Support for the Safe Use of Zinc Oxide Nanoparticle Sunscreens: Lack of Skin Penetration or Cellular Toxicity after Repeated Application in Volunteers
Zinc oxide is a widely used broad-spectrum sunscreen, but concerns have been raised about the safety of its nanoparticle (NP) form. We studied the safety of repeated application of agglomerated zinc oxide (ZnO) NPs [nanoparticles] applied to human volunteers over 5 days by assessing the skin penetration of intact ZnO-NPs and zinc ions and measuring local skin toxicity.
Representative multiphoton tomography multispectral images of in vivo human skin after repeated hourly (over 6 hours) and daily (over 5 days) topical application of uncoated and coated ZnO-NPs are shown in Figures 1a and 2a, respectively. The lack of a ZnO-NP SHG signal (red) at any depth in the viable epidermis (pseudocolored green-blue) shows that ZnO-NPs were not present in the viable epidermis after either of these treatment regimes.
Repeated application of ZnO-NPs to human skin in vivo over several days did not result in NP penetration through the stratum corneum or cause any visible morphological or redox changes. This finding extends the earlier observations that single ZnO-NP applications to human skin in vivo are associated with minimal skin penetration and local toxicity.