Chapter V 



MODES OF ENTRY INTO ANIMALS AND MAN 



A. Ingestion 



Gastrointestinal absorption is the most important route of uptake of such radio- 

 nuclides as Sr^O, Cs-'-'^'^, and I . The levels of these nuclides in animals and man 

 are correlated with their levels in the diet. 



It is extremely important to establish whether these and other radionuclides get 

 into plants largely by way of the soil or largely through direct foliar contamination. 

 Insofar as entry into the diet is due largely to direct contamination of plants, the 

 levels in man will depend on the rate of injection of radionuclides into the environment. 

 Insofar as it is due largely to uptake from the soil through the root system of plants, 

 levels in man will depend on the integrated contamination level of the soil. Knowledge 

 of the ecological characteristics of specific radionuclides is most important for as- 

 sessing the potential hazards of general environmental contamination. * 



Another factor bearing on ingestion as a potential route of entry is whether the 

 radionuclide is an isotope of an element required by the body or of one chemically 

 similar to a required element. The actinide and lanthanide rare earth series of ele- 

 ments have no chemically similar counterparts among the required body constituents 

 and are usually poorly absorbed by plants and animals. For these radionuclides, in- 

 halation may be relatively more important than ingestion, although the direct irradia- 

 tion of the intestinal surface must be given consideration. 



Whereas some generalizations are possible on the basis of similarities of ele- 

 ments within families of the periodic table and similarities to required body consti- 

 tuents, each radioelement exhibits its own metabolic properties. There is a continuing 



* Since the 1956 report, considerable understanding of the ecological behavior of 

 Sj;.90 and Cs^'^''' has been gained. Burton et aL of the United Kingdom have reported^ ' 

 that soil uptake was responsible for 20 percent or less of Sr^O in the British diet 

 during 1958 and that 80 percent or more was a result of direct foliar contamination. 

 While this ratio will undoubtedly vary with time, this suggests that the equilibrium 

 level of Sr90 in the bones of the population from environmental contamination will be 

 lower than that previously predicted. 



Measurements of Csl37 in human beings and in milk, carried out at Los Alamos 

 since 1956, suggest that contamination levels are largely dependent on the rate of 

 nuclear weapons testing and not on the total fission energy releasees. During 1960, 

 Csl37 levels in milk failed to show the broad spring peak characteristic of past years. 

 Some of the 1960 Cs^^? levels have already dropped to the lower limit of detection, and 

 the average level in the U.S. population was 10 to 15 percent lower than during 1959. 

 It is too early, however, to say whether the average yearly level in the population will 

 be proportional to the rate of decay of the integral soil level or whether it will be pro- 

 portional to the rate of stratospheric fallout. 



