EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 285 



ever, the simple laws of probability predict that, when ionizing radiations 

 strike an oxygenated aqueous solution in which the solutes are in small 

 concentration, water will be the system mainly "hit" by the radiation, 

 and the effects observed on the solute will be those produced by the 

 products of irradiation of water. As the radiation dose is increased, 

 the number of molecules of solute directly "hit" mil also increase. As 

 a consequence, under those conditions there will be both reaction with the 

 free radicals produced on water irradiation and direct colHsion. 



It is dangerous, however, to extrapolate from experiments with large 

 doses of radiations to what might take place with small doses. In biolog- 

 ical systems the effect of ionizing radiations differs qualitatively when the 

 radiation dose is changed. Small doses act by indirect action and pro- 

 duce mainly oxidations ; large doses act by the two mechanisms. A sim- 

 ilar phenomenon occurs when hving cells are subjected to the action of 

 sulfur and nitrogen mustards, two radiomimetic agents. 



PRODUCTS OF IRRADIATION OF WATER 



Biologists are not primarily concerned with whether "pure water" is 

 decomposed by ionizing radiations. It is generally agreed that, when 

 distilled water containing dissolved oxygen is irradiated with X rays, 7 

 rays, a particles, or /3 particles, there is ultimate formation of hydroxyl 

 radicals and hydrogen atoms: 



H2O -> HO + H 



The net effect of ionization is regarded as the production of a high con- 

 centration of hydroxyl radicals along the ionization track and of a much 

 lower concentration of hydrogen atoms contained in a cylinder of very 

 much larger radius than the track, which is the axis of the cylinder. In 

 the presence of dissolved oxygen the hydrogen atoms combine to form 

 O2H radicals: 



O2 + H ^ O2H 



Furthermore, the O2H radicals may be further reduced by hydrogen 

 atoms to hydrogen peroxide, 



O2H + H = H2O2 

 and the hydroxyl radicals may also give H2O2, 



OH + OH = H2O2 



Although the formation of these radicals on irradiation of water has 

 not yet been demonstrated, indirect evidence is so overwhelming that it 

 seems reasonable to take for granted the formation of OH and O2H. The 

 formation of hydrogen peroxide depends on the kind of irradiation. 

 When water is irradiated with X rays, H2O2 formation in the absence of 



