.-,0 RADIATION BIOLOGY 



lesser exposures and yet cover a thousandfold exposure range. The sig- 

 moid form of the curves is inherent in the scales used. 



REACTIVATION BY HEAT AM) I.KIIIT 



The j)ractical signilicaiu'e of the reactivating effect of time, heat, and 

 light on ultraviolet-injured bacteria and molds was overemphasized by 

 the manner in which the experimental data were first presented by Kelner 

 (1949). For example, a definite 20 per cent revival of the total initial 

 number of irradiated bacteria per experimental unit volume was presented 

 as a 3000 per cent increase in the number of viable organisms from the 

 indefinite few left after a theoretical killing of 99.99994 per cent. By this 

 method of presentation, if the "killing" had been complete, the per- 

 centage increase in viable organisms would have been infinite regardless 

 of the actual revival. 



The 20-25 per cent revivals of bacteria resulted from light exposures of 

 the order of 5-8000 ft-c-hr, exposures provided only by 2 3 days of the 

 highest levels of practical indoor illumination. The eciuivalent of a 2- to 

 3-hr exposure to 100 ft-c provided less than a 0. 1 per cent revival, and few 

 practical germicidal applications involve exposures of bacteria to as many 

 foot-candle-hours. This suggests that the reactivating effect of light is 

 of little or no significance indoors but may somewhat reduce the apparent 

 susceptibility of organisms to the ultraviolet of the sun. 



GERMICIDAL ACTION OF ULTRAVIOLET OF WAVE LENGTHS GREATER 



THAN 2800 A 



Buchbinder ct al. (1941) have shown that sunlight, direct and through 

 window glass, as well as the ultraviolet from common artificial light 

 sources, has measurable germicidal effects on bacteria exposed to common 

 illumination intensities for a day or two (Fig. 2-4). The daylight inten- 

 sities and exposure times may have been somewhat comparable with 

 those used in the reactivation experiments, in which case the killing must 

 have been the difference between the germicidal action of wave lengths 

 greater than 3000-3200 A and the reviving or protective action of wave 

 lengths greater than 3600 A. 



COMPOSITE OF KILLING FACTORS 



Only a three-dimensional model would completely represent the rela- 

 tions of wave length and exposure to killing. However, the outstanding 

 practicality of wave length 2537 A suggested making its plane representa- 

 tive of the wave length and plotting on it a fourth indeterminate variable, 

 the .susceptibility of organi.sms to killing by that wave length, as typical 

 of the killing by other wave lengths. The result was a consolidation of 

 Figs. 2-1 and 2 as Fig. 2-3. 



