APPLICATIONS AND SOURCES OF ULTRAVIOLET 69 



and bacterial contaminants to be a continuous hazard to the products. In 

 these instances, ultraviolet air disinfection can perform a job which is not 

 yet possible by available methods of air washing and filtration. 



ROOM AIR DISINFECTION 



Ultraviolet disinfection of air is accomplished in occupied rooms by 

 germicidal tubes in cylindrical parabolic refiectors which are designed 

 to project the energy for maximum distances through the air of the room 

 above the head level of the occupants. It is accomplished in vacated 

 rooms or where protection of the occupants can be provided by bare 

 germicidal tubes centrally placed in the rooms or on the ceilings. Because 

 of the greater distances the variations in ultraviolet intensities are much 

 greater through irradiated rooms than in irradiated air ducts. Fortu- 

 nately the convective circulation and the relatively low-intensity long- 

 time exposures practical in irradiated rooms provide an average intensity 

 exposure such as is obtained in air ducts only by induced turbulent flow 

 or by the use of many separate sources of ultraviolet. 



Unoccupied Rooms or Rooms with Occupants Protected. In the rela- 

 tively simple case of the vacated room or where occupants may be ade- 

 quately protected, effective ultraviolet intensities may be provided by 

 centrally placed bare ultraviolet sources. The effective intensity is then 

 determined entirely by the time available to disinfect the air. Assuming 

 this time to be 5 min, the intensity for an exposure of ultraviolet ^w- 

 min/cm- needs to be only 5 ultraviolet /xw/cm'. This intensity which 

 can be provided by the G36T6 tube of Fig. 2-9 at a distance of 12 ft will 

 disinfect not only the air but also the walls. Bacteria which might be 

 deposited on the walls from the air are thus subjected to only the mini- 

 mum intensity present in a 20- by 20-ft room, such as a hospital room 

 between occupancies or a room in a pharmaceutical factory. In rooms 

 where only air-borne bacteria are a problem the average rather than the 

 minimum intensity becomes the basis of installation, and the same bare, 

 centrally placed tube will provide an average intensity of 5 mw^sq ft 

 throughout a 40- by 40-ft room to disinfect the air in only 5 min. 



Occupied Rooms. Occupants in a room add three serious complications 

 to ultraviolet air disinfection that compel entirely different approaches 

 to the theory and the practice. In the occupied room the problem is not 

 the simple one of cleaning up the residual contamination but the dynamic 

 one of killing or removing air-borne microorganisms as rapidly as they 

 appear from the noses, throats, and clothing of the occupants. The kill- 

 ing or removal must be in such a way as to reduce to a minimum their air- 

 borne life under an equilibrium condition of origin, of necessity several 

 feet from the place of their killing or removal. In the occupied room the 

 maximum intensities tolerated on more sensitive faces range from 0.1 

 ultraviolet mw/sq ft for continuous exposure to 0.5 ultraviolet mw/sq ft 



