NEW YORK – Zinc oxide and titanium dioxide have a long history of use – from paint pigment to diaper rash remedy. They’ve even made a beach-side fashion statement among sun seekers who protect their ears and nose with a thick, white slathering of the light-reflecting mineral cream.
But these inert particles may someday form the basis of the "perfect sunscreen," Dr. Zoe Draelos said at the American Academy of Dermatology’s Summer Academy meeting. Such a compound would completely protect against both ultraviolet B and the more-damaging ultraviolet A while being completely invisible on skin types I-VI.
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Young people may be forever burying their faces in zinc oxide and titanium dioxide if the promise of nanoparticle sunscreen becomes a reality.
Nanoparticles are highly refined substances with a particle size of 15-100 nm. Theoretically, Dr. Draelos said, nanoparticulate zinc oxide and titanium dioxide could be dispersed in a comfortable vehicle that would be invisible even on dark skin, easy to apply, last a long time, and provide full-spectrum protection – all attributes that could increase user compliance and, therefore, overall skin safety.
Unfortunately, like most things that seem too good to be true, nanoparticles lug around some potentially serious baggage, said Dr. Draelos, a clinical dermatologist and researcher in High Point, N.C.
"We already know that some nanoparticles cause health risks," she said. "The smog that we breathed in on the way to this meeting contained carbon and other nanoparticles, which are so small that they escape phagocytosis, making it virtually impossible for the body to ever remove them."
Prolonged exposure to inhaled nanoparticles can contribute to lung disease, and there is even preliminary research that smog and some nanoparticles may even impact cardiovascular function.
"Because we already know some things about the potential harm of inhaled nanoparticles, it’s very important that any topical products don’t contain nanoparticles that could be absorbed into the skin," said Dr. Draelos, who is also vice president-elect of the American Academy of Dermatology.
To address the issue, the Food and Drug Administration established a nanotechnology task force in 2007, she said. "But no one has heard from them since." Other groups have cited environmental concerns, pushing the issue to the forefront. But scientific literature about skin absorption has come to conflicting results, some of which may not be applicable to humans.
Risks Associated With Nanoparticles
A 2009 study suggested that titanium dioxide nanoparticles, applied topically to pigs’ ears and to hairless mice, not only penetrated the stratum corneum but later showed up in other organs, including the liver and the brain. The study "indicates that nano-sized titanium dioxide may pose a health risk to humans after dermal exposure over a relative long time period" (Toxicol. Lett. 2009;191:1-8).
But a review published the same year found conflicting evidence (Dermatoendocrinol. 2009;1:197-206). The review examined the absorption of various nanoparticle types, including titanium dioxide and zinc oxide. Citing nine studies on humans, pigs, and mice, the authors concluded that the minerals in sizes of 10-100 nm did not penetrate the human stratum corneum beyond five layers, although they could be seen in the openings of the hair follicles.
Any nanoparticle smaller than 13 nm could potentially penetrate the stratum corneum, Dr. Draelos said, but anything larger would stay on the surface.
Dr. Zoe Draelos
"The follicular infundibulum is about 170 micrometers, so nanoparticles can get in there but, as sebum is produced, the particles are washed out onto the skin’s surface. If you had a nanoparticle that was really soluble, it could dissolve into the sebum, but nanoparticles made of these metal oxides don’t dissolve."
Applying a nanoparticle sunscreen to damaged skin could potentially increase the likelihood of absorption, and no one is quite sure what would happen when such a compound concentrates in the body’s creases and folds. Would the additional pressure on the inside of the elbow, for example, facilitate absorption? "No one really knows," she said.
For a sunscreen to remain invisible on all skin types, the nanoparticles would have to be no larger than 10 nm, "putting us right back where we started. So it’s going to be difficult to make an effective nanoparticle sunscreen that will be invisible on all skin types," she added.
To further complicate matters, nanoparticles themselves can be damaging. "When a photon of ultraviolet B radiation strikes a nanoparticle, the particle undergoes photocatalysis, which generates secondary free radicals," said Dr. Draelos. Researchers are trying to tackle this problem with polyester or nylon coatings that block the process.