In vitro assessment of sunscreen photostability: the effect of radiation source, sunscreen application thickness and substrate.

The photostabilities of four sunscreen products have been assessed in vitro by applying sunscreen to a substrate and measuring the spectral transmission prior to, and after exposure to a source of ultraviolet (UV) radiation. Results were independent of whether an application thickness of 1 or 2 mg/cm ; 2 was used, and whether the UV source was natural sunlight or a xenon arc solar-simulator. There were significant differences, however, between results obtained on a roughened quartz substrate and those obtained on excised human epidermis. It is unlikely that any substrate will give an exact representation of the in vivo situation and, indeed, both quartz and excised human epidermis have disadvantages associated with their use. However, the ranking of the four products in terms of their photostability was the same for both substrates. This implies that transmission spectroscopy, with either a quartz or a human epidermis substrate, can be used successfully to compare the photostabilities of different sunscreen products.