oT 40 established. Furthermore, it is not known what fraction of an ingested chemical carcinogen is actually retained in the body cells in a chemically potent form. If one takes a speculative viewpoint and assumes that less than 10% of a chemical carcinogen is retained in potent form and that cells can repair more than 90% of the initial radiation damage when delivered at low dose rate, then it becomes conceivable that for the more important radiocontaminants (tritium, radium, strontium-90, cesium-137, ete.) long term detrimental effects on a gram for gram basis may not be appreciably greater | than those for the most potentent chemical agents. The estimates presented in Tables I and II are based on various sources ‘of data. Animal studies were applied to man by assuming that the same con- centration of agent would produce the same effect. This is common pharma- _cological practice and suggests that if man weighs 100 times more than the test animal then the total amount of agent required for man is 100 times that of the test animal. No correction has been made for the relative lifetimes of man and the test animals. It is obvious that if man lives longer than the test animal he will be exposed to the detrimental effects of the agent for a longer period of time and therefore may be able to tolerate only & correspondingly lower concentration level. Indeed this appears to be the case for tumor induction in mice, dogs, and man by radium-226. It is found that the necessary body burden concentration levels are in inverse ratio to the relative life span (or exposure periods) of the different animals. It is obvious that many uncertainties becloud our ability to specify a body burden level for the production of long term effects. This is particu- larly true for very low exposure levels where it is unknown how effectively the body can negate or repair initial damage. The uncertainties occur for