■ULTRAVIOLET LIGHT — GERSHENFELD 215 



certain germs, especially those producing disease, and to reduce the 

 total numbers of all forms of life p>resent, without damaging the 

 product under treatment. Pasteurization does not kill all germs and 

 accordingly does not effect a complete sterilization. 



Ultraviolet light as commonly used does not render materials sterile. 

 The term "sterilization" is used incorrectly when applied to the use of 

 ultraviolet light, since the complete destruction of all forms of life is 

 rarely attained by its application in practice. Furthermore this is 

 never sought, and in fact from a sanitary viewpoint is not necessary. 

 Ultraviolet radiation, when used, is employed to reduce the total germ 

 content and especially for the destruction of those organisms causing 

 disease or capable of decomposing materials. It is not practical to 

 effect complete sterilization. It is on this account that for want of 

 more suitable terms the words "sanitize" and "sanitization" have been 

 advocated in place of sterilization and disinfection. Sanitization is 

 that process which renders materials free of organisms that produce 

 disease, or are capable of decomposing substances, or which are indica- 

 tive of insanitary conditions. The term describes more satisfactorily 

 the action of ultraviolet radiation and has come into use especially 

 when the latter is employed as a sanitary aid. 



FACTORS CONCERNED IN EFFECTIVE RADIATION 



There is a variation in the apparent sensitivity to ultraviolet radia- 

 tion not only by different species of organisms but this variation is 

 found in the same species at different stages of its life cycle. The thick- 

 ness of a layer of organisms to be radiated and the fact that individuals 

 in a clump may shield others from radiation are to be considered. 



The amount of water vapor present has an influence on the effective- 

 ness of ultraviolet I'adiation. At ordinary room temperatures, much 

 more radiation is required to destroy germs at a high humidity than 

 at a low humidity. 



The action of ultraviolet light is practically a surface action. It 

 possesses only very slight penetrative powers. Radiation therefore 

 must reach all surfaces to be effective. In order to maintain proper 

 intensity, lamps should be cleaned, and regular and thorough cleaning 

 of materials and environments wherever practical will be most helpful. 



The intensity is influenced by the distance that the surface to be 

 treated may be from the ultraviolet radiations. The distance and the 

 time of exposure are important factors to be considered at all times 

 when using ultraviolet light for the destruction of germs. An indica- 

 tion of what this means is to note that a 30-inch lamp generating 

 mainly 2537 Angstrom unit wave lengths will kill almost all of the 

 commonly observed non-spore- forming bacteria within 1 minute, when 

 at a distance of 1 foot, but it will require 4 minutes if at a distance of 



