You can’t even see the titanium dioxide! Image courtesy flickr user HB Art.
Summer officially began yesterday across the northern hemisphere (happy winter, Australia!), so it's time to get serious about sunscreen. Last week the U.S. Food and Drug Administration issued new guidelines designed to reduce consumer confusion around the wall of products on the market. If last June’s bottle of SPF 100 gave you a smug feeling of superiority over all the other 15-ers on the beach, be prepared: manufacturers can now only tout protection up to SPF 50. Nothing higher has demonstrated any increase in protection – protection that is provided, by the way, by our friend titanium dioxide.
Titanium, like most metals in nature, is found bonded with oxygen. In its pure elemental state, it is gray. Oxidized, it turns black. But when titanium is oxidized under just the right conditions, it becomes the whitest compound in existence. In fact, turning this black sand into white pigment is a nine billion-dollar global industry. The majority of the pigment goes into paint; other big markets are plastic and paper.
When oxidized, titanium dioxide is a quarter of a micron long. That just happens to make it a half-wave antenna for visible light. So when light strikes that nice crisp resume you just printed out, for example, it is reflected back rather than absorbed, making the page a brilliant white. Ultraviolet (UV) radiation, in the meantime, is half the wavelength of visible light. So when titanium dioxide pigment is ground into particles half their original size, the result is a UV half-wave antenna – perfect for a day at the beach.
These titanium dioxide nanoparticles, present in almost all sunscreens, are so small they are invisible – in fact, manufacturers of sunscreen often add more white pigment back into their product because it sells better than the clear stuff. Nanoparticles’ size has recently been the subject of concern, however; critics have argued that they may be able to penetrate the skin, while their health risks remain unknown.
In last week’s report, the FDA cited their own and other studies demonstrating the safety of nanoparticles, but said they would continue research in this area. So feel free to indulge in a golf ball-sized (the recommended amount) glob of titanium dioxide before you head for the shore.
Neil Gussman is Strategic Communications and Media Relations Manager at CHF.
FDA Strengthens Sunscreen Regulations [C&EN]
Episode 28: Summer [Distillations]
Nano Pop [Chemical Heritage]