Invited feature articleHow the sun protection factor (SPF) of sunscreen films change during solar irradiation

- We measure and calculate how sunscreen SPF varies with solar irradiation time.
- We estimate photochemical reactant and product spectra and quantum yields.
- Photochemical kinetics in emulsion sunscreen films is successfully modelled.
- SPF prediction during “standard” sunlight irradiation gives a useful sunscreen design tool.

We have investigated how the sun protection factor (SPF) of different types of sunscreen film varies with “standard” solar irradiation due to photochemical processes. We have used a combination of chemical actinometry, measurement and modelling to estimate the overall quantum yields for the photoprocesses occurring for avobenzone (AVB) and isopentyl p-methoxycinnamate (MC) in either propane-1,2-diol (PG) or squalane (SQ) as solvent. Using the obtained parameters, we have developed models to calculate the evolution of the film spectra and derived SPF values for both non-scattering sunscreen films consisting of solutions of multiple UV filters and for highly scattering Pickering emulsion based sunscreen films. Model calculations for all films are in excellent agreement with film spectra measured as a function of irradiation time using different laboratory light sources. Finally, using the estimated parameters and experimentally validated models, we are able to quantitatively predict how the in vitro SPF values for different film types containing any set combination of UV filter concentrations will vary with time due to photochemical processes induced by irradiation with “standard” sunlight. This provides a useful tool for the rational design and optimisation of new sunscreen formulations.

Changes in SPF due to sunlight-induced photochemical changes can be predicted for both solution and emulsion sunscreen films.272