APPLICATIONS AND SOURCES OF ULTRAVIOLET 



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in the room of one air change per minute, such as is thought to be desir- 

 able in very crowded rooms. Only one-tenth that intensity (0.5 ultra- 

 violet mw/sq ft) is, however, tolerated on sensitive faces for 7 hr per day 

 without objectionable "sunburning." So only about one-tenth of an air 

 change eciuivalent per minute is practical by this method. This is, 

 however, equivalent to the six air changes per hour, or 30 cu ft of out- 

 door air per child per minute sometimes specified for school rooms. 

 An average intensity of 5 ultraviolet mw/sq ft throughout a whole room, 

 with only 0.5 permitted in the lower 

 part of the room, can be obtained 

 b}^ an average of 14 mw/sq ft in an 

 upper third of the room or 18.5 in 

 an upper fourth. Since the ultra- 

 \'iolet in the lower part of the room 

 is due mostly to the diffuse reflec- 

 tion from the upper side walls and 

 ceiling, its intensity may vary little 

 throughout the entire area, especi- 

 all}^ under 10- to 12-ft ceilings. 

 However, since the ultraviolet in 

 the upper part of the room is pro- 

 jected through it from a few sources 

 (usually in cylindrical parabolic re- 

 flectors on the walls), the intensity 

 may vary from 2 or 3 ultraviolet 

 mw/sq ft to 2 or 3 ultraviolet 

 watts/sq ft (a thousandfold varia- 

 tion). Such an uneven distribu- 

 tion of the energy in the room is 

 effective only because the convective circulation exposes nearly all the 

 air to nearly the entire range of intensities to provide an integrated lethal 

 exposure, as with turbulent flow in an air duct. 



Hospital Room Disinfection. The first uses of ultraviolet for air dis- 

 infection in hospitals were to provide the equivalent of local curtains or 

 barriers between the surgeon and his operation (Hart, 1936; Overholt and 

 and Betts, 1940) and across the front of infant cubicles (Sauer et al., 

 1942; Del Mundo and McKhann, 1941; Robertson et al., 1939, 1943). 

 Ultraviolet intensities ranging from 20 mw at the floor to 200 mw at 

 head level can readily be provided. The 20-mw intensity becomes as 

 effective as the 200-mw intensity because the width of the di\'ergent beam 

 and the distance of the bacterial travel through the beam, and thus the 

 average exposure time, are ten times as great at the floor. These inten- 

 sities may produce erythema at head level in 1 min and at floor level in 

 10 min, so that such installations are limited to pharmaceutical plants 



Fig. 



and 



0.001 



10 20 30 40 50 



mwatt-min/sq ft OR mwatt-min/cu ft 

 2-12. Comparison of linear 

 logarithmic plotting of percentage sur- 

 vival of air-borne bacteria as a function 

 of dilution by air or by equivalent ultra- 

 violet irradiation. 



