A novel in vitro technique for measuring the water resistance of sunscreens

A novel in vitro technique has been used to assess the water resistance of topical sunscreens. The technique is based upon transmission spectrometry using excised human epidermis as the substrate. The sun protection factors (SPFs) of four sunscreens obtained prior to water immersion and after two immersions were compared with in vivo data determined using human volunteers. In every case there was close agreement between the SPFs obtained on dry skin and on skin that had been immersed in water. Not only does the technique provide a less expensive and less time consuming alternative to in vivo testing, it also offers the possibility to study the effect that water immersion may have on the absorption spectrum of products which cannot be determined in vivo. This can give valuable information on which, if any, of the active ingredients present in sunscreens are being preferentially removed by water. In addition, the method can be easily adapted to study the effects on sunscreen substantivity of other factors such as water temperature, salinity and turbulence.