Exploitation of long‐lasting ultraweak photon emission to estimate skin photodamage after ultraviolet exposure

Background Establishing a noninvasive method to estimate skin damage immediately after ultraviolet (UV) exposure is required to minimize the anticipated severe symptoms triggered by early phase UV‐induced reactions in the skin. To develop a suitable method, we focused on ultraweak photon emission (UPE) immediately after UV exposure to characterize the relationship of UPE to skin photodamage caused by the UV exposure. Materials and Methods Analysis of the correlation between UV‐induced UPE and erythema formation characterized by skin redness was conducted in a clinical study. To clarify the source of UPE, time‐dependent lipid oxidation was analyzed in human epidermal keratinocytes in vitro using a fluorescence indicator as well as the lipid hydroperoxide (LPO) assay. Results The average amount of UV‐induced long‐lasting UPE per second, especially from 1 to 3 minutes compared to other time periods after the UV radiation, increased in a dose‐dependent manner and was highly correlated with the intensity of cutaneous redness 24 hours after UV exposure. In addition, cellular examinations elucidated that both the long‐lasting UPE signals and the increased amounts of LPO 2 minutes after UV radiation were significantly suppressed by Trolox (a vitamin E derivative), which has been shown to inhibit UV‐induced erythema formation in human skin. Conclusion Long‐lasting UPE generated between 1 and 3 minutes immediately after UV exposure, which is associated with LPO production, is a valuable indicator to estimate and/or avoid severe cutaneous photodamage.