Sec. 24.2] CRITIQUE OF BIOLOGICAL APPLICATIONS OF ISOTOPES 505 



the effects of their ionizing radiations. Again it is physical properties of 

 atomic nuclei that are basic to the phenomena studied. 



The biological application of isotopes can be conveniently classified as 

 follows: 



I. Use as labeling agents ("tracers") of normal and abnormal physiology 



A. As tracers of metabolism 



1. Of intrinsic constituents of the organism 



a. Water 



b. Minerals, including trace elements 



c. Organic substances 



2. Of extrinsic substances (abnormal to the organism in form or 



amount) 



a. In pharmacological studies 



b. In toxicological studies 



B. As tracers of nonmetabolic processes 



1. Involving cells and other intrinsic complex constituents 



2. Involving foreign substances 



a. Dissolved gases 



b. Colloids and particles of greater than colloidal size 



c. Foreign cells and organisms 



II. Use in the study of the behavior and effects of isotopes per se 



A . Differential fates of isotopes 



B. Effects of isotopes 



1. Of abnormal concentrations 



2. Of radiation (including radiotherapy) 

 III. Use as tools for biochemical analysis 



24.2. Survey of Useful Isotopes. The 102 isotopes that have been used in 

 biological and medical studies as tracers or for purposes of irradiating 

 biological systems belong to 60 of the 96 known elements and are listed in 

 Tables 46 and 47. Tracer studies in biological systems have been carried 

 out for 55 (87 isotopes) of these 60 elements and are listed in Table 46. 

 Isotopes of the remaining 5 elements (as well as a few of the other 55) have 

 found use in irradiating organisms, tissues, or cells; but of these elements, two 

 (thallium and actinium) are represented only because they have isotopes in 

 the natural radioactive series, which thus contribute to the biological effects 

 of the radiation from radium and radon samples. Nontracer studies are 

 given in Table 47. 



In Table 48 are listed a large number of potentially useful isotopes for 

 which no published reports on biological tracer application have as yet 

 appeared. These include promising species belonging to elements repre- 

 sented in Table 46 and also species of the 41 elements for which no tracer 

 studies have been reported. This listing is not meant to imply that all known 



