SUNBURN 



519 



stance is cut iivly liypothetical. Also indicated as a result of the injury to 

 epidermal cells is the formation of new melanin which is the other factor 

 contributing to melanization of the epidermis. A dotted line suggests the 

 direct participation of ultraviolet radiation in the formation of melanin as 

 a possible minor factor in melanization. Indicated on the extreme right 

 of the diagram is the darkening of bleached melanin which is brought 

 about by the longer wave lengths of the ultraviolet and the near-visible 

 spectrum, approximately 0.3-0.42 n. This is an oxidation of bleached 

 melanin by molecular oxygen as shown by ]\Iiescher and Minder (1939). 



1.0 I.I 1.2 1.3 1.4 



WAVE LENGTH, ^ 



Fig. 13-12. Spectral distribution of sunlight. 0, outside the atmosphere (air mass 0); 

 1, with the sun at zenith (air mass 1); 2. with the sun 60° from zenith (air mass 2). 

 Curves R and C indicate, respectively, the sensitivity of the human rods and cones; 

 the ordinate units are arbitrarily chosen. {After Blum, 1945; data from Moon, 1941.) 



Sunburn may logically be classed with other types of burn. There is a 

 ciuantitative aspect to be recognized in that the primary damage in sun- 

 burn is very superficial because of the low penetration of ultraviolet radi- 

 ation. As in the case of other superficial burns, some systemic involve- 

 ment may be expected when the burned area is extensive enough. Studies 

 of such systemic effects will not be discussed here, nor will the various 

 claims for therapeutic effects of ultraviolet radiation. For an account of 

 some of these, and of other effects of sunlight on man, the reader is referred 

 to an earlier review (Blum, 1945). 



Sunburn by Natural Sunlight. Many misconceptions about sunburn 

 arise from failure to take into consideration the character of sunlight as an 

 environmental factor. It is natural to evaluate the intensity of sunlight 

 in terms of perception by the human eye, but this may be very misleading 

 as regards the intensity of the sunburn-producing portion. The situation 

 is best illustrated by referring to the diagram in Fig. 13-12. which shows 



