Abstract
It has been well known that the UV radiation in sunlight accelerates premature skin aging, and this feature of skin aging is commonly termed photo-aged skin. Cosmetic companies have recently recommended that consumers use sunscreens to prevent the risk of photo-aged skin. In general, sunscreen effects have been realized by formulating UV-scattering agents such as TiO2 and ZnO, and organic UV-absorbing agents. Among sunscreen agents, octyl methoxycinnamate (OMC) and 4-tert-butyl-4'-methoxydibenzoylmethane (BMDBM) are widely used and are known to cause photo-degradation during UV irradiation. However, the photo-degradation of UV absorbers on topically treated skin during sun exposure leads to a decrease of their photo-protective capability, which results in sunburn formation and the acceleration of photo-aged skin formation. In order to reduce the photo-degradation of UV absorbers and to prevent skin damage, it is critical to understand the basic behavior of UV absorbers in a thin layer such as when topically applied on the skin. This study was conducted to find clues for solutions to preventing these phenomena. Among UV absorbers, BMDBM gave the highest degree of photo-degradation. Furthermore, the degree of photo-degradation of BMDBM showed a negative relationship with the polarities of oil solvents in the following systems: BMDBM simply solubilized in oil and W/O emulsions without silicone oil. In combinations of BMDBM formulated with different UVB absorbers, the pair of BMDBM and OMC showed an equivalent photo-degradation in UVB and UVA regions. However, the pairs of BMDBM and 2-hydroxy-4-methoxybenzophenone (HMBP) or ethylhexyl triazone (EHT) gave a higher photo-degradation in the region of UVA than of UVB. Photo-stabilizers such as octcrylene(OCR)and methoxyoctocrylene(MOCR)required more than the same concentration of UV absorber to prevent photo-degradation. These results suggest that the photo-degradation of BMDBM occurs partially through the formation of some radicals.
