Zinc Oxide Sunscreens: The Role of Formulation and Residual Film in Photoprotective Performance

Zinc oxide (ZnO) is increasingly used as a mineral UV filter, but its efficacy may be underestimated by conventional spectrophotometric in vitro methods based on mass application. This study investigated whether this underestimation reflects intrinsic ZnO performance or the residual film formed on the test substrate. Two formulations containing 10% w/w ZnO—an oil-in-water (O/W) emulsion and an anhydrous formulation—were compared with an O/W formulation containing organic UV filters, all targeting SPF 15. Dry-residue analysis showed approximately 99% mass retention for the anhydrous formulation versus 41% for the O/W ZnO emulsion, corresponding to estimated residual film thicknesses of ~12 and ~4 µm, respectively. ISO 24443-derived SPF values were 13.8 and 7.1, whereas the Helio2 method yielded 15.2 and 17.3, respectively. UVA-induced intracellular ROS in HaCaT keratinocytes showed significant protection with the anhydrous ZnO and organic-filter formulations, but not with the O/W ZnO emulsion. The BASF Sunscreen Simulator also underestimated both ZnO formulations (SPF 5). These findings suggest that poor performance of the O/W ZnO emulsion primarily reflects formation of a thin, discontinuous residual film rather than lower intrinsic ZnO efficacy, highlighting limitations of current in vitro and in silico approaches.