•46 



RADIATION BIOLOGY 



^ 0.00002 



^ 00001 



7000 



"2000 3000 4000 5000 6000 



WAVE LENGTH, A 



Fig. 2-4. Relative bactericidal action extended to the near-ultraviolet and visible 

 regions for E. colt on agar. (Luckiesh et al., 1947 ; Hollacndcr and Claus, 1935-36.) 



no effects on fungi are reported, the curve probably applies only to the 

 more susceptible organisms, at wave lengths greater than 3600 A. 



THE UNIQUE 2537 A MERCURY LINE 



Sixty per cent of the electrical input to a low-pressure mercury arc is 



converted directly into radiation of wave length 2537 A. Wave length 



2537 A produces 85 per cent of the maximum germicidal effect on most 



bacteria, fungi, and viruses which is possil)le at ^2650 A. This efficient 



production of ultraviolet of nearly optimum germicidal wave length is 



one of the more unusual coincidences in biophysics. The shape ot the 



germicidal-action curve is su(^h that the effectiveness of 2537 A ultraviolet 



is only 10-20 per cent less than the maximum effectiveness possible at 



2650 A, an uncertainty well within the variations in the action curves for 



various organisms and within the experimental errors inherent in the 



determination of the curves. For these reasons the low-pressure mercury 



arc has been selected as the practical source of ultraviolet for germicidal 



effects. About the only practical interest in the germicidal-action curve 



is to appraise the relative inefficiency of high-pressure mercury arcs in 



fused (luartz glass, the only artificial sources of ultraviolet that are at all 



comparable with the low-pressure arcs. In Fig. 2-5 the action curves 



are superposed on block diagrams of the relative line intensities of typical 



high- and low-pressure mercury arcs. The block diagrams are calculated 



on the basis of etjual amounts of power (in watts) into th(> two types of 



arcs in order to show graphically their relative ultraviolet efficiencies as 



well as their germicidal and erythemal effectiveness. 



