Painted metal roofs are cheap, convenient, and usually very durable. But over the past two years, a rash of accelerated ageing has blighted pre-painted steel roofing in Australia. And intriguingly the ageing—which affects the coating—seems to be localized to small patches, taking on the form of fingerprints, handprints and even footprints.
The culprit it seems is sunscreen that is spilt or otherwise transferred to the roofing by construction workers during installation. And not any old sunscreen—this would appear to be a uniquely nano phenomenon. But I get ahead of myself…
Pick up a bottle of sunscreen and there is a fair chance these days that it contains nanoparticles, engineered to absorb and reflect away harmful UV radiation. Many manufacturers are introducing lines of nanoparticle-containing sunscreens as alternatives to those using more conventional organic chemicals, and it’s not hard to see why: the active ingredients in these nano sun blocks are generally more gentle on the skin than their non-nano counterparts; they are made to sit on the surface of the skin rather than penetrate into it; and if designed well, they continue to block UV radiation for several hours after application. And of course, they go on clear, giving a product that works well and looks good.
But each year as the sun and the sunscreen come out, questions over the safety of nano-formulations are raised. Can these nanoscale particles penetrate through the outer layers of the skin to the underlying living cells, and even the bloodstream? And if they get there, what harm could they cause? So far, most studies suggest that nanoparticles in sunscreens stay where they are supposed to—on the skin, not in it. Yet there is another question that has been bobbing along just under the surface for the past few years: could mixing nanoparticles, sun and moisture lead to a chemically corrosive mix that is bad for the skin?
The issue in question is photocatalytic activity. Titanium dioxide, and to a lesser extent zinc oxide, are photoactive—they have the ability to absorb UV, and in the presence of moisture convert benign water molecules into chemically active hydroxyl free radicals. These highly reactive chemicals could spell bad news for sunscreen users if they are generated in large amounts—eating away the components that hold the sunscreen together, and even possibly causing skin damage if they get below the surface and into cells.
Fortunately, manufacturers and users of titanium dioxide have long been aware of this propensity to generate free radicals, and have found ways of suppressing it in sunscreens. Photocatalytic activity depends on the crystalline structure of titanium dioxide. Anatase and rutile forms of titanium dioxide have the same chemical formula but different crystalline structures. And, as it turns out, different properties. Make nanoparticles from anatase titanium dioxide, or a mix of anatase and rutile, and you have a powerful source of harmful hydroxyl radicals in the presence of water and UV. But make nanoparticles out of rutile titanium dioxide alone, and photocatalytic activity is reduced substantially.
However, even rutile titanium dioxide particles show some photocatalytic activity. Early uses of rutile titanium dioxide as a white pigment in outdoor paint were plagued by the paint turning chalky after too much sun exposure. The problem was tracked down to hydroxyl radicals being produced and degrading the paint’s binder. The solution: coat the particles with a material that prevents free radical formation—no more chalky paint, and coatings that will last for years in the fiercest sun.
Makers of titanium dioxide-based sunscreens use a similar trick to retain the functionality of nanoparticles while avoiding the potentially harmful photocatalytic properties. For instance Optisol—a UV blocking agent made by the company Oxonica—incorporates a minute amount of manganese into the crystal lattice of rutile titanium dioxide nanoparticles. This doping allows the absorbed UV energy to be dissipated while virtually eliminating the formation of free radicals. Not only does this make sunscreens using Optisol potentially safer; they also last longer in the sun, as there are fewer free radicals to break down other ingredients in the product.
So all looks rosy for nano-enabled sunscreens. At least, it did until the publication of a recent paper. And this is where we get back to pre-painted steel roofs. Since mid 2006, researchers in New South Wales Australia have noticed unusual defects developing in newly installed pre-painted steel roofs. The damage is typically localized to areas of pressure contact, often taking the form of fingerprints or shoe impressions. And it results in accelerated weathering—in one example, patches of a roof appeared to age an equivalent of 15 years in only 18 months. The culprit? Nanoparticle-containing sunscreens, which are accidentally transferred to the roof during installation by touching or splashing.
