L. H. GRAY 



cles, one-twentieth by three particles, and so on. The proportion of the cells 

 which are traversed by two particles relative to the proportion traversed by 

 one particle, falls rapidly as the dose is reduced below D^. For example, 

 when exposed to \ D^ the proportions traversed by one, two, and three 



Disturbances at the Molecular Level 



Inorganic Radicals 

 [very short life 



Organic Radicals 



Damage to Biological Structure 



Pre-prophase Arrest 



Cell Division 



Genetic vhibalance 



PARTIAL 



OR COMPLETE 



RECOVERY 



Metabolic Disturbances 



MUTATION 



Morphological Change 



\ 



STERILIZATION 



DEATH DIFFERENTIATION 



Figure 1. Stages in the development of radiobiological damage 



particles are as 1 : 0- 17 : 0-02. If, therefore, a particular biological response 

 is due to the combined effect of two or more particles, this will be clearly 

 revealed by experiments within the dose range \ D^ to 3 Z)^, as a lack of 

 proportionality between dose and effect. Conversely, if proportionality 

 is found experimentally at this dose level, it may confidently be inferred at 

 all lower dose levels, because lowering the dose only affects the proportion of 



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