Sunscreen effects in skin analyzed by photoacoustic spectroscopy

Abstract
In this work, photoacoustic spectroscopy (PAS) was employed to characterize samples of commercially available sunscreen (SPF15) and the system formed by sunscreen plus skin (topically applied sunscreen). Measurements were performed at 70Hz, in the wavelength range that corresponds to most of the ultraviolet (UV) radiation that reaches Earth. The absorption spectrum of sunscreen was obtained in vitro and in situ., showing that the sunscreen analyzed presents an effective absorption of the UV radiation After that, the PAS technique was used to monitor the absorption kinetics of sunscreen applied to human skin (abdomen) samples, characterizing alterations in the human skin after application of sunscreen. This was done by applying the sunscreen in a skin sample and recording the absorption spectra in regular time intervals, up to 90 minutes after application. Measurements show that light absorption by the system sunscreen plus skin stabilizes between 25 and 45 minutes after sunscreen application. This agrees with the instructions given by the producers about the need of applying the sunscreen at least 30 minutes before sun exposition. The requirement to periodically reapply the sunscreen is confirmed by the progressive decrease in the level of UV absorption as a function of time.