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May 23rd, 2007

Screen Test: Reading the Micro-Fine Print

Mineral sunblocks that contain titanium dioxide (TiO2) or zinc oxide (ZO) are preferable to chemical sunscreens, because rather than being absorbed into the skin, the minerals lie on top of the skin, reflecting UV rays before they cause damage. The choice of most lifeguards, these sunblocks are famous for giving off that unattractive "white" mask.

But this is where problems with minerals arise. In order to reduce the visibility of sunscreen, many manufacturers use nanometer-sized particles of TiO2 and ZO. A nanometer (nm) is about a billionth of a meter—a unit so small that a single human hair is about 80,000 nm in diameter. The U.S. government has defined nanomaterials as particles smaller than 100 nm, and according to the Australian government, most nano-sized sunscreens use particles that size or smaller because the sunscreens become transparent on skin.

Nanoparticles are unpredictable because their small size and high ratio of surface area to volume can produce chemical or physical properties that are very different from their larger counterparts. For instance, once TiO2 nanoparticles enter the bloodstream, they are at risk of infiltrating the brain where they can damage cells, whereas larger micron-sized (millionths of a meter) particles of TiO2 are blocked by the blood-brain barrier, which protects the brain from harmful substances in the bloodstream. Fortunately, the consensus in the scientific community, as demonstrated by a 2006 Australian government literature review on the topic, is that neither TiO2 nor ZO penetrate the skin deep enough to actually enter the bloodstream.


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