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Are nanoparticle sunscreens safe? The jury is still out.


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 beachside 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. 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.

Nanoparticles are highly refined substances with a particle size of 15-100 nm. Theoretically, Dr. Draelos said, nanopar-ticulate 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.

But 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 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, 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.

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. The researchers wrote that it "indicates that nanosized 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 (Dermato-endocrinolology 2009;1:197-206). The review examined 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. "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 in-visible 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.

Although research continues, no nanoparticle sunscreens are available in the United States. Two can be found in Europe. "One uses a 15% concentration of 50-nm zinc oxide and the other, a 5% concentration of 30-nm titanium dioxide," she noted

The Swedish government, however, has blocked 10 companies from marketing such sunscreens because of possible toxicity to aquatic flora, she said.

Dr. Draelos disclosed financial relationships with numerous pharmaceutical companies.
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Title Annotation:DERMATOLOGY
Author:Sullivan, Michele G.
Publication:Internal Medicine News
Date:Sep 1, 2011
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