Ultraviolet A irradiation induces ultraweak photon emission with characteristic spectral patterns from biomolecules present in human skin.

Oxidative stress is associated with photoaging of the skin as well as with skin cancer, and is therefore, critical to monitor. Ultraweak photon emission (UPE) is extremely weak light generated during the oxidative process in the living body and has been used as a non-invasive and label-free marker for the evaluation of oxidative stress. However, the mechanism of UPE generation is not clear. Therefore, we aimed to elucidate the molecular mechanism underlying UPE generation by analyzing the spectra of UPE generated from biomolecules in the skin during ultraviolet A (UVA) exposure. The spectra of UVA-induced UPE generated from linoleic acid, linolenic acid, elastin, phospholipids, and 5,6-dihydroxyindole-2-carboxylic acid were measured, and the spectrum of human skin tissue was also obtained. The spectral patterns varied for the different biomolecules and the peaks were distinct from those of the skin tissue. These results suggested that the UPE generated from skin tissue is a collection of light emitted by biomolecules. Moreover, we proposed that UPE is generated through a photosensitization reaction and energy transfer. The identified characteristic spectral patterns of UPE can be useful to elucidate UVA-induced oxidative stress in the skin, with implications for prevention and treatment of photoaging and skin diseases.