494 HAUIATION lilOLOGY 



ivaches the mali)if;hiaii layer itself, where the principal photochemical 

 reaction of sunburn takes place. Rottier (1952) arrives at a similar pic- 

 ture from reflection measurements. An exact determination of the 

 absorption in that layer when in aitu would recjuire that its thickness and 

 that of the corneum, as well as the attenuation coefficients, be accurately 

 known. The corneum is formed from dead cells which have been pushed 

 up from the malpighian layer as new cells are formed there; the elements 

 of the corneum which so effectively scatter the radiation represent the 

 casts of these dead cells. There is no very exact line of demarcation 

 i)et\ve(Mi the two layers, the thicknesses of which vary from place to place 

 on the body of a fi;iven person and from person to person. \'arious factors 

 cause thickening of both layers, one of which is exposure to ultraviolet 

 radiation. The effect of this on the erythemal threshold will be discussed 

 later. 



Of particular interest with regard to the erythemal spectrum is the 

 correspondence of the maximum of absorption at 0.28 m with the sharp 

 minimum in the erythemal spectrum, clearly shown in Fig. 13-2. The 

 strong absorption l)y the corneum in the region of 0.28 m probably 

 accounts to a considerable extent for the minimum of effectiveness of 

 these wave lengths in producing erythema, a point first made by Hauisser 

 in 1928. It is obvious that with a spectrally selective absorbing layer 

 (the corneum) superficial to that in which the photochemical reaction 

 occurs (the malpighian layer) the measured erythemal spectrum does not 

 reflect directly the nature of the absorbing substance which is concerned 

 in that reaction. To reach the true action spectrum at the level of the 

 malpighian layer, it would be necessary to make appropriate corrections 

 for the attenuation of the radiation by the corneum, but this can be done 

 only roughly. Some attempts at this will be discussed later on, but the 

 inexactness of the measurements on which they must be based should 

 not be lost sight of. 



The interpretation of the erythemal spectrum is fuither complicated by 

 the findings of Rottier (1952, 1953) and Rottier and Mullink (1952), who 

 present evidence that the corneum participates actively in the erythemal 

 response. They suggest that two photochemical reactions are concerned, 

 one in the corneum and one in the malpighian layer. Dilator substance is 

 contributed to the eryth(>mal lesponse by both reactions. })ut, since there 

 appear to be two different light absorbers involved, the dilator substances 

 presvmial)ly differ also. These investigators suggest that the light 

 absorber in the corneum may be a sterol. This active participation of the 

 corneum does not, of course, prevent it from acting also as a spectral filter 

 for the radiation reaching the malpighian layei', so that what has been said 

 here about the effect of absorption by the conieuni is .still pertinent. It 

 simply adds one more complicating factor that must be taken into 

 account. 



