Sunscreen in vitro spectroscopy: application to UVA protection assessment and correlation with in vivo persistent pigment darkening

In the present study, we have described an in vitro spectroscopic method to evaluate the sunscreen products for UVA sun protection factor. The roughened PMMA plates have been used as a transparent substrate on to which the test product is spread. The UVA protection factors have been deduced from the UV-transmittance data measured in the UVA area. In order to be as close as possible to the in vivo protection factors, issued from the PPD end-point, the treated polymethylmethacrylate (PMMA) plates are submitted to different UV-irradiation doses, before the measurement. The correlation in vitro/in vivo is poor when the sunscreens are not irradiated. A UV dose of about 2 minimal pigmenting dose (MPD) is enough to achieve a good correlation between in vitro and in vivo data issued from the 13 tested sunscreens. These results are consistent with the fact that the photostability of sunscreens is challenged during an in vivo PPD test.