514 RADIATION BIOLOGY 



■with the mercury arc. Mcusuromcnts made with the mercury arc gave 

 much higher \ahies of /■• than those obtained on the same persons witlj 

 the carhoii aic The six'cl rum of neither of these sources closely 

 resembles that of sunHght, and it is obvious that, with such wide varia- 

 tions in the ol)tainal)le values of /-*. tests of sunburn preventives with 

 sources other than sunlight can have little practical significance as (|uanti- 

 tative measures. Sunlight itself is a variable (juantity, its intensity and 

 spectral distribution changing radically with latitude, season, and time of 

 day, and the validation of laboratory tests against actual field conditions 

 is therefore beset with many unavoidable difficulties. Moreover, the 

 diflRculties of assembling an adequate number of human subjects under 

 conditions appropriate for such validation may be overwhelming. It 

 may be tempting to choose some simpler method without validation (see 

 Giese and Wells, 194Ga, b), but the dangers inherent in doing so must be 

 obvious from what has just been stated. Eventually, physical methods 

 such as that of Luckiesh et al. (1946) may replace the laborious testing on 

 human skin, but these too will recjuire validation (see Blum c( «/., 194(). 

 for further discussion of methods). It should be obvious that tests with 

 methods chosen uncritically and without validation may be misleading. 



MECHANISMS 



Up to this point sunburn has been treated as a complex physiological 

 response further complicated by difficult optics. Only descriptions of 

 various aspects of this complex have been given, with little attempt to 

 explain the more intimate underlying mechanisms. It is now time to 

 attempt to put together a general picture of the whole. In the scheme 

 arranged in Fig. 13-11, it is represented that various physiological 

 responses are mediated by specific substances elaborated as a result of the 

 action of ultraviolet on living cells. In erythema this seems certainly to 

 be the case, since wave lengths completely absorbed in the epidermis pro- 

 duce dilation of vessels in the underlying dermis, and such action seems to 

 be interpretable only in terms of a transportable mediating substance. 

 In other cases the presence of a mediating substance in the scheme can be 

 considered as little more than a guess, and the tentativeness of the whole 

 must be emphasized. 



In the diagram (Fig. 13-11) the central theme evolves from an initiating 

 photochemical reaction taking place in the malpighian layer, which results 

 in the elaboration of a number of substances by the cells of that layer. 

 Each of these substances induces a specific physiological response in the 

 immediate locus of its elaboration or in a neighboring tissue. Consider 

 first the nature of the initiating photochemical reaction in terms of the 

 light-absorbing substance. As has been shown, the possibility of identi- 

 fying this substance by means of the action spectrum is complicated by a 



