On the other hand, this same U. N. Committee (1958) stated that 

 "the possibility cannot be excluded that our present estimates exagger- 

 ate the hazards of chronic exposure to low levels of radiation". There 

 is obviously need for much intensive research on all aspects of the 

 problem. For the present, it is evident that the only proper course to 

 pursue, in regard to exposure of man to radiation, is a cautious one. 



It should be noted that the maximum permissible concentrations 

 in drinking water tabulated in Handbook 52, and in similar publications, 

 are calculated on the assumption that this is the only source of man's 

 intake of an isotope. If a man is ingesting quantities from several 

 sources, the total intake should not exceed the intake corresponding to 

 the mpc for drinking water times Z,ZOO (the daily water intake, in mil- 

 liliters, assumed in Handbook 5Z). Therefore, in estimating the max- 

 imum permissible level in any particular source (such as in food from 

 the sea), account must be taken of all other sources. 



On the other hand, care must also be taken to appreciate the full 

 significance of the terms "maximum permissible exposure" and "max- 

 imum permissible concentration". These terms, as used here, are 

 based on the effects of continuous exposure to a large human population 

 over the average life span. Thus local, short term conditions in which 

 concentrations of radionuclides exceed the mpc values may be per- 

 missible, provided that the total exposure over a relatively long period 

 of time remains below the required average for that period. 



PROCESSES DETERMINING THE CONCENTRATION OF NUCLIDES IN 

 THE MARINE ENVIRONMENT AND IN SEA FOOD 



The physical, chemical, and biological processes affecting the 

 distribution of radio-isotopes in the sea have been considered in some 

 detail by Revelle, et al . (1957), and the processes in coastal waters 

 particularly have been reviewed by Carritt, et al . (1958). It is, how- 

 ever, desirable to consider some of these matters here, with particular 

 reference to the problems of disposal of wastes from nuclear-powered 

 ships. 



Physical effects : The oceans are stratified in approximately a 

 two layer system, with an upper, mixed layer about 100 meters thick, 

 separated from the deeper waters by a region of rapid increase in den- 

 sity, which acts as a barrier to vertical mixing. In partially enclosed 

 basins (harbors and estuaries) interchange between the waters thereof 

 and the w^aters of the upper mixed layer of the open sea is often inhib- 

 ited, so that the rate of dilution of pollutant in the water of the basin is 

 a great deal less than the rate of dilution due to eddy diffusion in the 

 open sea. Where the basin has a shallow entrance, there usually will 

 be developed in the basin below the entrance depth a body of water which 

 stagnates seasonally or permanently, and in which there can be a con- 

 siderable accumulation of elements which fall out as particles from the 

 upper layer. The interchange of water froin partially enclosed basins 

 with the open sea depends on a number of factors including (1) the to- 

 pography of the basin, (2) the amount of fresh water runoff into the 



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