Sunscreen tests: Correspondence between in vitro data and values reported by the manufacturers

Background: In vitro sunscreen tests are diffusively used to test both the sun protection factor (SPF) and the photo-stability of filters. Spectrophotometric measurements of the absorbance of ultraviolet radiations through a sunscreen applied on a suitable substrate allow a rapid evaluation of its protection factor both at short and long wavelength ultraviolet radiation (UVB and UVA). Objectives: The objective of this study has been to demonstrate if Teflon can be adopted as substrate both for SPF evaluation and photo-stability tests. Moreover, we have investigated if there is a correspondence between in vitro SPF measurements and values reported by manufacturers on sunscreens. Material and methods: Teflon has been used to perform several photo-stability tests by irradiating the filters with different wavebands and analyzing the combined effect of UVand infrared (IR) light. Similar analyses have been carried out using PMMA Plates, which is the standard substrate for UVA in vitro test. Results: We have demonstrated that it is possible to establish a good correspondence between in vitro SPF and values reported by manufacturers on sunscreens. We have also verified that the in vitro/label SPF correlation curve depends on the quantity of product applied while this does not seem to be true for other parameters like Critical Wavelength and UVA ratio. With regard to photo-stability studies, our results indicate for the first time that IR irradiation may have a role on photo-degradation. Conclusions: The results show that there is a good correlation between the in vitro SPF determined by the present method and the SPF reported by the manufacturer.