DEGREE OF ERYTHEMA 135 



Degree of Erythema 



o 



Using the homogeneous emission line of mercury at 2967 A as a source, 

 a minimum perceptible erythema is denned as that produced by 180,000 

 ergs per square centimeter. For an exposure of 15 minutes a radiant 

 flux intensity of 20 microwatts per square centimeter would be required 

 (Council on Physical Therapy [1932], [1934]). 



Medical authorities usually recognize four stages of erythema. From 

 the viewpoint of therapy the first three stages may be beneficial, depend- 

 ing upon the objective sought. The final blistering stage may even 

 be curative locally; it is, however, in general pathological. The first 

 three vary visually from a faint redness to a vivid redness which is the 

 more lasting. For midsummer sunlight the relative exposure times, for 

 an assumed average untanned skin, are as follows : 



Degree 1 , minimum perceptible erythema 1 . 



Degree 2, vivid, producing moderate tan 2.5 



Degree 3, painful " burn " 5.0 



Degree 4, blistering 10.0 



From this point of view the minimum perceptible erythema is one 

 that disappears in 24 hours. These relative values are only approximate 

 and are modified by the degree of pigmentation, but in general they 

 represent a good working range for time of exposure and erythema pro- 

 duced. In order to save time of exposure ultraviolet-ray therapy has 

 been generally investigated and practiced with high-intensity sources 

 of ultraviolet radiations in which exposures of a few minutes are usually 

 resorted to. The conclusions gained from high-energy exposures for 

 short periods of time, however, cannot be applied to the production of 

 erythema by moderately intense sources over longer periods of time. 



TABLE IV-1 



Erythemal Effectiveness of Radiation in Range 

 2400 to 3300 A, 2967 A Taken as Unity 





