Metal oxide nanoparticles are commonly used in personal-care formulations as protective agents against exposure to ultraviolet radiation. Although previous research has concluded that nanoparticles do not penetrate healthy skin, it remains contentious whether this conclusion holds under normal conditions of sunscreen use. Humans (n = 20) were exposed to sunscreens containing zinc oxide (ZnO) particles to determine if Zn from the particles was absorbed through skin over five consecutive days under outdoor conditions. Two sunscreens were tested-"nano sunscreen" containing 19-nm nanoparticles and "bulk sunscreen" containing > 100-nm particles. Venous blood and urine samples were collected 8 days before exposure, twice daily during the trial, and 6 days post-exposure. As the first application in nanotechnology studies, stable isotope tracing was used where the ZnO, enriched to > 99% with the stable isotope (68)Zn, allowed dermally absorbed zinc to be distinguished from naturally occurring zinc. The overwhelming majority of applied (68)Zn was not absorbed, although blood and urine samples from all subjects exhibited small increases in levels of tracer (68)Zn. The amount of tracer detected in blood after the 5-day application period was ∼1/1000 th that of total Zn in the blood compartment. Tracer levels in blood continued to increase beyond the 5-day application phase in contrast to those in urine. Levels of (68)Zn in blood and urine from females receiving the nano sunscreen appeared to be higher than males receiving the same treatment and higher than all subjects receiving the bulk sunscreen. It is not known whether (68)Zn has been absorbed as ZnO particles or soluble Zn or both.

Sunscreens containing metal oxide nanoparticles appear transparent on the skin and provide excellent protection against sunburn caused by UV radiation. While it is likely that nanoparticles remain on the surface of the skin of healthy adult humans, and thus are considered safe for use in sunscreens, there has been no comprehensive assessment of the impact on human health from exposure to the metal oxide nanoparticles destined for use in sunscreens, either in the workplace during the manufacturing process, in long-term use across a range of skin conditions, or upon release into the broader environment, either accidentally or consequent of normal sunscreen use. In this review, we focus on zinc oxide nanoparticles destined for use in modern sunscreens, and discuss the potential for human exposure and the health hazard at each stage of their manufacture and use. We highlight where there is a need for further research.

The relationship between the toxicity of zinc oxide (ZnO) nanoparticles (NPs) and their surface chemistry was investigated. Cytotoxicity, genotoxicity, and the ability to generate reactive oxygen species (ROS) were assessed for well-characterized ZnO NPs whose surface chemistry was varied from its pristine state by coating with oleic acid (OA), poly(methacrylic acid) (PMAA), or components adsorbed from cell culture medium (medium-soaked). It was found that uncoated NPs showed ROS accumulation and diminished cell viability whereas all tested surface coatings assisted in reducing ROS production and cytotoxicity. The ability of coatings to reduce the cytotoxicity of ZnO NPs was ranked in the following order: medium-soaked ≈ PMAA > OA. However, PMAA-coated ZnO had significant genotoxicity compared to uncoated ZnO and the other coated NPs, highlighting the need to investigate thoroughly the effects of NP surface modification on both cytotoxicity and genotoxicity assays. The lowest toxicity was achieved with a surface coating of components from a cell culture medium.

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