570 



BIOLOGICAL EFFECTS OF RADIATION 



Table 1. — Comparison of Biological Effectiveness of Alpha Particles with 



Their Ionizing Ability 



" Based on the assumption that the biological effectiveness varies as the five halves power of the 

 ionization per unit path. 



Although biological effectiveness is clearly not a linear function of 

 ionization per unit path, there is apparently a simple mathematical 

 relationship between the two. This can be expressed (Zirkle, 28) by the 

 empirical equation B — kP-'°, where B is biological effectiveness, k a 

 constant, and / the ionization per unit path (compare columns 3 and 4, 

 Table 1). Since the effective radius of the path is roughly the same 

 throughout the length of the path, it follows that / is roughly propor- 

 tional to the local concentration of ions in the path. It thus appears 

 that in different portions of the path, the biological effectiveness of the 

 alpha particle varies with the ion concentration raised to some power of 

 the order of the square or the cube. 



These results immediately suggest the investigation of a possible 

 concentration factor in the action of alpha particles upon other biological 

 material. It would also be of great interest to determine whether or not 

 the concentration effect can be modified by biological factors, such as 

 water content. 



SUMMARY 



Practically all of the reported effects of alpha particles have been 

 lethal or harmful. The few reports of favorable stimulation are all open 

 to serious criticism. 



The qualitative effects of alpha particles have usually been found 

 similar to those of beta, gamma, and X-rays. Moreover, the doses of 

 these various radiations necessary to produce a given degree of biological 

 effect have been found to produce the same order of magnitude of calcu- 

 lated total ionization per unit cellular volume. However, the total 

 ionization per unit cellular volume is not the only ionization factor deter- 

 mining the degree of biological effect, for with alpha particles it has been 

 shown that a given number of ions may produce widely different degrees 

 of biological effect if one uses various linear portions of the ionizing paths 

 along which the concentration of ions varies considerably. 



