RADIATION INJURY AND LETHALITY 449 



2. Under exposure to radiation the resultant Gompertz function is the 

 sum of contributions due to aging and injury. 



3. A dose dehvered in a short time causes a displacement upward 

 without change in slope (after an initial accumulation period which we 

 neglect here) . 



4. The Gompertz function for a succession of doses is given by the 

 sum of the effects of the separate doses involved. 



The third hypothesis, that of indefinite accumulation of injury and 

 lethality, is not a necessary consequence of the empirical findings. Re- 

 covery constants on the order of thousands of days cannot at present 

 be discovered. Should they exist, there would be a profound divergence 

 from present expectation in a favorable direction, but they are not justi- 

 fiable on the basis of any present data. Even if there are such slow 

 recovery constants, it is reasonable to admit the necessary existence of 

 a residue of non-recovering injury having analogs in genetic injury and 

 in degradation of order in tissues — a sort of increasing entropy such as 

 probably accompanies the natural aging process. 



It must be understood that "aging" functions discussed here as con- 

 sequences of irradiation may or may not represent processes identical 

 qualitatively with normal aging processes. In the case of cataracts, for 

 example, the correspondence may be a partial one. Incidentally, it is 

 w^orth recalling that the crystalline lens is a transparent tissue, and that 

 changes of a similar nature may occur in other tissues but be invisible, 

 and in fact may contribute to the actuarial picture of aging. 



Calculations based on the above, using empirical constants deduced 

 from mouse and dog survival data (33, 34) (see Fig. 9), indicate that 

 a continuously accumulated tolerance dose might decrease the hu- 

 man life expectation by 10 per cent (less if we assume an unexplored 

 late recovery function) and that even the background exposure due to 

 cosmic rays and natural radioactivity may be responsible for the loss of a 

 year or so of our "ideal" Hfe expectation (Figs. 3, 4). It must, of course, 

 not be forgotten that ionizing radiation is but one of many possible in- 

 sults, and that the literature of vital statistics gives us numerous ex- 

 amples of comparable degrees of harm, although the experimental ap- 

 proach has been generally neglected except in radiobiology. 



We now consider the remainder of the lethality picture, with reference 

 to the physiological situation. If we are to hope to complete our under- 

 standing of these matters, it is urgently important that physiology and 

 pathology be practiced, preferably on the same experimental material 

 as is used for actuarial study. 



The relation between dosage rate and survival of two strains of mice 

 (33) shows a roughly hyperbolic form characteristic for all species and 



