98 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



is, if a given body burden of radium takes 15 yr to produce a malignancy 

 in man, does it take 15 yr or 1 yr to produce the same effect in the rat? 

 The usefulness of radiotoxicit}' experiments with laboratory animals will 

 depend to a large extent upon such matters. 



Investigations of this sort, which are critically needed, require large 

 numbers of animals and long periods of time and most often will lead to 

 negative results. These considerations have discouraged the undertaking 

 of such studies, which, however, must eventually serve as a basis for tol- 

 erance or maximum permissible values. 



The complexity of the problem may be indicated by the following: 

 (a) Internally deposited radioisotopes irradiate the body continuously 

 until they are eliminated, (b) The pattern of absorption and behavior 

 in the body is characteristic for each element and is even dependent upon 

 the chemical form of the element; in addition, the behavior pattern may 

 depend upon the age or rate of growth of the individual, (c) The dis- 

 tribution may not be homogeneous even in a given tissue, as demonstrated 

 in Chap. 7 for the bone seekers, (d) The radioisotope may become an 

 integral part of an important metabolite or structure, and thus the radia- 

 tion may have a greater effect than anticipated, as discussed in Chap. 2. 



The National Committee on Radiation Protection has calculated, from 

 available data on various isotopes, the maximum permissible amount of 

 radioisotopes in the total body and the maximum permissible concentra- 

 tions in air and water for continuous exposure (8). These values, except 

 for radium and strontium as noted above, are based upon an estimation of 

 the uniform concentration of the radioisotope in the critical organ which 

 will irradiate it at a dose rate of 0.3 rep/week, corrected for the biological 

 efficiency of the particular radiation. The critical organ is that which 

 receives radiation damage that results in the greatest insult to the total 

 body. It is usually but not always the organ of greatest concentration of 

 the radioisotope. Values are listed in Chap. 6 for the individual radio- 

 isotopes. It must be emphasized that, on account of the many assump- 

 tions necessary in calculations of this type, the values must be considered 

 only as approximations — even though they are the best available and 

 probably on the conservative side. It is to be hoped that support for 

 these values will he forthcoming from experiments of a toxicologic nature. 



Avoidance of Internal Hazards. Radioisotopes may enter the body by 

 ingestion with food and water, by inhalation, through wounds and abra- 

 sions, or even by absorption through intact skin. The basic protection lies 

 in good housekeeping and good work habits. In general, procedures 

 should be such as to eliminate any possibility of accidental ingestion. 

 Volatile or dusty materials should be handled in a closed system. Special 

 care should be taken to clean up spills immediately so that the 

 radioactive material does not dry down and become disseminated by air 



