508 RADIATION BIOLOGY 



photoclu'inical reaction that is postuhittHl by the hiochoinical schomo just 

 discusst'd, hut both ioiii/.iufj; radiation and ultrax'ioh't injure the epidermal 

 cells. Moreover, melanization of human epidermis tollows injur}' by such 

 diverse agents as heat, lubbinji;, photodynamie action (photosensitized 

 oxidation) and wounds. I'nt'oitunatcly, there is no very good explana- 

 tion i'oi- melanization ot the epidermis caused by any of these agents, but, 

 until such matters are more clearly understood, the idea of direct forma- 

 tion of melanin by photochemical reaction, attractive though it may be, 

 should not be accepted without reservation. 



PHOTOSENSITIZATION 



Sensitization of the skin to light by exogenous agents cannot })e dis- 

 cussed here at length. It may be pointed out, however, that in human 

 skin there are at least two different types of such action. As examples, 

 sulfanilamide sensitization and the photodynamie action of eosin (the 

 cause of photosensitization by lipstick) may be chosen. vSulfanilamide 

 does not act as a true photosensitize!" — i.e., it does not participate as the 

 light absorber — but seems only to increase the injury to the skin brought 

 about by ultraviolet radiation. Eosin, on the other hand, acts as a photo- 

 sensitizer, absorbing the light and bringing about photochemical oxidation 

 of skin constituents. Photosensitization by eosin is inhibited by occlu- 

 sion of oxygen; photosensitization by sulfanilamide is not (I^lum, 1941b). 

 Photosensitization of human skin is discussed elsewhere by the author 

 (1941a), and other aspects of photooxidation will be treated by Norman 

 Clare in volume III of this series. 



PROTECTION AGAINST SUNBURN 



Natural Protection. Following even a mild degree of sunburn the ery- 

 themal threshold rises and may remain above its previous level for about 

 two months. The decrease in sensitivity to ultraviolet radiation takes 

 place at approximately the same time that suntan is developing. This 

 observation and the relative insensitivity of Negro skin to suidight led 

 long ago to the idea that the melanin pigment serves to protect the skin 

 against sunburn by absorbing the sun's rays (e.g., Davy, 1828; Wedding, 

 1887; Bowles, 1889). The experiment of painting the skin with an 

 opacjue material, e.g., India ink, and so obtaining protection from sun- 

 burn, has been repeatedly interpreted as supporting the idea of protection 

 by the melanin pigment (e.g., Davy, 1828; Finseii, 1900) which persists as 

 one of the most popular misconceptions regarding sunburn. 



The idea apparently remained unchallenged until about 1920, after 

 which objections were raised by a inimber of workers. Probal)ly the first 

 of these was With (1920) who found that areas of vitiliginous skin, which 

 do not develop melanin i)igment, can be rendered less sensitive to ulti-a- 



