Study of the photostability of 18 sunscreens in creams by measuring the SPF in vitro.

The target of this research was to evaluate the photostability of various sunscreen agents incorporated into an O/W emulsion. The concept of photostability is very important in the field of solar protection. The effectiveness of the anti-solar products is quantified using a universal indicator: the sun protection factor (SPF). This number which can be found on packaging can be given in two different ways: by methods in vivo (Colipa method) and in vitro. It is this last method which was adopted for this study. According to selected filter UVB (currently directive 76/768/EEC modified authorized 18 filters UVB), we can obtain more or less effective creams. We chose the irradiation of sun lotions formulated using the authorized filters, used with their maximum amount of employment, in a Suntest, with an irradiance of 650 W/m(2) throughout variable time. With interval of regular time, one carries out a measurement of SPF in order to establish for each filter the kinetics SPF=f(time). An indicator of stability (t(90)) is then given. In this way, we could classify the filters by order of increasing photostability.