APPLICATIONS AND SOURCES OF ULTRAVIOLET 77 



Air disinfection would seem to have most of the possibilities and 

 limitations in health value of unusual amounts of ventilation with outdoor 

 air, amounts ordinarily impractical in the cost of moving and heating. 

 Ultraviolet energy in germicidal barriers across openings would seem to 

 provide a bacterial isolation of rooms and people where doors or glass 

 partitions may be impractical, as in some hospital infant wards. Ultra- 

 violet barriers in air ducts can supplement and in some cases serve the 

 purpose of excessive filtration of duct air, as in pharmaceutical factories. 



FLUID DISINFECTION 



When intervening media are gases of negligible absorption and licjuids 

 of so great an absorption that the effective penetration distance is 

 negligible, the intensity at the irradiated surface is obviously dominated 

 entirely by the distance through the gas. When, however, there is 

 irradiation through a gas such as air and into the mass of a licjuid of low 

 or intermediate absorption, the intensity at any given point in the licjuid 

 is determined primarily by the total distance from the source to the 

 point, and secondarily by the absorption from the surface of the liciuid 

 to the same point. This absorption can vary over a 10,000-fold range, 

 from water of low iron content which can be disinfected in a duct in much 

 the same way as air, when due allowances are made for its greater absorp- 

 tion and the increased ultraviolet tolerance of wet bacteria, to milk and 

 serum which must be processed in films of thicknesses less than a few 

 thousandths of an inch. 



WATER DISINFECTION 



Water was the first liciuid to be disinfected by ultraviolet and with 

 commercial equipment using high-pressure mercury arcs. The method 

 could not compare with chlorine disinfection, economically, and it did not 

 provide any evidence of effective use as does "residual chlorine." 

 Although low-pressure sources of germicidal ultraviolet have greatly 

 reduced the cost, ultraviolet is now used only in instances where chemical 

 methods or boiling cannot be tolerated and where there may be routine 

 bacteriological control, such as in beverage, food, and pharmaceutical 

 processes. 



Water-borne E. coli recjuire an 8-10 times greater ultraviolet exposure 

 for a given kill than when air-borne, 150-200 ultraviolet /iw-min/cm- for 

 a theoretical 99 per cent kill in distilled water (Luckiesh and Holladay, 

 1944). The ultraviolet absorption of all water is much greater than that 

 of air and of itself varies more than tenfold from a 90 per cent absorption 

 in 5 in. to the same absorption in 50 in. (Fig. 2-16, from data by Luckiesh 

 ('/ a/., 1947, 1949). This variation is apparentl}^ almost entirely due to 

 dissolved iron salts. Since water readily acciuires sufficient iron for such 

 \ariations in absorption from contact with iron pipes and storage tanks, 



