On the assessment of photostability of sunscreens exposed to UVA irradiation: From glass plates to pig/human skin, which is best?

Photostability of suncare products is a great area of interest since several sunscreens on the market are photounstable, and this is primarily a problem concerning the UVA region (320-400 nm). Here we report a comparative study on the photostability assessment of two commercial sunscreens with same SPF, spread onto glass plates or onto full thickness pig ear skin or human/pig SCE membranes, and exposed to 183 kJ/m2 UVA. Absorbance spectra and lipid peroxidation (measured by TBARS production) were determined. The results indicate: (a) sunscreen performance consequent to UVA exposure is independent of whether it is spread onto a non-biological and chemically inert substrate such as glass, or on biological substrates such as skin/SCE membranes; (b) despite the same SPF, sunscreen performance and photostability can be very different; (c) the data on human SCE membranes are similar to those on pig SCE membranes, indicating the suitability of the latter as a model for human skin. However, since the results obtained using skin membranes, akin to the more realistic conditions of use in vivo, do not substantially differ from those obtained on glass plates, the method proposed here using the latter may be applied for rapid, inexpensive, efficacy screening of photostability of sunscreens. Photostability testing should be a mandatory requirement for safer sunscreen protection products, since the results clearly show that some are still far from perfect.