Background/Aims: Exogenous factors (e.g. physical: UV irradiation; or chemical: hydrogen peroxide) and endogenous metabolic processes (e.g. cellular respiration, oxidative burst, etc.) generate oxidative stress in living tissues which are in balance with enzymatic antioxidative systems and ingested antioxidants under normal conditions. These complex biological reactions are accompanied by chemiluminescence (ultraweak photon emission). However, knowledge about the chemiluminescence decay characteristics of human skin and the modulatory influence of topically applied antioxidants is still scarce. Methods: Using ICL-S (induced chemiluminescence of human skin), a highly sensitive in vivo method, the decay characteristics of UVA-induced photon emission caused by different UVA doses were investigated in detail. In addition, modulatory properties of topical antioxidant pretreatment were examined for 2 weeks. Results: UVA-induced chemiluminescence signals were generally characterized by two distinct decay phases: an initial burst (0–5 s), contributing approximately 80% of the complete signal with an inverse dose-response relationship (UVA dose vs. chemiluminescence intensity), followed by a second decay phase (delayed chemiluminescence, 5–200 s) showing a direct correlation. Antioxidant pretreatment caused a reduction in signal intensity of approximately 50%, which was calculated by signal integration and confirmed using the modulation of the intersection point of decay curves resulting from irradiation with different UVA doses at constant intensity with and without treatment. Conclusion: In addition to the established UVA filter testing (independent from UVB filter content) on human skin in vivo, ICL-S is also a valuable tool for the efficacy testing of topically applied antioxidants under in vivo conditions in humans. The first rapid, but short, decay phase not only provides approximately 80% of the complete chemiluminescence signal, but is also essential for the investigation of antioxidant-mediated effects. Chemiluminescence signal modulations induced by UVA intensity reduction (e.g. UV filters in daily care products) can be clearly distinguished from antioxidant-mediated signal modulations. The probe head dimensions permit comprehensive in vivo testing in humans on practically every skin area (e.g. arms, legs, back, abdomen and face).

Background/Aims: Topically applied antioxidants (AOs) are widely used in cosmetic products – especially in day and sun care – to help reduce oxidative stress caused by exogenous influences such as ultraviolet (UV) radiation. Despite several advances in recent years, little is known about the duration of protective effects by application of topical AOs, AO protection capacity (APC) or the activation of an endogenous protection capacity (EPC). Methods: By measuring oxidative-stress-induced photon emission of human skin in vivowith the ICL-S method (induced chemiluminescence of human skin), the protective effect of daily AO treatment for 2 weeks was examined on 4 consecutive days after treatment. UVA-dose-independent effects were investigated by decay curve intersection point analysis. In addition, chemiluminescence signal integration was used to investigate the influence of different UVA doses for stimulation on the determined APC as well as the modulation of the EPC by repetitive UVA stimulation both forming the skin protection capacity (SPC). Results: The SPC showed a strong dependency on the UVA dose used for stimulation. AO pretreatment was more effective against lower UVA doses. Over the course of 4 days, the AO-induced SPC did not change significantly for a given UVA dose. Analyzing the decay curve intersection point for 2 different UVA doses, however, revealed a decrease in SPC with time. In addition, we found that a repetitive UVA irradiation of 1 J/cm² caused a statistically significant protective effect against UVA irradiation by stimulation of endogenous mechanisms. Conclusion: Topically supplemented AOs provide a protective effect against oxidative stress for at least 3 days, supporting their widespread use in cosmetic products. Especially their interaction with cutaneous protective mechanisms should be investigated in more detail for maximal protection, as endogenous defense mechanisms are already triggered by 2 low-dose UVA irradiations within 24 h. In summary, the in vivo measurement of UVA-induced cutaneous chemiluminescence permits the UVA-dose-independent determination of the AO efficacy for better comparability of the results while also taking endogenous defense mechanisms into account.