Multicenter methodology comparison of the FDA and ISO standard for measurement of in vitro UVA protection of sunscreen products.

In vitro standard methods are available and accepted worldwide to assess UVA protection of sunscreen products. Though, harmonisation of methods has made progress in the last decade, still two differing methods - one by FDA the other by ISO - are in use. In a multicentre study including 9 centres in Germany, 4 different commercial sunscreen products were assessed using both methods to discover their similarities and differences. UVA protection factor and Critical Wavelength were detected at various substrate type (sandblasted versus moulded PMMA plates), at different surface roughness of the plates as well as at different product application dose using two different irradiation spectra. Results: The strongest influence on UVA protection factor results from the surface roughness of the plates. Depending on the roughness (accepted range of 2 to 7 µm in the FDA method) a variability in the UVA protection factor of up to 25% was observed, while the much narrower definition of plate roughness by ISO (4.5 to 5.2 µm) had no relevant influence on the test results. Sandblasted plates in our assessment led to higher UVA protection factors and produced less scattered results compared to moulded plates. These differences were not pronounced. Application dose and spectra of the irradiation source were of negligible influence on UVA protection factor results for the investigated UV-filter combinations. The UVA protection factor which is the endpoint of the ISO method was found to be a parameter with a high potential to differentiate among different test products. The endpoint of the FDA method - the Critical Wavelength - was found to be an unambitious endpoint. Insensitivity to all described modifications of the method was observed. All investigated products performed similar and passed the Critical Wavelength criteria independent of method and parameters.

In vitro water resistance testing using SPF simulation based on spectroscopic analysis of rinsed sunscreens.

OBJECTIVE: Water resistance retention (WRR) is the third performance attribute of sunscreens. Today, the standardized method for testing WRR is performed in vivo. For screening purposes, an in vitro method is highly preferable. Up to now, however, available methods have failed in accurately predicting in vivo WRR. In this study, we examine a novel in vitro approach aimed at enhancing in vivo prediction of WRR. METHODS: We investigated two approaches, including a traditional procedure referred to as the 'plate method', which involves measuring in vitro sun-protection factor (SPF) before and after water immersion, and a new approach termed the 'solution method'. The latter employed a computational method to calculate the SPF using UV transmittance measurements of a sunscreen solution, obtained by rinsing the substrate without water immersion and with water immersion. It also had a model function for describing film irregularity. The aim was to avoid the effect of substrate-to-product affinity on the film distribution, which impacts the measured absolute value of the in vitro SPF occurring in the conventional approach. For both methodologies, we assessed two substrates for the sunscreen application based on different polymers, the widely used polymethylmethacrylate (PMMA) and the ethylene methacrylate acid copolymer (EMA). RESULTS: The agreement between in vivo and in vitro WRR values obtained from each of the four test conditions was analysed using different sunscreens. No correlation was found between in vivo and in vitro WRR using the plate method, independently of the plate type. In contrast, the solution method, using EMA plates, revealed a significant correlation between in vivo and in vitro results, with an especially high correlation for in vivo non-water-resistant sunscreens. The results of two operators were comparable. The solution method was unsuitable for PMMA plates. CONCLUSION: Despite the small discrepancy remaining between WRR values obtained from the in vitro solution method and in vivo method, which itself shows variability in results, this work provided a new insight into the in vitro testing of water resistance of sunscreens.