Chapter 2 



COMPARISON OF SOME NATURAL RADIATIONS RECEIVED BY 



SELECTED ORGANISMS^ 



Theodore R. Folsom, Scripps Institution of Oceanography, La Jolla, California 



and 

 John H. Harley, Health and Safety Laboratory, U. S. Atomic Energy Co?nmission 



In attempting to consider in numerical terms 

 possible consequences to populations from mu- 

 tations caused by very low levels of artificial 

 radioactivity, it is instructive to collect for quick 

 comparison some estimates of the natural doses 

 to which certain organisms have been exposed 

 for geological periods. These data emphasize 

 that doses from natural sources vary widely and 

 depend not only upon the habitat but also upon 

 the physical size of the organism; this natural 

 radiation background varies particularly widely 

 amongst aquatic organisms. 



A very useful summary of natural and arti- 

 ficial radiation to which human beings are now 

 exposed has been published by Libby (1955) ; 

 it has already been quoted and some of his com- 

 parisons will be repeated here. Nevertheless, 

 additional radiological factors must be included 

 whenever the natural exposures of marine or- 

 ganisms are to be evaluated. 



Only sources contributing substantially to 

 the average dose to the organisms as a whole 

 will be listed here. The major contributors are 

 (a) cosmic rays, (b) radioactivity in local sur- 

 roundings, and (c) radioactivity spread through 

 the tissue inside the organism itself. 



Cosmic rays 



Cosmic ray intensity decreases far more rap- 

 idly from sea level downward than it increases 

 with increasing elevation above the earth. Fig- 

 ure 1 and Table 1 show the trend of the ioniz- 

 ing component of these rays with elevation 

 above sea level, and with depth in water. The 

 absolute dose which is used in Table 3 and 

 Figure 2 is the average of the two values Libby 



1 Contribution from the Scripps 

 Oceanography, New Series, No. 904. 



Institution of 



(1955) uses for the geomagnetic equator and 

 for 55° geomagnetic north latitude. (See Fig- 

 ure 1 and Table 1.) 



External activity 



Most organisms live close to either (a) igne- 

 ous or metamorphic rock, (b) sedimentary rock, 

 or (c) water. Sea water has a characteristic 

 natural radioactivity — much lower than that 

 of terrestrial rocks but quite appreciable when 



ELEVATION 

 (FEET) 

 20,000 r- 



10.000 - 



400 MRAD/YR 

 40 MRAC YR 



200 1- 



OEPTH IN SEA 

 (METERS) 



Figure 1 



28 



