Methods to Detect Antioxidants and Free Radicals in Human Skin and Its Applications with Focus on Asian Skin

The interaction between antioxidants and free radicals has become increasingly important in cosmetic research. This review is not a conventional literature overview but is primarily based on our own experimental data and methodological developments.

The first part describes noninvasive spectroscopic techniques, such as resonance Raman spectroscopy are highlighted as robust tools for the in vivo determination of carotenoid concentrations, serving as indirect markers of the skin's antioxidant status. The next section addresses the detection of free radicals. Electron paramagnetic resonance (EPR) spectroscopy is presented as the only direct and quantitative method for detecting free radicals in biological systems. Its application in vitro, ex vivo, and in vivo enables controlled assessment of radical formation and antioxidant efficacy, including under defined irradiation conditions. The introduction of the radical protection factor (RPF) provides a standardized parameter to compare the scavenging capacity of individual compounds and complex formulations, thereby supporting product development and claim substantiation. In the last section, in vivo and ex vivo measurements are compared. It is shown that the amounts of free radicals generated in the UV and infrared spectral ranges of solar radiation differ significantly between different skin types. For sun protection, this means that the protective effect of a sunscreen must extend across the entire spectral range of solar radiation, since 50% of the free radicals generated by the sun originate in the visible and infrared spectral ranges.

These findings led to the development of a sunscreen important for Asian skin (skin type IV).