Human Epidermal Zinc Concentrations after Topical Application of ZnO Nanoparticles in Sunscreens

Zinc oxide nanoparticle (ZnO NPs) sunscreens are generally considered safe because the ZnO NP do not penetrate through the outermost stratum corneum (SC) layer of the skin. However, cytotoxicity of zinc ions in the viable epidermis (VE), after release from ZnO NP and penetration into the VE, is ill-defined. We therefore quantified the relative concentrations of endogenous and exogenous Zn using a rare stable zinc-67 isotope (67Zn) ZnO NP sunscreen applied to human skin and the cytotoxicity of human keratinocytes (HaCaT) using multiphoton microscopy, zinc-selective fluorescent sensing and laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) methodology. Multi-photon microscopy with second harmonic generation showed that 67ZnO NPs were retained on the surface or within the SC.  Zn fluorescence sensing revealed higher levels of labile and intracellular zinc in both SC and VE relative to untreated skin, confirming that zinc permeated through the SC into the VE as ionic Zn and not as ZnO NPs. Importantly, the LA-ICP-MS estimated exogenous 67Zn concentrations in the VE of 1.0 ± 0.3 mg/mL are much lower than that estimated for endogenous VE zinc of 4.3 ± 0.7 mg/mL. Further, their combined total zinc concentrations in the VE are much less than the exogenous zinc concentration of 21 to 31 mg/mL causing VE cytotoxicity, as defined by the from half maximal inhibitory concentration of exogenous 67Zn found in human keratinocytes (HaCaT). This speaks strongly for the safety of ZnO NP sunscreens applied to intact human skin and the associated recent US FDA guidance.