The Penetration of Ultra Violet Light Into the Human Skin.

Abstract

The problem of the penetration of ultra violet light into the human skin is not yet solved. This problem is of great interest, however, particularly from the point of view that certain limited parts of the ultra violet spectrum produce certain characteristic biological effects, such as erythema, pigmentation, bactericidy, antirachitic action, and others. For the scientific explanation of this reaction it is of interest to know to what depth of the skin these biologically active rays penetrate and by what substances they are selectively absorbed. In order to contribute to the solution of this problem absorption measurements were made for the various parts and biological constituents of the human skin.

Two methods have been used, the photographic and the photoelectric, both in connection with a mercury quartz arc and a Hilger spectrograph. The photoelectric method has the advantage that the whole spectrum can be obtained within a few minutes, and that it gives a crucial test about the end absorption in the far ultra violet. The photoelectric method gives more exact quantitative results, except at the farthest ultra violet end, where a trace of spectral impurity lowers the exactness of the measurements. Besides, it takes a long time to go over the whole spectrum, and it is difficult to keep the conditions of biologic specimens constant for such a length of time. Both methods used are superior to the thermoelectric method as their pronounced ultra violet selectivity makes them more independent from scattered visible and infrared light.

Results: (1) The difference between living and dead tissue kept in Ringer solution and on ice for one or two days is smaller than can be exactly measured. (2) The penetration of ultra violet light is stronger than given in the older literature, particularly by Hasselbalch, 1 and smaller than suggested by Macht and his co-workers.^2,3 (3) The various layers of skin (horny layer, epidermis, corium, sub-cutaneous fat, connective tissue, plasma, blood, melanin, etc.) exert a very different absorption and show characteristic selective absorption bands. (4) The corrected skin sensitivity curve toware erythema, as observed by Hausser and his co-workers 4 can be explained by the passive absorption of ultra violet by the horny layer and the active abfsorption by the proteins in the skin.