In the paper “The interaction of modern sunscreen formulations with surface coatings,” [Progress in Organic Coatings62: 313:320. 2008] authors Phil Barker and Amos Branch systematically track down the underlying cause behind the unsightly blemishes. Out of ten sunscreens tested—four containing no nanoparticles, five containing titanium dioxide nanoparticles, and one containing zinc oxide nanoparticles—all but one of the nanoparticle-based sunscreens consistently degraded samples of pre-painted roofing surface exposed to sunlight for 12 weeks. In contrast, the non-nano products had no obvious deleterious effect. In the worst case, the roofing lost over 85% of its gloss (a measure of degradation) in just six weeks.
Digging a little deeper, Barker and Branch pinned the effect to nanoparticles in all but one sunscreen acting as photocatalysts, and generating hydroxyl radicals in the presence of UV radiation and water. Despite assumptions that nanoparticles in sunscreens are engineered not to produce significant amounts of free radicals, these products were generating them fast enough to significantly damage roof coatings in a matter of weeks!
So have we had the wool pulled over our eyes? Are these supposedly benign nano-sunscreens we are slathering on our skin adding to our wrinkle-count before our time, and perhaps more besides?
Before jumping to conclusions, it is worth taking stock of what is known, and what is not. While the study showed all but one of the nanoparticle-based sunscreens had some adverse effects on the roofing, these effects varied greatly between products. The sunscreen using nano-zinc oxide particles led to a 55% reduction in gloss over 12 weeks, while in the worst case, a sunscreen containing 4% titanium dioxide led to a 95% reduction in gloss over 12 weeks. Assuming that the reduction in gloss is associated with the formation of hydroxyl radicals (and the evidence presented by Barker and Branch arising from a logical sequence of laboratory experiments is pretty convincing), there is still uncertainty over how harmful these would be when generated on the skin of a sunscreen-user. To cause damage, the hydroxyl radicals would need to penetrate deep into the skin and into cells before loosing their potency, and if the nanoparticles stay on top of the skin where they are supposed to, significant penetration may not occur.
Then there is the anomalous nano-sunscreen that didn’t show an appreciable effect. A nifty piece of X-ray diffraction analysis in the Barker and Branch paper showed that the titanium dioxide nanoparticles in the roof-damaging sunscreens were an anatase/rutile mix, while the nanoparticles in the benign sunscreen were comprised of rutile titanium dioxide alone. Clearly crystalline form matters, as Oxonica realized when they selected the less-active rutile form of titanium dioxide as the basis for Optisol.
This study demonstrates that it is possible to create nanoparticle-based sunscreens that do not generate significant amounts of hydroxyl free radicals. But the bottom line here is that some nano-based sunscreens are being sold (in Australia at least) that contain photoactive nanoparticles which generate hydroxyl radicals in the presence of water and sunlight. This raises questions about the impact of these products on users over time and, perhaps more significantly, their impact on the environment. A photocatalytic titanium dioxide particle released into the environment will continue to generate hydroxyl radicals as long as it is exposed to UV radiation—because this is a catalytic process, the particle is not destroyed in the process, but just carries on doing its stuff; day after day, year after year.
But perhaps the biggest question here is one of regulation. In the US, the Food and Drug Administration does not currently discriminate between anatase and rutile titanium dioxide particles in sunscreens, or doped and un-doped particles [Sunscreen Drug Products For Over-The-Counter Human Use: Final Monograph. May 21 1999. PDF, 144 KB]. This may change following further consultation on the use of nanoscale titanium dioxide and zinc oxide in sunscreens [see Sunscreen Drug Products For Over-The-Counter Human Use; Proposed Amendment of Final Monograph; Proposed Rule. August 27 2007. PDF, 424 KB]. But in the meantime, what is to stop manufacturers using potentially harmful forms of titanium dioxide in sunscreens? And how will consumers be able to distinguish between companies that have got it right, and those that have not?
It seems that if we are not careful, nano-sunscreens could be making their mark on more than just pre-painted steel roofing.
This post first appeared on the SAFENANO blog in June 2008