Film thickness frequency distribution of different vehicles determines sunscreen efficacy

Sun protection factor (SPF) frequently differs between sunscreens containing the same composition of ultraviolet (UV) filters that primarily define sunscreen efficacy. We tested the hypothesis that the thickness frequency distribution of the sunscreen film is also responsible for and can explain the divergence in the measured SPF. For this, we developed a method to measure film thickness from the difference of topography before and after application of of sunscreen on pig ear epidermal membrane. The influence of five vehicle formulations and of application pressure and spreading time on mean thickness ( ), to median ratio, and SPF in vitro was investigated. The vehicle had a significant impact, low vehicle viscosity resulting in a smaller , larger to median ratio, and lower SPF in vitro than high viscosity; continuous oil phase produced the largest and SPF values. A long spreading time reduced and SPF and increased application pressure reduced SPF. There was a positive correlation between and SPF in vitro, underlining the relevance of film thickness for interpreting UV protection differences of formulations with the same filter composition. This work demonstrated a strong influence of vehicle and application conditions on sunscreen efficacy arising from differences in film thickness distribution.