Chapter VIII 



MAXIMUM PERMISSIBLE EXPOSURE (MPE) LEVELS 

 FOR RADIOACTIVE MATERIALS 



The establishment of MPE levels for radioactive materials presents many 

 problems similar to those associated with maximum exposure levels for external 

 radiation. The biological effects of ionizing radiation are qualitatively similar whether 

 the source is inside or outside the body. Although the standards applicable to external 

 sources can be and are utilized for internal emitters to a considerable extent, there 

 are several important differences. 



1. The spatial and temporal distribution of the radiation dose from internal 

 emitters is a function both of the half-life and of the physiological behavior of the 

 particular radionuclide and the compound in the particular organism. 



The lack of detailed information concerning the physiological behavior of these 

 radioisotopes in man is a source of uncertainty not encountered in the evaluation of 

 external irradiation hazards. The ICRP and NCRP have recently published an ex- 

 tensive tabulation of the metabolic data on which they base their estimates of the 

 hazards of exposure to approximately 240 radionuclides 3, 8, These data, which 

 necessarily form the basis of recommendations about exposure levels, have many 

 shortcomings. 



For many radioelements, there are simply no data of the type needed for hazard 

 evaluation, and assumptions must be made on the basis of known behavior of pre- 

 sumably similar elements. For those internal emitters of principal concern, a sub- 

 stantial body of metabolic data are available. In most instances, however, there is 

 a lack of such detailed information as the effect of the chemical or physical form of 

 the radionuclide, and the effect of physiological variables such as age, nutritional 

 status, etc. While gross distribution patterns are known, the fine structure of 

 radionuclide distribution within tissues, which may be of critical significance in 

 determining toxicity, is often unknown. For nearly all radioelements, the available 

 data cover only brief periods of exposure or brief periods of retention following a 

 single exposure. For most radioelements, there are few or no data directly appli- 

 cable to man, and animal data may be misleading. 



2. Application of external radiation standards to radionuclides in the body has 

 required use of the concept of the "critical organ". Theoretically, the critical organ 

 should be the most vulnerable tissue under the given circumstances of deposition and 

 retention, and with external radiation it can indeed be so chosen. However, with 

 radionuclides, the organ with the highest concentration is very frequently selected 



as "critical"'^' ^. This is conservative inasmuch as, with few exceptions, no organ 

 system is permitted a dose greater than that permitted for whole-body external 

 radiation. However, the organ of highest concentration varies with time, route of 

 administration, etc. As a result, as many as twelve potentially critical organs may 

 be chosen for each radionuclide'^ •> °. The one yielding the minimal MPC value is usually 



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