APPLICATIONS AND SOURCES OF ULTRAVIOLKT ()0 



The mercury column in the tube absorbs almost all the energj^ redirected 

 to it by the reflector and prevents higher utilization. 



OZONE FORMATION 



One-tenth to 5 per cent of the mercury line 1849 A energy is trans- 

 mitted by the glasses used for the tubes of germicidal lamps as indicated 

 in Fig. 2-oa. Energy of this wave length, transmitted only a few inches 

 through air, easily breaks the weak bonds of the oxygen molecule to 

 permit the formation of ozone near the ultraviolet source. Roller 

 (1946) found such ozone to have a half life of 15 hr, from concentrations of 

 several hundred parts per million, in the dry glass containers in which it 

 had been formed. Ewell (1942) had found, however under more prac- 

 tical conditions, that humidity, light, 2537 A ultraviolet, and surface 

 absorptions greatly catalyzed the reversion to oxygen. Under such 

 conditions he found ozone to have a half life of 2-3 min when irradiated 

 with 2537 A ultraviolet and of 6-7 min when not irradiated, in both cases 

 from concentrations of 3-4 ppm. 



Since this ozone diffuses throughout an irradiated space, its inherent 

 instability is considerably increased by 2537 A energy, and short-lived 

 atomic oxygen occurs in a uniciue manner. Commercial sources permit 

 air disinfection with eciuilibrium ozone concentrations less than the 1 

 part per 10 million considered permissible by the American Medical 

 Association (1948). Other sources provide the somewhat higher concen- 

 trations traditionally used in certain food-storage applications, where the 

 odor-masking effects of ozone and its concentration by absorption on 

 moist surfaces may be of some value. 



PHOTOCHEMICAL EFFECTS OF 2537 AND 1849 A ENERGY 



The photochemical actions of the 2537 and the 1849 A energy are out- 

 side the scope of this chapter except in so far as they are incidental to some 

 of the practical applications in other fields. The 2537 A energy con- 

 siderably increases the normal oxidizing action of oxygen without its 

 obvious ionization. This is greatly increased wherever there is ioniza- 

 tion and ozone formation by 1849 A energy. In all germicidal applica- 

 tions of the ultraviolet the possibility of objectionable chemical changes 

 should be investigated, e.g., formation of phosgene and hydrogen chloride 

 in poorly ventilated dry cleaning rooms where carbon tetrachloride may 

 be used, formation of hydrogen sulfide and mercaptans in egg-drying 

 plants where egg powder may be in the air, and modification of the flavor 

 of irradiated foods such as meat, milk, cheese, and butter. 



TEMPERATURE AND VENTILATION 



Like fluorescent lamps, commercial germicidal lamps are designed to 

 operate under average conditions of room temperature and ventilation. 



