APPLICATIONS AND SOURCES OF ULTRAVIOLET 67 



ULTRAVIOLET DISINFECTION 

 Practical uses of the lethal action of ultraviolet energy are limited only 

 by the practicality, in each case, of providing an adequate exposure 

 (nitensity times time) on microorganisms. Exposure time is usually 

 defined by mechanical conditions leaving little chance for radical change, 

 and the exposure intensity must be adjusted for an adequate exposure 

 product. Provision of an adequate ultraviolet intensity is conditioned 

 on two basic and independent factors: (1) the variation with distance 

 from the source or sources and (2) the absorption of the ultraviolet by 

 intervening media. The variation with distance has been represented in 

 general terms as insignificant within a few inches from a relatively large 

 assembly of sources and reflectors, as inverse with the distance for a few 

 inches from single tubular sources and reflectors and a few feet from large- 

 area sources, and as inverse with the square of the distance at greater than 

 source-length distances from single tubular sources and reflectors, as well 

 as at considerably greater distances from large areas. The absorption 

 of germicidal ultraviolet by air is entirely negligible even for the irradia- 

 tion distances of large auditoriums. 



AIR DISINFECTION 



Ultraviolet air disinfection is commonly accomplished by placing germi- 

 cidal lamps in the rooms or in the air ducts serving such rooms. The two 

 methods well illustrate the definition of exposure times by the mechanics 

 of the problems, with adequate exposure intensities to be provided if pos- 

 sible. As detailed later, an effective exposure for air disinfection is 

 15,000 ergs/cm^ or 25 ultraviolet /xw-min/cml In the upper air of 

 occupied rooms, practical exposure times may be 1-5 min, and eff'ective 

 intensities may be 25-50 ultraviolet Mw/cml In air ducts the exposure 

 times may be H-}i sec, and the corresponding intensities may be 10,000- 

 25,000 ultraviolet /xw/cm^. 



DUCT AIR DISINFECTION 



The disinfecting exposure of duct air is defined by its transit time 

 through an ultraviolet-filled zone of definite length and by the average 

 ultraviolet intensity during the transit time throughout that zone. In 

 Fig. 2-2, 15 ultraviolet MW-min/cm^ is suggested as a disinfecting exposure 

 for most air-borne microorganisms except fungi. In an air duct with a 

 cross section of 2 sq ft and a rating of 1200 cu ft/min, the linear flow of 

 600 ft/min through an ultraviolet-filled zone 3 ft long provides an expo- 

 sure time of 0.005 min. The average ultraviolet intensity throughout 

 the zone must then be 3000 ultraviolet yuw/cm^, or about 1800 ultraviolet 

 mw/sq ft for an exposure of 15 ultraviolet ^w-min/cm-. The average 

 intensity throughout a cylindrical zone radially irradiated by a linear 



