Observations on the mechanism of the protective action of sunscreens

A method is described for measuring the entrance into excised skin of ultraviolet radiation absorbing chemicals (UVRACs) following their application to the cutaneous surface in volatile, partially volatile or nonvolatile vehicles. Also a method is presented for observing changes in optical density (OD) of a sheet of stratum corneum subsequent to the application of an UVRAC and then washing it from the surface. Using these methods, p-aminobenzoic acid (PABA) and 2-ethylhexyl p-dimethyl aminobenzoate (O-PABA) have been studied. In the presence or absence of the nonvolatile vehicle, isopropyl myristate (IM), significant amounts of PABA enter the skin but almost all of the O-PABA remains on the surface. Nevertheless subsequently PABA is more easily removed by water than is O-PABA. When either UVRAC is applied to excised stratum corneum, the OD of the tissue increases immediately; only with PABA is there a further increase as it enters the skin. In vivo, delayed erythemal responses to 280-400 nm radiation of persons to whom the UVRACs are applied correlate well with the observations made on excised skin.