396 



INFLUENCE OF EXPOSURE FACTORS 



usually resistant, but more have a sensitivity approaching the mean 

 value. Dose increments in the latter group have a greater effect in re- 

 ducing the percentage surviving. The test period is usually about 30 

 days. The part of the dose-survival curve that changes rapidly with in- 

 crease in dose is widely used for comparing effectiveness of different con- 

 ditions of irradiation and is usually expressed as the LD^q (lethal dose for 



100 



90 



80 - 



70 



^ 60 



M 



c 



I 50 - 



to 



"^ 40 — 



o 



'^ 30- 

 20 

 10 







6^ 



1 



2 3 4 5 6 7 



Exposure, hundreds of roentgens 



10 



Fig. 2. Schematic dose-survival curve for whole-body exposures to x-rays, based 

 primarily on data obtained with mice (10, 13, 27, 29, 49, 52, 53, and 54). The dose- 

 effect curve may be seen to be sigmoid. It exhibits a tlii-eshold at about 300 r and a 

 median effect (LD50) at approximately 600 r. The shape of the curve may be in- 

 terpreted as representing the frequency of individuals with a certain degree of radio- 

 sensitivity as shown in the block diagram. 



50 per cent of the population). See Elhnger (10), Evans (13), Henshaw 

 (31), Lamarque (39), and Quastler (51, 53) for dose-effect curves of dif- 

 ferent mammals after single exposures. 



The LD50 for whole-body irradiation is lower than that for local ir- 

 radiation or for general but superficial exposures. Both physical and 

 biological factors are involved. In the first place, more energy is ab- 

 sorbed, and in the second, tissue destruction is more generalized. Organs 

 which would otherwise aid in recovery (either by removing noxious ma- 

 terials or by replacing destroyed cells) are themselves impaired. The 



