SUNBURN 491 



whether cells are living or dead. The corneum acts as a filter, preventing 

 a large proportion of the incident ultraviolet radiation from reaching the 

 malpighian, and until very recently it was thought to take no active part 

 in the sunbiu'n complex. Rottier (1952, 1953) and Rottier and Mullink 

 (1952) have now presented strong evidence, however, that photochemical 

 changes in the corneum may contribute erythema-producing substances. 



The Eryfhemal Threshold. The most obvious criterion for quantitative 

 studies of sunburn is the appearance of erythema. The erythema! 

 threshold, i.e., the amount of ultraviolet radiation required to produce 

 just perceptible reddening, has been quite generally used, and is up to now 

 the only feasible measure available. As will be seen, however, it is not a 

 very accurate one and should be interpreted with caution. The erythemal 

 threshold has been measured in various waj^s, but all involve more or less 

 the same type of operation. Usually the skin is covered with a template 

 having a series of small holes through which the ultraviolet radiation may 

 reach the skin. A different dose is applied through each of these holes. 

 The threshold is usually determined after some arbitrary period, say, 24 

 hr. At this time, if the doses have been chosen appropriately, the highest 

 ones will be represented by red areas corresponding to the positions of the 

 holes in the template. The intensity of the redness will fall off with the 

 dose, those areas having received the lowest doses showing no erythema 

 whatsoever. The dose which has elicited a just perceptible erythema, or 

 the average betw^een this and the next lower one which has produced no 

 erythema, is usually chosen as the threshold. The exact value for the 

 threshold will depend, of course, on the time elapsed between the exposure 

 to ultraviolet radiation and the time of observing, since the erythema 

 l)uilds up rather slowly and then falls off. Studies of the rate of increase 

 and decline of erythema have shown that these factors may vary widely in 

 different individuals (e.g., Schall and Alius, 1926). 



There are obvious sources of error in determining the erythemal 

 threshold. The presence or absence of a very slight reddening is a diffi- 

 cult end point and is subject to considerable uncertainty in reading since 

 the degree of flushing or tanning of the skin affect the contrast. On the 

 whole, photography has not helped to standardize such measurements, the 

 unaided eye being more sensitive and accurate. It is probable, in any 

 case, that inherent variability in the erythemal response is greater than 

 the errors in reading. ^^ The threshold not only varies widely among indi- 

 viduals but also from one area to another. For example, the palm of the 

 hand or the sole of the foot can hardly be sunburned. On the other hand, 

 the skin of the torso is rather uniformly sensitive. The threshold is 

 affected by the amount of previous exposure to ultraviolet radiation. It 

 may vary widely from one observation to another without known cause 



" For a discussion of errors of measurement of the erythemal threshold, see Blum 

 and Terus (19-46b). 



