SYDNEY: Zinc from sunscreens can penetrate healthy adult skin, according to two Australian studies. The results raise questions over the safety of sunscreens containing zinc oxide nanoparticles – which may be dangerous in the body if still in a nanoparticle form.
Zinc oxide and titanium dioxide block UV light across a wide spectrum of wavelengths, making them suitable for use in sunscreens. Sunscreens containing large bulks of these particles of the metal oxides reflect light and appear white on the skin, and so they are not popular with consumers.
Many sunscreens for sale in Australia now contain nanoparticles of zinc oxide and titanium dioxide, measuring just 20 to 30 billionths of a metre across. The nanoparticles don’t reflect sunlight, making the lotion appear clear when rubbed into the skin.
While such particles have the potential to cause damage to DNA and cells, scientists have been unsure until now whether the particles were penetrating through the skin.
Zinc gets into the bloodstream
Now new research shows that zinc from the sunscreens reaches the bloodstream, said geochemist Brian Gulson from Macquarie University at the International Conference on Nanoscience and Nanotechnology (ICONN) in Sydney last week.
Having zinc in the bloodstream is not necessarily dangerous – especially as the amount they found was much smaller than the recommended daily dietary intake for zinc.
“The amounts we are seeing compared to the amount in blood are really quite tiny,” said Gulston. “It’s a thousand times lower than what’s actually floating around in our bodies.”
He suggested that until more is known about skin penetration of the metal oxides that we continue to follow the Cancer Council’s advice of slip, slop slap.
Blood and urine samples show zinc gets in
In the experiment, the researchers concocted two types of sunscreen zinc oxide, using a rare non-radioactive isotope of zinc, zinc-68, which has four more neutrons than common zinc 64. One sunscreen contained ‘nano’ particles of zinc oxide and the other contained ‘bulk’ particles at least five times larger.
Test volunteers were separated into two groups and applied sunscreen twice daily: one group used sunscreen with bulk particles and the other used sunscreen with nanoparticles.
Gulson’s team then took blood and urine samples from volunteers to see whether levels of zinc 68 increased compared to levels measured before the test.
Gulson found higher levels of Zinc 68 in all volunteers, but it isn’t known exactly how the zinc is getting in. The question that remains is whether the zinc is in the form of a zinc oxide nanoparticle or if it had dissolved into harmless zinc ions.
Slip, slop, slap … for mice
Sydney-based biologist Megan Osmond, from the science research body, CSIRO, is also conducting tests with the same sunscreen containing zinc 68 on a special breed of hairless mice. The mice skin is easier to penetrate than human skin, so was considered a worst-case scenario for the effect of nanoparticles.
Early results show that zinc 68 is getting into the bloodstream of the mice, but as with Gulson’s study it is not known whether nanoparticles are penetrating the skin, she also said at the ICONN conference.
“All we can really say is that we can see the tracer,” said Osmond. “Whether it’s dissolved zinc or tracer attached to a zinc oxide nanoparticle is something we can’t answer at this stage.”
“Finding nanoparticles in a complex matrix [such as bodily tissues] is really hard to do, especially if they are there at low concentrations,” said nanoscientist Maxine McCall, from the CSIRO.
“So you’ve got to search, search, search, and you have got to develop methods to detect them.”
McCall said it’s not yet possible to say whether nanoparticles in sunscreens are safe: “We have to finish our current studies, and when we know the results we’ll certainly be making them public.”
Are nanoparticles dangerous?
Previous studies have shown that nanoparticles can penetrate human cells. And a study by BlueScope Steel concluded that when applied to roofing material and subjected to UV light, titanium dioxide nanoparticles operated as potent ‘photo-catalysts’, accelerating the production of hydroxyl radicals. Hydroxl radicals are known to damage protein and DNA in the body.
Georgina Miller, a spokesperson for Friends of the Earth Australia (FoE), said the Australian Therapeutic Goods Administration (TGA) needs to bring in mandatory safety testing for sunscreens containing nanoparticles. FoE is also calling for the introduction of labelling of nanoparticle content on sunscreens to give consumers a choice.
After reviewing the scientific literature in 2009 the TGA concluded that nanoparticles do not penetrate far enough into the body to do damage. Miller said that now bodily penetration of Zinc 68 has been shown in these two studies, the TGA needs to rethink its position.