78 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



Hazards to human beings from a radioactive nuclide in seafood will result from radiation 

 in the gastro-intestinal tract as the food passes through the body, and from radiation in other 

 body organs which accumulate the radionuclide. As shown in National Bureau of Standards 

 Handbook 69(5), the maximum permissible concentrations in drinking water for many iso- 

 topes, including Cr', Fe'", Co"", Zr'', Nb"\ Ru""\ Ce'", and Ta''*-, are limited by the radia- 

 tion exposure of the gastro-intestinal tract; for others, including P*-, S^\ Ca^% Fe'', Zn" ', Sr"°, 

 I"\ and Cs"', the accumulated burden in body organs is limiting. 



For the first group of isotopes, the concentration per unit volume of marine food must be 

 held below a certain value; for the second group, the specific activity (that is, the ratio of the 

 radioactive to the non-radioactive species in the seafood) must be controlled. 



Organisms do not, in general, distinguish significantly between the radioactive and the 

 non-radioactive isotopes of a particular element. Hence, if the total uptake of any element 

 by a human body organ comes from eating seafood, the specific activity in the body cannot, 

 in general, exceed the specific activity in the ocean. (Exceptions may possibly occur if the 

 radioactive and the non-radioactive species are in different chemical states in sea water.) 

 Indeed, the specific activity of the radioactive isotope accumulating in the body will be much 

 less than in the ocean if the isotope has a short radioactive half-life, because the concentra- 

 tion in marine food organisms and in body organs will be reduced by radioactive decay. Con- 

 sequently, whenever the gastro-intestinal tract is not the critical body organ, the permissible 

 specific activity of a radioisotope in sea water will be greater than the permissible specific 

 activity in the body, often by a large factor for isotopes with a short radioactive half-life. 

 (Obviously, this will also be true when the concentration in sea water of the stable species of 

 the element is very small. In these cases, the ratio of the radionuclide to a non-isotopic carrier 

 used by the body must be considered.) 



These qualitative statements can be put in quantitative terms through the following cal- 

 culations: 



Case I — Critical body organ not the gastro-intestinal tract. 



Let Ir and I„ be respectively the radioactive and non-radioactive isotopes of a particular 

 element. 



Irh and Ire are the maximum permissible concentrations of Ir in the critical human body 

 organ and in sea water, and Irf is the concentration of h in marine food organisms. Values of 

 Irb for the total body, corresponding to the concentration in the critical organ for different 

 radioactive isotopes, can be computed from Table 1 of NBS Handbook 69, referred to above. 

 Inb, Inf, and Ino arc respectively the concentrations of I,, in the critical body organ, in marine 

 organisms, and in sea water. 



K is the radioactive decay constant of Ir. 



B and Br are the corresponding constants for the biological elimination of Ir and L from 

 the body and from marine organisms. 



K=^i^(T„i = radioactive half-life of Ir) 



B = -^^(Tbi = biological half-life of Ir and L in the human body. Preliminary values 



Tb2 



of Tbi are given in National Bureau of Standards Handbook 52(6). 

 Bt=7F-T(Tbfi = biological half-life of Ir and I„ in marine organisms) 



