This paper presents the results of the Alt-SPF consortium ring test, obtained by the in-silico methodology, and discusses these results in the context of existing standards. To evaluate the effectiveness of the in-silico methodology in correlating with these standards, a comparison was made between the in-silico sun protection factor (SPF) and the UVA protection factor (UVA-PF) and the values derived by standard in vivo SPF (ISO24444) and in vitro UVA-PF (ISO24443). The model utilises the same algorithm as in vitro measurements of SPF and UVA-PF, with the measured UV transmission substituted by calculated transmission simulated through an applied sunscreen film. The in-silico approach necessitates quantitative UV absorbance data of all UV-filters, their photodegradation and photointeraction properties, oil/water phase synergies and a model describing the irregular distribution of the film thickness on the skin. The performance factors are calculated using an initial analytical evaluation of the 32 test formulations of the ring test, to ascertain the presence and concentration of individual UV absorbers. The filter concentrations obtained are then processed via computational analysis to calculate the SPF and UV-PF values. It has been demonstrated that, in accordance with defined statistical parameters to characterize the ability of a method to correlate with the in vivo SPF standard, the in-silico methodology is a highly reproducible and accurate tool for SPF prediction. The alignment with the lowest measured in vivo SPF values serves to ensure the safety of the end consumer and is not a weakness of the method. Additionally, it has been demonstrated that the methodology facilitates precise prediction of UVA protection in comparison to the in vitro standard ISO 24443. These calculations can be employed to engineer novel sunscreens, thereby diminishing the necessity for ethically questionable and extensive laboratory measurements. In case still in-vivo measurements are needed, the methodology can deliver the frequently required SPF/UVA-PF values and may also support functions such as those of responsible persons, including safety assessors, in their evaluations.

Sunscreens for the photoprotection of human skin often are prepared as emulsions, containing organic UV-absorber molecules dissolved in the oil phase. The solubility of such oil-soluble UV-absorbers can be a limiting factor when aiming for high protection against UV-radiation. Possible synergistic effects of combinations of oil components toward UV-absorber solubility are therefore of great interest. Since a multitude of different combinations of oil components are possible, it would be desirable to predict synergistic effects by computational methods. As a model system, the solubility of a hydroxyphenyl triazine type UV-absorber was studied in several binary oil mixtures, experimentally and also by using a computational procedure based on density functional theory (DFT) and the continuum solvation model COSMO-RS. We have found good agreement of experimental and computational results. Computational methods may thus be employed to predict synergistic behaviour of solubility for systems containing two or more solvents.

Sunscreens are used to protect human skin against harmful UV radiation. Today there is a trend towards high sun protection factors (SPF) and good UVA protection. Methods for the assessment of SPF and UVA protection involve irradiation of the product, and the photostability properties of the sunscreen have an influence on its performance. Sunscreens often contain more than one UV filter. Some photolabile UV absorbers may be stabilized by the presence of other photostable UV-absorbers. Stabilization can be achieved just by a certain optical density due to the presence of such UV-filter substances. However, photostabilization may also be caused by quenching mechanisms, such as singlet–singlet or triplet–triplet energy transfer. Investigation of butyl methoxy dibenzoylmethane and ethylhexyl methoxycinnamate as photolabile sunscreens in the presence of either octocrylene or bis ethylhexyloxyphenol methoxyphenyl triazine showed that both mechanisms may apply. With the systems butyl methoxy dibenzoylmethane plus octocrylene and ethylhexyl methoxycinnamate plus bis ethylhexyloxyphenol methoxyphenyl triazine the quenching mechanism appears to be predominant.

2′-Ethylhexyl-4-Methoxycinnamate (EHMC), also designated as octinoxate, is an oily UV-absorber used in sunscreens for the protection of human skin against solar UV-radiation and represents one of the most employed UVB absorbers for that application. In water-in-oil emulsions EHMC was adjusted at a constant overall concentration, while changing the EHMC concentration in the oil droplets by adding a non-absorbing oil. In that way the EHMC concentration could be varied at constant optical thickness. Here we show that the kinetics of the photoreaction follows a second-order rate law, in line with the UV-induced [2 + 2]-cycloaddition reaction mechanism known for this UV absorber. The second-order rate constant decreased with higher overall EHMC concentration. This can be explained by the fact, that at higher overall concentration of the UV absorber not every EHMC molecule will absorb a photon due to the increased optical density, so that on average less photons are absorbed per molecule. On the other hand, the rate constant increases with decreasing polarity of the surrounding oil. Since the molar fraction of the trans -isomer of EHMC is augmented at lower polarity, more photons are absorbed in this case, as the strength of the absorption band of the trans -isomer is significantly higher than that of the cis -isomer. In conclusion, our experiments show that a high polarity of the oil phase and a high concentration of EHMC are advantageous for the photostability of this compound.

The sun protection factor (SPF) is the most important quantity to characterize the performance of sunscreens. As the standard method for its determination is based on clinical trials involving irradiation of human volunteers, calculations of sunscreen performance have become quite popular to reduce the number of in vivo studies. Such simulations imply the calculation of UV transmittance of the sunscreen film using the amounts and spectroscopic properties of the UV absorbers employed, and presuppose the validity of the Beer–Lambert law. As sunscreen films on human skin can contain considerable concentrations of UV absorbers, it is questioned whether the Beer–Lambert law is still valid for these systems. The results of this work show that the validity of the Beer–Lambert law is still given at the high concentrations at which UV absorbers occur in sunscreen films on human skin.

Photo-instability of common UV-filters is a well documented phenomenon. This study develops a method for concomitant measurement of photostability and photo-induced ROS generation in cosmetic formulations. Oil-in-water formulations containing three common UV filters (OMC, BMDBM, EHT), individually or combined, were further supplemented with phosphatidylcholine and exposed to UVA. All filters show spectral decrease after UVA exposure. OMC and EHT do not induce significant lipid-peroxidation (as measured by TBARS production) while BMDBM does. In the latter case, this is reduced when BMDBM is combined with OMC but not with EHT. Neither OMC nor EHT stabilize BMDBM with respect to loss of absorbance. ROS-generation assessed via TBARS formation was supported by EPR experiments. The UV-induced changes in UV-filter performance, as monitored in the model formulations and in commercial sunscreens, demonstrate that this is a simple and effective method for stability assessment of sunscreen filters under conditions of use.