Repetitive Application of Sunscreen Containing Titanium Dioxide Nanoparticles on Human Skin

Titanium dioxide (TiO₂) has for decades been approved for use in sunscreens as a physical sunblock. It is not known whether reducing the size of TiO₂ particles in sunscreens creates new issues of safety and/or effectiveness. A study was therefore conducted to investigate the relative risk of skin penetration by TiO₂ nanoparticles in healthy fair-skinned individuals.

The protocol for the study was approved by the Research Involving Human Subjects Committee of the US Food and Drug Administration, and informed written consent was obtained from each of the study participants. The participants were 6 fair-skinned individuals with phototypes II and III skin as defined by a UV-sensitivity questionnaire ¹ and dermatological evaluation, who were recruited from the Washington, DC area. Daily applications of sunscreen with and without TiO₂ nanoparticles at a concentration of 2 mg/cm² were made to 2 test sites (5 x 5 cm) on the lower back of each participant, with a separate site as an untreated control. Except for the absence of tocopheryl acetate, the 2 custom-prepared sunscreen formulations used in the study (sunscreen with uncoated TiO₂ nanoparticles [sunscreen plus TiO₂] and [sunscreen only]) were identical to previously reported formulations.² The sunscreens were applied once daily for 3 days to 2 of the participants. To further detect penetration of TiO₂ nanoparticles, sunscreen was applied once-daily for 8 days to the 4 other participants. One day after the final sunscreen application, 5 shave biopsy specimens were acquired from each participant (2 from the sunscreen plus TiO₂ site, 2 from the sunscreen-only site; and 1 from the control site). Details of the procedures for biopsy and tissue specimen processing, and of the biomarkers for immunohistochemistry, have been described,³ as have details of the biopsy-sample preparation for electron microscopy.² Transmission electron microscopy was used to assess the penetration of TiO₂ particles through epidermal-dermal skin layers. Confirmatory identification was done with scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX) analysis as previously described.²

The study participants (4 female and 2 male) were healthy individuals with a mean age of 37 years and did not present with skin irritation or other adverse effects. All skin specimens contained TiO₂ particles in the stratum corneum layer, probably caused by normal rubbing of clothing and resulting in cross-contamination of the study preparations to other sites. The TiO₂ nanoparticles in the specimens appeared agglomerated and ranged from 20 to 100 nm in diameter, as found previously.² Generally, the skin sites receiving sunscreen plus TiO₂ contained more TiO₂ particles than those treated with sunscreen only, as would be expected, and therefore were sites of greater scrutiny for dermal penetration. The deep epidermis and dermis of the control sites and at the sunscreen-only sites were not examined because the level of TiO₂ in the stratum corneum of these samples was much lower than in the sunscreen plus TiO₂ skin samples. The TiO₂ nanoparticles were confirmed with SEM-EDX as present mainly in the dermis surrounding hair follicles but at very low qualitative levels and with no obviously decreasing gradient with skin depth.

Fewer than 30 confirmed TiO₂ nanoparticles or their aggregates were detected in all of the sunscreen plus TiO₂ skin specimens, mainly in the dermis surrounding the hair follicle. The detectable numbers of TiO₂ nanoparticles were several orders of magnitude lower than those in the applied doses of sunscreen plus TiO₂ in the area with maximal dermal presentation (Figure), or about 0.00014% of the total applied dose of uncoated TiO₂ on the basis of the assumptions previously described.² These results are in accord with previous findings in minipigs and corroborate findings for nanoparticles surrounding the hair follicles in humans.^2,4-6 Our findings depend on selected “regions of interest,” which does not allow a detailed quantitative assessment of dermal penetration of TiO₂ nanoparticles. Notwithstanding its limitations, however, the study described here provides valuable data on the dermal penetration of TiO₂ nanoparticles and the parameters for informing the design of future clinical studies.

Corresponding Author: Sergio Coelho, PhD, Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, Bldg 37, Room 2132, Bethesda, MD 20892 (sergio.coelho@fda.hhs.gov).

Accepted for Publication: December 1, 2015.

Published Online: February 24, 2016. doi:10.1001/jamadermatol.2015.5944.

Author Contributions: Drs Coelho and Miller had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Wokovich, Howard, Miller.

Acquisition, analysis, or interpretation of data: Coelho, Patri, McNeil, Miller.

Drafting of the manuscript: Coelho, Wokovich, Howard.

Critical revision of the manuscript for important intellectual content: Coelho, Patri, McNeil, Howard, Miller.

Statistical analysis: Patri.

Obtained funding: McNeil, Miller.

Administrative, technical, or material support: Coelho, Patri, Wokovich, McNeil, Howard.

Study supervision: Patri, McNeil, Miller.

Conflict of Interest Disclosures: None reported.

Funding/Support: This study was supported in part by the Office of Science and the Center for Devices and Radiological Health, US Food and Drug Administration (FDA), and in part by the Intramural Research Program of the National Cancer Institute, National Institutes of Health (under contract HHSN261200800001E).

Role of the Funder/Sponsor: The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Disclaimer: The content of this publication does not necessarily reflect the views or policies of the US Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the US government. This article reflects the views of the authors and should not be construed to represent FDA’s views or policies. The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the US Department of Health and Human Services.

Additional Contributions: We are indebted to our colleague, Janusz Beer, PhD, DSc, Center for Devices and Radiological Health, FDA, who created the clinical facility in which this work could be completed and is survived scientifically through the collaborations he nurtured. In addition, we thank Lucinda Buhse, PhD, Center for Drug Evaluation and Research, FDA, for helping to design and create the cream formulation. Nakissa Sadrieh, PhD, Center for Drug Evaluation and Research, FDA, published the seminal paper on which this work is based and reviewed the clinical protocol. We would also like to extend special thanks to Boris Lushniak, MD, MPH, who at the time of the study was Rear Admiral and Assistant Surgeon General, US Public Health Service (and now is retired Acting Surgeon General) for review of the study design, and Barbara Zmudzka, PhD, Center for Devices and Radiological Health, FDA, and Vincent Hearing, PhD, National Cancer Institute, National Institutes of Health, for their help during the clinical study. None of these individuals received additional compensation for their work in the study.