42 



RADIATION HIOLOGY 



The practical biological applications of ultraviolet are those utilizing its 

 erythenial effects, its ergoslerol activation, and its inactivating and 

 mutational effects on bacteria, fungi, and viruses. The action spectra 

 describing all these effects as functions of wave length are subjects of 

 other chapters; this chapter is concerned with one outstandingly practical 

 biological application, the germicidal effect. There arc included dis- 

 cussions of commercially available sources of ultraviolet for this effect and 

 for research on this and other effects. 



GERMICIDAL-ACTION CURVES 



If bacteria are irradiated with ultraviolet of various wave lengths and 

 with an identical exposure (intensity times time) for each wave length, 





1.0 

 0.9 

 0.8 

 0.7 

 0,6 

 0.5 

 0.4 

 0.3 

 0.2 

 0.1 



2000 



2200 



2400 



2600 2800 



WAVE LENGTH, A 



Fig. 2-1. Bactericidal- and orytluMiial-action curves. 

 Illumination, Berlin, 1985.) 



3000 



3200 



3400 



{International Commission on 



which is sufficient to give a convenient unit of killing, oO, 63.2 (lethe; see 

 p. 49), or 90 per cent, at the optimum wave length, the data may be 

 plotted as a germicidal-action curve. Such a curve has not been stand- 

 ardized as have the curves for luminosity and the erythemal action, but 

 Gates (1929-30), Hollaender et al. (1940), Jones ct al. (1940), and others 

 have studied the action of specific wave lengths on specific organisms. 

 Caspersson (1931, 1937) associates the germicidal-action curve with the 

 ultraviolet-absorption curve of the nuclear protein. For most bacteria 

 and fungi and for some viruses, the optimum killing w^ave length is at 

 ^2650 A. The relative effects at longer and shorter wave lengths are so 

 similar that a single tentative action curve for the average germicidal 

 effect on various bacteria and fungi and on many viruses is shown in Fig. 

 2-1 along with an erythemal-action curve standardized by the Inter- 



