924 RADIATION BIOLOGY 



the efficiency varies inversely with specific ionization. It is inferred that 

 only a few ions are required for these all-or-none effects, additional ioniza- 

 tion being wasted. 



Further evidence pointing to the similarity of the effects produced by 

 the different radiations is given by additivity studies. Obviously, incom- 

 plete additivity of two radiations would indicate some difference in 

 mechanism, whereas complete additivity would suggest that at least the 

 events that are directly responsible for observed effects are identical. 

 When the energy distribution is similar and the exposure times are rela- 

 tively brief, complete additivity of the various radiations is observed 

 (Stapleton and Zirkle, 1946; Zirkle, 1950). If the exposure time is pro- 

 longed (24 to 48 hours instead of 1 hour) additivity of y rays and fast 

 neutrons is incomplete (Mitchell, 1947). This may be related to differ- 

 ences in the recovery times, for there is reason to believe that recovery 

 may be most rapid with and y rays, less with conventional X rays, and 

 least with neutrons (Quastler and Lanzl, 1950; Lasnitzki, 1948). This 

 point requires further amplification. Incomplete additivity is also 

 observed after external irradiation with beta and hard gamma rays 

 (Raper, 1947). In view of the great difference in tissue distribution of 

 these two radiations given externally, one would hardly anticipate more 

 than a partial additivity. It is of interest that an isotope with osseous 

 distribution and one with reticuloendothelial distribution are synergistic 

 with respect to lethality (Friedell and Christie, 1951). 



In the production of some biological effects (e.g., gene mutations or 

 lethal effects in Triton) a given dose produces the same degree of effect 

 regardless of the rate or time during which it is delivered (Lea, 1946; 

 Brunst and Sheremetieva-Brunst, 1949). In a few instances, the radia- 

 tion effect diminishes with very brief exposure times : the lethal effect on 

 Drosophila eggs exposed to 165 r is reduced by 20 per cent when the 

 radiation is delivered in 0.4 second instead of 1.2 seconds (Sievert and 

 Forssberg, 1936). For most responses, especially in mammals, the 

 effectiveness of a given dose decreases as the rate of exposure decreases. 

 Yet the events in even this category display reciprocity over a part of 

 the intensity spectrum, and there is little information relating to very 

 intense instantaneous exposures, such as might be encountered in an 

 atomic explosion. Reduction in the biological effect with protraction of 

 the irradiation is generally explained by assuming that the recovery rate 

 becomes appreciable during the exposure. Increasing the period of 

 exposure 10 times reduces the lethal action in mice for a given dosage of 

 gamma radiation to about 70 per cent (Henshaw et al, 1947). The acute 

 LD 50 of gamma radiation for mice is 840 r when the dosage rate is 30 r per 

 minute and 1200 r when the rate is reduced to 3 r per minute (Henshaw 

 et al, 1946). In man, the dosage required to produce an equivalent 

 cutaneous reaction is doubled when the period of irradiation is increased 



