RADIATION INJURY AND LETHALITY 



451 



100 



0.10 



0.01 



Present tolerance 

 level (0.042 r/day) i 



0.001 



0.00001 0.0001 



0.001 0.01 



Dose rate, r/day 



0.10 



Fig. 4. Per cent reduction of after-expectation of life under continuous exposure as 

 a function of dose rate, for a hypothetical population with a normal expectation of 

 40 years, under various assumptions about the accumulation of radiation injury: 

 A, Chronic lethality coefficient k = 0.00035 r~^ (experimental dog value) and injury 

 accumulates throughout Hfe; B, chronic lethality coefficient k = 0.00017 r~^ (ex- 

 perimental mouse value) and injury accumulates throughout life; C, chronic lethality 

 coefficient k = 0.00017 r~\ and there is a recovery constant of 1000 days. 



for various qualities of radiations (Fig. 5) . The mean accumulated dose 

 to death plotted against dose rate (Fig. 6) shows a shallow minimum 

 centering at 86 r per day, which corresponds to a mean survival time of 

 18 days and a minimum mean accumulated dose of about 1500 r. A 

 similar maximum of lethality is manifested by the rat (35), and we have 

 sufficient supporting evidence to conclude that this is a general charac- 

 teristic of the survival of mammals under duration-of-life exposure. 

 Maximum daily-dose lethality occurs a little later than the peak of single- 

 dose killing, and the clinical and pathological features of the two pat- 

 terns of exposure are almost identical in this time period (36). 



These and similar observations quite early suggested that the effects 

 of continuous exposure could be interpreted as arising from a summa- 

 tion of the effects of single dosages (35). This has been demonstrated 

 on some suitable mouse data bj' an extension of the actuarial approach 



