80 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



For most elements, eq. (4) and consequently eq. (5) are much oversimplified, because 

 accumulation by food fishes and edible marine invertebrates does not take place directly from 

 sea water but through a complex food chain, starting with marine plants. Where there are n 

 links in the food chain, we may write. 



Irb^ A K""^+K"2B, +K-^2B,B,-n,+ 



■ Ine I i H" ' 



^) 



However, so little is known about the biological half lives in different marine organisms 

 and the mode of accumulation of minor and trace elements that this complexity is not war- 

 ranted at the present time. 



In the absence of adequate knowledge, it is conservative to assume that Bt is greater than 

 either B or K; in other words, that the hold-up time in marine organisms is short. Conse- 

 quently, for those elements of long radioactive half life where K<B<Bf, eq. (5) approaches. 



Ire«= r^ Ine (5a) 



Inh 



That is, the permissible specific activity in the oceans is about equal to that in the critical body 

 organs for isotopes of long radioactive half-life. 



WhenB,>K>B 



'b-H) 



Ire-f^lJl+^ (5b) 



That is, for isotopes of short radioactive half-life and long biological half life in the human 

 body, the permissible specific activity in the ocean is much greater than in the body. Ex- 

 amples are P^- and P", which have biological half-lives in the human body 20 to 100 times 

 greater than the radioactive half-life. (If the biological half-life in marine food organisms is 

 also known, the permissible activity in the sea water will be further greately increased. ) 



So far, we have discussed only adult organisms for which the rates of uptake of different 

 substances are roughly constant. For a rapidly growing organism, we must return to equations 

 of the form of (2a) and (2b). If the growth rate is exponential, as in the early stages of the 

 human fetus, the equation for Ire may very well be 



^""iSH^^GTbJ l-e-'«*K*B,^ (5c) 



where G= „ (Ts=the doubling time for exponential growth). Comparison of eq. (5b) 



and (5c) shows that, for rapidly growing organisms, the effect will be to reduce the concentra- 

 tion below that allowable for adults. 



The above considerations apply when the gastro-intestinal tract is not the critical organ. 

 They rest on the assumption that the amounts of non-radioactive isotopes of the radio- 

 active species in the critical body organs are not markedly different for persons on a seafood 

 diet than for the rest of the population, and that the radioactive and the non-radioactive iso- 

 topes have a similar biological behavior. 



Case II — Critical body organ is gastro-intestinal tract. 



When the gastro-intestinal tract is the critical body organ, we are no longer concerned 

 with specific activity, but only with the concentration per unit volume of the radioactive 

 isotope in the food. Returning to eq. (4), we see that 



