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“Nanoparticles” may seep through skin

Sept. 30, 2008
Courtesy University of Rochester Medical Center
and World Science staff

Sci­en­tists are find­ing that par­t­i­cles that are barely there—ti­ny ob­jects known as na­no­par­t­i­cles that have found a home in elec­tron­ics, food con­tain­ers, sun­screens, and other ap­plica­t­ion­s—can breach our most per­son­al pro­tec­tive bar­ri­er: the skin.

The par­t­i­cles un­der scru­ti­ny by Li­sa De­Louise at the Uni­ver­s­ity of Roch­es­ter Med­i­cal Cen­ter, in New York, are less than one five-thou­sandth the width of a hu­man hair.

In the Sep­tem­ber is­sue of the re­search jour­nal Nano Let­ters, a team led by De­Louise pub­lished a pa­per re­port­ing that nanopar­t­i­cles pass through the skin of a liv­ing or­gan­ism, a type of mouse com­monly used as a mod­el to study the dam­ag­ing ef­fects of sun­light.

The health im­plica­t­ions of na­no­par­t­i­cles in the body are un­cer­tain, said De­Louise. Oth­er sci­en­tists have found that the par­t­i­cles can ac­cu­mu­late in the lymph sys­tem, the liv­er, the nerv­ous sys­tem, and in oth­er ar­eas of the body. In her stu­dy, she found that the par­t­i­cles ac­cu­mu­late around the hair fol­li­cles and in ti­ny skin folds.

De­Louise, a chem­ist, notes that her study did not di­rectly ad­dress the safe­ty ques­tion. “We simply wanted to see if nanopar­t­i­cles could pass through the skin, and we found that they can un­der cer­tain con­di­tions,” she said.

While nanopar­t­i­cles are be­com­ing widely used in the ma­n­u­fac­ture of con­sum­er prod­ucts, they are al­so un­der a great deal of study in re­search labs, and there are some pro­cess­es—in­clud­ing or­di­nary can­dle flames—that pro­duce them nat­u­ral­ly. Some of the ob­jects are so small, less than 10 nanome­ters wide (a na­no­me­ter is one-millionth of a mil­lime­ter), that they are nearly as small as the gaps be­tween some skin cells.

De­Louise’s team stud­ied the pen­etra­t­ion of na­no­par­t­i­cle known as quan­tum dots that flu­o­resce un­der some con­di­tions, mak­ing them eas­i­er to see and track. The sci­en­tists looked at the dis­tri­bu­tion of quan­tum dots in mice whose skin had been ex­posed to about the same amount of ul­tra­vi­o­let light as might cause a slight sun­burn on a per­son. The team found that while the na­no­par­t­i­cles were able to breach the skin of all the mice, they pas­sed more quickly through “sun­burnt” skin.

De­Louise plans lat­er to study ti­ta­ni­um di­ox­ide and zinc ox­ide, widely used in sun­screens and cos­met­ics to help block ul­tra­vi­o­let light. In re­cent years the met­al ox­ide par­t­i­cles used in many con­sum­er prod­ucts has be­come smaller and smaller, so that many now are na­no­par­t­i­cles. The re­sults are vis­i­ble to an­y­one who takes a walk on the beach or stops by a cos­met­ics count­er: The ma­te­ri­als are of­ten com­pletely trans­par­ent when ap­plied to skin. A trans­par­ent lip gloss that pro­tects against UV light, for ex­am­ple, or a see-through sun­screen may con­tain na­no­par­t­i­cles, De­Louise said.

“A few years ago, a life­guard at the swim­ming pool wear­ing sun­screen might have had his nose com­pletely cov­ered in white,” she said. That’s be­cause “old­er sun­screens have larg­er par­t­i­cles that re­flect vis­i­ble light. But many newer sun­screens con­tain na­no­par­t­i­cles that are one thou­sand times smaller, that do not re­flect vis­i­ble light.” Many peo­ple apply sun­screen af­ter they’re sun­burnt, she added.

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Scientists are finding that particles that are barely there—tiny objects known as nanoparticles that have found a home in electronics, food containers, sunscreens, and a variety of applications—can breach our most personal protective barrier: the skin. The particles under scrutiny by Lisa DeLouise at the University of Rochester Medical Center, in New York, are almost unfathomably tiny. The specks are less than one five-thousandth the width of a human hair. If the width of that strand of hair were equivalent to the length of a football field, a typical nanoparticle wouldn’t even reach the one-inch line. In the September issue of the journal Nano Letters, a team led by DeLouise published a paper reporting that nanoparticles pass through the skin of a living organism, a type of mouse commonly used as a model to study the damaging effects of sunlight. The health implications of nanoparticles in the body are uncertain, said DeLouise. Other scientists have found that the particles can accumulate in the lymph system, the liver, the nervous system, and in other areas of the body. In her study, she found that the particles accumulate around the hair follicles and in tiny skin folds. DeLouise, a chemist, notes that her study did not directly address the safety question. “We simply wanted to see if nanoparticles could pass through the skin, and we found that they can under certain conditions,” she said. While nanoparticles are becoming widely used in the manufacture of consumer products, they are also under a great deal of study in research labs, and there are some processes—including ordinary candle flames—that produce them naturally. Some of the objects are so small, less than 10 nanometers wide (a nanometer is one-millionth of a millimeter), that they are nearly as small as the gaps between some skin cells. DeLouise’s team studied the penetration of nanoparticles known as quantum dots that fluoresce under some conditions, making them easier to see and track compared to other nanoparticles. The scientists looked at the distribution of quantum dots in mice whose skin had been exposed to about the same amount of ultraviolet light as might cause a slight sunburn on a person. The team found that while the nanoparticles were able to breach the skin of all the mice, they passed more quickly through “sunburnt” skin. DeLouise plans later to study titanium dioxide and zinc oxide, widely used in sunscreens and cosmetics to help block ultraviolet light. In recent years the metal oxide particles used in many consumer products has become smaller and smaller, so that many now are nanoparticles. The results are visible to anyone who takes a walk on the beach or stops by the cosmetics counter at a department store: The materials are often completely transparent when applied to skin. A transparent lip gloss that protects against UV light, for example, or a see-through sunscreen may contain nanoparticles, DeLouise said. “A few years ago, a lifeguard at the swimming pool wearing sunscreen might have had his nose completely covered in white. Older sunscreens have larger particles that reflect visible light. But many newer sunscreens contain nanoparticles that are one thousand times smaller, that do not reflect visible light,” said DeLouise, noting that many people apply sunscreens after they’re sunburnt.