Background: Solar UV radiation (UVR) is composed of UVB (290-320 nm) and UVA (320-400 nm) wavelengths. Only two sunscreen active ingredients approved in the US, avobenzone (butylmethoxydibenzoylmethane) and zinc oxide (ZnO), provide true broad-spectrum protection against UVA wavelengths >360 nm. Although effective against shorter UVR wavelengths <360 nm, titanium dioxide (TiO(2)) is also often believed to confer broad-spectrum protection and is substituted for ZnO or avobenzone. To sustain its absorption capacity within a sunscreen film during UVR exposure, avobenzone needs to be formulated into sunscreen products using sound formulation strategies.
Objectives: To characterize the efficacy of avobenzone, ZnO, and TiO(2) in terms of their abilities to provide broad UVA protection and to demonstrate the effectiveness of the different formulation strategies used today to maintain the efficacy of avobenzone even during prolonged exposures to UVR.
Methods: UVA efficacy was assessed by measuring absorbance profiles in vitro using Vitro Skin® (IMS Inc., Orange, CT, USA) as an inert substrate and by determining UVA protection factors (PFA) on human skin. The impact of avobenzone loss on sun protection factor (SPF) and PFA values was evaluated by serially reducing avobenzone concentrations in an otherwise photostable product. The photostabilizing influence of specific formulation ingredients was monitored by measuring the extent to which they prevented UVR-induced degradation of avobenzone, whereas photostability of commercial sunscreen products was quantified by measuring the percentage change in absorbance within the UVB and UVA spectral regions following irradiation of thin product films on inert substrates.
Results: Model formulations containing 3% avobenzone or 5% ZnO provided superior attenuation of UVA wavelengths >360 nm compared with formulas containing 5% TiO(2). Additionally, sunscreen products of similar SPF containing avobenzone or ZnO exhibited significantly higher PFA values than those containing TiO(2). The addition of photostabilized avobenzone or ZnO increased PFA values nearly 3-fold, whereas the addition of TiO(2) increased PFA values only modestly. Judicious selection of sunscreen actives alone or in combination with extra stabilizing agents maintained the photostability of avobenzone in formulations to deliver sustained broad-spectrum absorbance during 4 hours of exposure to UVR. Small losses (<20%) of avobenzone did not significantly reduce a product's protective effects as measured by SPF and PFA values on human skin.
Conclusions: TiO(2) provided neither the same level of UVA attenuation nor the same degree of UVA protection on human skin as did products containing photostabilized avobenzone or ZnO. Hence, TiO(2) cannot be considered a substitute for avobenzone or ZnO in providing high levels of UVA protection to human skin. Use of proper formulation strategies can ensure that avobenzone losses are minimized to the extent that they have no impact on a product's ability to deliver sustained protection, even over periods of prolonged exposure to UVR.