APPLICATIONS AND SOURCES OF ULTRAVIOLET 



53 



erythemal exposures are about 10 times the germicidal and are comparable 

 with the fungicidal exposures, so that the time element becomes important 

 in practical applications. Intensities, which are germicidal within the 

 few seconds and minutes reciuired, in many cases become erythemal with 

 10-15 times longer exposures of the face and eyes, thus making some form 

 of protection usually necessary. As suggested in Fig. 2-76, an exposure to 



1,0 00 500 



80 



250 



LINEAR AIR SPEED, ft/mm 

 100 50 25 to 5 



10,000 

 8,000 



6,000 

 5,000 

 4,000 

 3,000 



2,000 



80,000 



60,000 

 50,000 

 40,000 



30.000 

 20.000 



q: 



UJ 



< 



O 

 < 

 O 



o 



a: 

 o 



t,000 

 800 



< 



u. 



CO 



o 



o 

 S 



o 

 > 



< 



0.001 0.0O2 0.005 OOI 002 0.05 01 0.2 



EXPOSURE TIME IN AIR DUCTS, min/ft 



Fig. 2-8. Reciprocity of time and intensity for short-time exposures in air ducts. 



2537 A energy of 450,000 ergs or 750 MW-min/cm- will produce a minimum 

 perceptible erythema of many skins (see Chap. 13, this volume). 



AMERICAN MEDICAL ASSOCIATION TOLERANCE 



The American Medical Association (1948) specifies 0.5 M^/cm"^ as the 

 maximum permissible ultraviolet intensity for 7-hr day exposures, an 

 exposure of 126,000 ergs, or 210 MW-min/cm-, but they reduce this to 144 

 MW-min/cmVday (an intensity of 0.1 fiw/cm^) for contimious exposure 

 and acknowledge that the specification is based on experience with wave 

 lengths 2800-3200 A rather than wave length 2537 A. 



