48 KADI ATION lUOLOr.Y 



gest suppression of tlio mycdia without serious damage to the spores, buf 

 the mechanism should tx' ;iii interesting subject for research. 



COMPARISON OF SUSCKl'l I HlLl'l^ TO TLTRAVIOLET AK\) TO OTHIOR 



ij/niAi, a(;kxts 



The intrinsic susceptil)ility of \arious species of bacteria, fungi, and 

 viruses to killing by ultraviolet of 2537 A, or any other wave length, varies 

 over an exposure range of as much as 1-3. The extrinsic susceptibility, 

 determined by acquired tolerance and age, may also \'ary over a range of 

 1-3. In contrast with liacteria and viruses, for any arbitrary percentage 

 kill of various mold spores, the exposure may be 15-300 times that 

 required for the same kill of dry air-borne Escherichia coli, with a difTer- 

 ence as great as 1 to 1000 between the most susceptible bacteria and the 

 most resistant mold spores. The exposure necessary to kill any one kind 

 of organism may vary considerably, depending on its environment, tem- 

 perature, illumination, and physical condition, illustrated in Pig. 2-2 by 

 humid water-borne E. coli which requires four times the exposures of dry 

 air-borne E. coli for comparable killings. 



Although the effects of radiation of shorter wave lengths on various 

 microorganisms seem to be very similar to those of the ultraviolet, there 

 is little similarity between the effects of ultraviolet and of other lethal 

 agents such as heat, dryness, or chemicals except in the logarithmic nature 

 of the kill. Markedly thermoduric organisms, for example, are readily 

 killed by ultraviolet energy. 



LOGARITHMIC NATURE OF KILL 



The practical ultraviolet killing of nearly all microorganisms is more 

 or less logarithmic in nature. Wyckoff (1932) and Rahn (1932, 1945) 

 have discussed this rule and its exceptions in detail. The logarithmic 

 nature of kill is in accord with the general exponential attenuation law, 



,V = AV-", (2-1) 



where 



iVo = the initial concentration of organisms, 



,V = concentration of organisms after an exposure I'or tinu- / to an ultra- 

 violet intensity, 

 / = the ultraviolet intensity, and 

 e = the base of the natural logarithms (about 2.718). 

 For a unit exposure, 7/ = 1, the concentration of survivors .V becomes 



N = Noe-' = 0.3(58 No, (2-2) 



and the kill, .Vo — N, becomes 



No- N = 0.632 .Vn. (2-3) 



I 



