assumes controlling importance. 

 Elements that behave like radium now have their MPC values determined by com- 

 parison of their potency and effects with those of radium. The choice of critical 

 organ is unnecessary; and little information is needed on organ deposition, retention, 

 etc. , except for the final calculation of permissible air and water concentration. 

 This provides the internal emitter field with a "double standard" for maximum per- 

 missible dose estimates. The clinical data from both external radiation experience 

 and the radium experience can be applied. The calculated MPC values allow somewhat 

 different radiation doses to the critical organ (bone) when estimated by these two 

 methods, but the difference is surprisingly small. This relative coincidence of values 

 lends credence to the soundness of both approaches. 



3. The genetic effects of internal emitters are presumed to be similar to those 

 caused by external radiation sources. If the element in question is rather uniformly 

 distributed in the body or if the gonads are the critical organ, then the potential 

 genetic effects control the MPC in the same way as for external radiation. The possi- 

 bility of a "transmutation effect" over and above the effect of ionizing radiation has 

 been considered theoretically^'^' ^^. Experimental information on the importance of 

 this effect and indeed on the genetic effects of potentially important internal emitters 

 is almost wholly lacking. 



17 



