70 RADIATION BIOLOGY 



for 7 lir of exposure per day. Altliough ideally placed, these intensities 

 are not high enough for rapid disinfection. In occupied rooms the value 

 of air disinfection is primarily that of the health value in removal of 

 microorganisms. In the absence of any criteria of the health value or 

 hazard of air-borne organisms, the natural ventilation believed of value 

 becomes a secondary criterion. 



Conrcrtirc Circulation and Upper-air Irradiation. The (;onvective cir- 

 culation of air, by which the heating of a room from a few localized 

 sources of heat is possible, involves the use of vertical components which 

 provide an interchange of air between the upper and lower parts of a room 

 equivalent to from several air changes per hour to several per minute. 1 n 

 occupied rooms the basic convective ciicul.itioii is increased by the body 

 heat, the breathing, and the movement of the occupants. These factors 

 increase the circulation in proportion to the crowding, the contamination, 

 and the need for air disinfection. Ultraviolet disinfection of the upper 

 third or fourth of a room can provide in these portions of the room a 

 reservoir of air for the dilution of the lower air at rates equivalent to 

 unusual natural or mechanical ventilation. Lacking more direct criteria 

 of value, it becomes convenient to consider ultraviolet air disinfection as 

 equivalent to and a substitute for outdoor air for sanitary ventilation 

 purposes. 



Upper-air Method of Disinfection. Luckiesh and Holladay (1942b) 

 treat the upper part of a room as a duct containing air in random circula- 

 tion at a velocity (5-10 ft/min) equivalent to about one one-hundredth 

 the linear velocity in wall ducts and room units and irradiated with an 

 average ultraviolet intensity (0.025 ultraviolet watt/sq ft) which is about 

 one one-hundredth that provided in air ducts. The upper part of the 

 room is then treated as a duct serving the lower part. There is, however, 

 no such definite separation between the two parts of the room as this 

 oversimplification suggests, and the following analysis (Buttolph, 1951) 

 is believed to be more realistic. 



An ultraviolet (2537 A) intensity of 5 ultraviolet mw/sq ft, effective 

 throughout a cubic foot, will kill respiratory and E. coli test organisms 

 at the same rate as they might otherwise be washed or diluted out of the 

 same cubic foot of air by one air change per minute. This is the theo- 

 retical reduction of 62.3 per cent of Fig. 2-2. An additional air change or 

 an additional 5 ultraviolet mw/scj ft can dispose of 62.3 per cent of the 

 remaining 37.7 for a theoretical reduction of 82.5 per cent. The effect 

 is the same whether by 5 mw for 2 min or 10 mw for 1 min, two air 

 changes in successive miimtes or two air changes in 1 min. These rela- 

 tions are plotted on linear and on semilogarithmic scales for comparison 

 in Fig. 2-12. 



An average ultraviolet intensity of 5 mw/ s(i ft throughout the entire 

 cubage of a room would theoretically provide the disinfection equivalent 



