Comparison of dermal absorption of zinc from different sunscreen formulations and differing UV exposure based on stable isotope tracing

In a pilot study to determine if zinc (Zn) from zinc oxide nanoparticles in sunscreen can penetrate human skin in vivo, nanoparticles (~30nm) of a stable isotope (52% ⁶⁸Zn enrichment) were incorporated into an essentially phytochemical-based formulation and applied to the backs of 3 human subjects twice daily for 5days during the Southern Hemisphere winter. Blood and urine were collected prior to application and at regular intervals and up to 50days. As observed in a larger outdoor trial following this pilot study but with a different formulation and with UV exposure: values of ⁶⁸Zn in blood continued to increase beyond the 5day application phase with the highest measurement at 14days after the first application; variable amounts of the ⁶⁸Zn tracer were observed in urine; and the amounts of extra Zn added to blood were small and indicate very low levels of absorption (minimal estimate <0.01% of the applied dose) through the skin. Reasons for differences in absorption detected in the stable isotope trials and previous investigations include: the sensitivity of the stable isotope method; the duration of the investigations; the number of applications of sunscreen formulation; in vitro methods with excised skin; lack of measurement of blood and urine; no skin flexing; and lack of UV exposure.