Al'l'LICATIONS AND SOURCES OF ULTRAVIOLET 



49 



The same attenuation law also covers the dilution of fluid-borne con- 

 tamination by the admixture of a neutral and sterile fluid when the unit 

 volume of diluent is substituted for / in the exponent. 



UNIT KILL 



This relation suggests 63.2 per cent as a basic unit of sanitation for 

 which the term "lethe" has been suggested. With air sanitation in mind, 

 Wells (1940) has also used the term for a unit ultraviolet exposure pro- 

 ducing a 63.2 per cent kill of a standard organism (E. coli) under elabo- 

 rately specified conditions. A lethal exposure then becomes equivalent 



/xw/cm^ TO KILL IN I min 



2 3 4 5 6 7 8 9 10 11 12 13 14 15 

 MINUTES TO KILL WITH I //watt/cm^ 



Fig. 2-6. Typical data from Fig. 2-2 shown on a logarithmic scale of percentage of 

 survivors and uniform scale of exposures. 



in effect to the air change of mechanical ventilation, as discussed later. 

 The lethe unit of kill is indicated in Figs. 2-2 and 6. 



The exponential form of the curve of killing as a function of exposure 

 has also led to the suggestion of a unit kill of 50 per cent, by analogy with 

 the half-life rating of radioactive materials. 



The logarithmic nature of the ultraviolet germicidal efi^ect is illustrated 

 in Fig. 2-6 where the data on dry and wet E. coli are plotted on a loga- 

 rithmic scale of survivors and on an arithmetic scale of exposures. Most 

 of the published data plot as straight lines on such scales within the 

 experimental errors of the measurements [see Lea (1946) for a discussion 

 of methods of plotting such data]. An arithmetic or linear scale of kill is 

 used in Fig. 2-2, along with a logarithmic scale to e.xpand the range of 



