XVII. RADIOACTIVE TRACERS 647 



the most serious is C^*02 which may occur as the respiration product 

 of animals treated with C^^ in some organic compoimd, or in some 

 chemical operation. Though this may prove difficult it is considered 

 much better practice to try to capture and retain such C'*, if present 

 in amounts greater than a few microcuries, than to dilute it and dis- 

 perse it. It is because of such problems as these that the apparent 

 advantage of C^* over C^^ as a tracer on closer observation becomes 

 less clear-cut. Gaseous wastes from other tracers may include I^^^ 

 as iodine or hydrogen iodide, S^^ in various forms, and any number of 

 organic molecules containing other tracers. The maximum level of 

 activity set by the National Committee {68) is 10~^ ixc. per liter of air. 



Chemical operations such as organic syntheses involving C^^ (and 

 other tracers) can be performed in a closed system with much more 

 safety than in an open system. Thus any C"02 or other radioactive 

 gas can be absorbed in some chemical trap rather than disposed to the 

 surroundings. With liquid wastes the problem again depends on the 

 nature of the isotope. For short-lived materials (half -life less than 

 one month), storage of active solutions as such until decayed is 

 highly feasible and is probably the best answer. With those of 

 longer half-life it is best to precipitate the activity in the form of some 

 insoluble compound, and store that, being sure to check the activity 

 of the supernatant before discarding it into the drain. In no case 

 should active solutions be allowed to get into a public drain without 

 monitoring or checking of some sort. The level of activity that is 

 safe to pour into a drain is another matter of controversy and any ex- 

 isting regulations or suggestions extant at the time should be checked. 

 Some laboratories may be fortunate enough to be situated so that an 

 auxiliary drain can be built to carry the active liquid waste to a point 

 from which it can under no circumstances get into a public water 

 supply. Such a drain may safely carry a somewhat higher level of 

 activity but the possibility of concentration of the radioactivity by 

 plant life to a dangerous point puts a fairly low limit on the maximum 

 quantity disposable in this way. If a new laboratory is being built 

 or if one has a possibility for remodelling, a tank to contain the possi- 

 bly radioactive liquid wastes to allow for monitoring should be in- 

 stalled. 



On the subject of radiochemical laboratory design, the only real 

 conclusion that can be reached at present is that the subject is not 

 static and that the suggestions and regulations that appear in the 

 literature between the date of this writing and the time this is being 



