Several methods and standards have been proposed to determine the protection factor of sun products using indicators such as SPF, UVA protection factor (UVA-PF) and critical wavelength (CW) or broad spectrum efficacy. Although in vivo tests have interesting advantages, in vitro methods are more frequently used for practical, economical and ethical reasons. Recently, several key parameters were identified1–4 for the improvement of in vitro sunscreen testing.
Despite large efforts by the cosmetics industry to determine the photostability of sunscreens, the conditions for UV exposure have not been well-studied. Without strictly controlling the characteristics of instruments, the methods used would hold little value and lack inter-laboratory reproducibility. Unfortunately, even if the curve aspect and level of irradiance is compulsorily checked, external characteristics of irradiation instruments such as the temperature at the substrate surface, beam uniformity and air flow during UV exposure are not yet controlled.
Focusing on these last points, all known parameters were fixed in the study described here, and 22 sunscreens were tested. This work is part of a larger program aimed at reproducibility optimization by identifying, demonstrating and controlling the parameters that influence in vitro UV values on a large selection of products.
Materials and Methods
Solar simulator: Two instruments were used in this study to highlight the importance of controlling external parameters. First, a long-arc xenon solar simulatora (Simulator A) with poor temperature control, poor beam uniformity and uncontrolled air flow was used. Second, a short-arc xenon solar simulatorb (Simulator B) with high temperature control during UV exposure, high beam uniformity and no air flow was used. To comply with the U.S. Food and Drug Administration’s (FDA) final rule for OTC sunscreens,5 a fixed irradiation dose of 800 J/m², an efficient equivalent to 4 MEDs, was used for both instruments.
Sunscreens: Twenty-two products with different properties and cosmetic presentations were selected. Among the samples, five were identified as photo-unstable, eight as thermo-sensitive and nine as sensitive to air flow, as they were presented in liquid form.