Zinc oxide nanoparticle distribution in excised human skin. The black line represents the surface of the skin (top), blue represents ZnO nanoparticle distribution in the skin (stratum corneum), and pink represents skin. Credit: Timothy Kelf, Macquarie University.

In June 1997, Chicago Tribune columnist Mary Schmich decided to write a high school commencement speech. Lacking an invitation proved to be no barrier to her writing one of the most memorable commencement speeches ever—delivered or not.

Popularly known as “The Sunscreen Speech,” her article (actually titled “Advice, like youth, probably just wasted on the young”) gained worldwide fame two years later when Australian Baz Luhrmann used the text in its entirety in his song (watch video), “Everybody’s Free (To Wear Sunscreen).”

She begins her “speech” to her nonspecific audience of the class of ’97,

“Wear sunscreen.”

“If I could offer you only one tip for the future, sunscreen would be it. The long-term benefits of sunscreen have been proved by scientists, whereas the rest of my advice has no basis more reliable than my own meandering experience. I will dispense this advice now.”

And she does. She has plenty of eloquent advice on how to live and where to live, etc. Realizing that her wisdom might be eschewed by her youthful readers, she concludes with “But trust me on the sunscreen.”

The question is, should you?

Zinc oxide is the active ingredient in today’s commercial sunscreens. Thanks to its high optical absorption in both UVA and UVB ranges, it provides very effective protection for the skin from sun damage. Driven by consumer demand for transparence, the ZnO particles are nanosize, with an average size of less than 20 nm.

Nanoparticles that are on the surface or the uppermost layer of skin—the stratum corneum—are nontoxic. However, deeper layers of the skin are “susceptible to toxicological hazards associated with extraneous nanomaterials,” according to a new paper published in the open access journal, Biomedical Optics Express.

The research team drawn from Australian and Swiss universities acknowledges that the toxicity of nanoscale ZnO is still a matter of debate. However, taking the view that it is better to be safe than sorry, they report on a quantitative microscopy method for imaging nanoparticle distribution and uptake in human skin.

Quantitative evaluation of the distribution of ZnO nanoparticles on the skin is not so easy. Heretofore, available methods are limited to in vitro studies, and many had drawbacks resulting from difficulties in handling skin specimens. For example, the skin’s transport properties can be distorted during electron microscopy.

The research team turned to nonlinear optical microscopy to image ZnO nanoparticle uptake in human skin. NLOM provides high-contrast images and, because it is noninvasive, it can be used for in vitro and in vivo studies.

NLOM capitalizes on the nonlinear optical properties of ZnO. The nonlinear optical properties of bulk ZnO are well known, but nanoscale ZnO exhibits different photoluminescent properties arising from quantum confinement effects. NLOM imaging is based on the principle of two-photon-absorption-induced photoluminescence. A laser beam scans axially through the sample, which elicits “two types of nonlinear optical interactions” at the focal volume. A light-induced variation in refractive index occurs and produces a detectable lensing effect.

The team studied human skin samples treated with a commercial sunscreen, Zinclear, with a mean particle size of 21 nm. Results showed that ZnO nanoparticles were found only on the skin surface, the stratum corneum and in skin folds. No detectable amounts were found in the deeper layers of the skin.

The paper goes into depth on the physics of two-photon absorption, ZnO nanoparticle photoluminescence and the ways in which the ZnO nanoparticle distribution varies depending on whether the sunscreen solvent is organic or polar.

The bottom line is that Schmich is right, you should wear sunscreen.

The paper is “Characterization of optical properties of ZnO nanoparticles for quantitative imaging of transdermal transport,” Zhen Song, Timothy A. Kelf, Washington H. Sanchez, Michael S. Roberts, Jaro Rička, Martin Frenz, and Andrei V. Zvyagin, Biomedical Optics Express (doi: http://dx.doi.org/10.1364/BOE.2.003321)