The weight of evidence
Current evidence suggests minimal risks to human health from the use of TiO2 or ZnO nanoparticles at concentrations up to 25% in cosmetic preparations or sunscreens, according to Schilling et al., regardless of coatings or crystalline structure. In a safety review of these ingredients, they noted that these nanoparticles formulated in topical products occur as aggregates of primary particles 30-150 nm in size, and bond in such a way that renders them impervious to the force of product application. Thus their structure remains unaffected, and no primary particles are released. The authors also noted that nanoparticles exhibit equivalence with larger particles in terms of distribution and duration and, therefore, recognition and elimination from the body (Photochem. Photobiol. Sci. 2010;9:495-509).
But in 2011, Tran and Salmon, in light of findings that nanoparticles may penetrate the stratum corneum under certain conditions, considered the possible photocarcinogenic results of nanoparticle sunscreens. They noted, though, that most such results were obtained through the use of animal skin models, not investigations with human skin (Australas. J. Dermatol. 2011;52:1-6). To this point, the weight of evidence appears to show that such TiO2 nanoparticles are safe when applied to intact human skin (Semin. Cutan. Med. Surg. 2011;30:210-13).
In response to the increased scrutiny and concern exhibited by the general public and government agencies regarding the safety of TiO2 and ZnO nanoparticles, Newman et al. reviewed the literature and position statements from 1980 to 2008 to ascertain and describe the use, safety, and regulatory state of such ingredients in sunscreens. They found no evidence of significant penetration deeper than the stratum corneum of TiO2 and ZnO nanoparticles, but caution that additional studies simulating real-world conditions (i.e., sunburned skin and under UV exposure) are necessary (J. Am. Acad. Dermatol. 2009;61:685-92).
Conclusion
Titanium dioxide is a well-established, safe, and effective physical sunblock. Nanotechnology has introduced some cause for concern regarding its use in physical sunblocks. In particular, evidence suggesting that photoexcitation of TiO2 nanoparticles leads to the generation of reactive oxygen species that damage DNA, potentially launching a cascade of adverse events, has prompted investigations into the safety of TiO2 in nanoparticle form. However, to date, multiple studies suggest that TiO2 nanoparticles do not penetrate or are highly unlikely to penetrate beyond the stratum corneum.
Dr. Baumann is chief executive officer of the Baumann Cosmetic & Research Institute in Miami Beach. She founded the cosmetic dermatology center at the University of Miami in 1997. Dr. Baumann wrote the textbook "Cosmetic Dermatology: Principles and Practice" (McGraw-Hill, 2002), and a book for consumers, "The Skin Type Solution" (Bantam, 2006). Dr. Baumann has received funding for clinical grants from Allergan, Aveeno, Avon Products, Galderma, Mary Kay, Medicis Pharmaceuticals, Neutrogena, Philosophy, Stiefel, Topix Pharmaceuticals, and Unilever.