752 RADIATION BIOLOGY 



active substances are probably involved in aberration production in the 

 presence of oxygen. It seems particularly likely that the H0 2 radical 

 [resulting from reaction (9)] would itself be highly active biologically and 

 should produce effects similar to H 2 2 . Furthermore, recent evidence 

 from experiments on the oxygen effect with fast neutrons has made it 

 clear that there is an inverse relationship between the specific ionization 

 (ionization density) for a given radiation and the magnitude of the 

 oxygen effect (Giles, Beatty, and Riley, 1952). The best data, for iso- 

 chromatid breaks, indicate that the nitrogen-oxygen dose ratios (the 

 ratio of the dose of a given radiation in nitrogen to that in oxygen produc- 

 ing an equal aberration yield) for a particles, fast neutrons, and X rays 

 are, in order, 1.0, 1.4, and 2.6. These data serve to reemphasize the 

 importance of ionization (and subsequent radical) distribution as a factor 

 in radiobiological effects. 



This inverse relationship between specific ionization and the oxygen 

 effect apparently can be interpreted best as arising from differences in the 

 distribution, and consequently in the interactions, of the primary radia- 

 tion products in water (H atoms and OH radicals) and dissolved oxygen. 

 Thus with a particles, the OH radicals and H atoms are closely spaced 

 and reactions (4), (5), and (6) are favored. Even with oxygen present 

 there is little opportunity for reaction (9) to occur, since H atoms are 

 rapidly removed by reaction (6). Thus no H0 2 radicals are produced 

 and no oxygen effect is noted. With X rays, however, the primary 

 radiation products are more widely spaced and dissolved oxygen can 

 react with H atoms to produce H0 2 [reaction (9)]. With radiations 

 having intermediate specific ionization, such as recoil protons produced 

 by fast neutrons, some oxygen effect might be anticipated, since reaction 

 (6) would be less favored in comparison with reaction (9) than is the 

 case with a particles. 



It seems quite possible that reactions produced by OH and H0 2 

 radicals (and possibly by H atoms) rather than by H 2 2 molecules, are 

 principally effective in producing chromosome aberrations and that a 

 certain average concentration of such radicals may be required for a 

 chromosome break to result. It should be noted, however, that the 

 probability also exists that H 2 2 molecules are involved in a-particle 

 effects, even if they prove to be relatively unimportant in X-ray effects, 

 since the close proximity of OH radicals along the paths of these particles 

 should favor their rapid reaction to form H 2 2 . At the present time it 

 does not appear possible to decide whether OH radicals (and possibly 

 also H atoms) or H 2 2 molecules are primarily responsible for the a-par- 

 ticle effects. Further, there is the additional possibility that active 

 products of secondary reactions, such as organic peroxides which are 

 known to be mutagenic (Dickey et al., 1949), are involved. This seems 

 rather unlikely, however, in view of the apparent localization of the 



