The most effective nanoparticles in some invisible sunscreens might also be the most toxic, suggests a new Australian modelling study.
The study, by Dr Amanda Barnard of CSIRO Materials Science and Engineering in Melbourne, appears in today's issue of Nature Nanotechnology.
Barnard carried out computer simulations of the properties of titanium dioxide nanoparticles, which are used to make some sunscreens transparent, increasing their appeal to consumers.
"There's a whole range of demographics that would never ever use sunscreens if they were ugly," says Barnard.
"The transparent ones do increase usage and protection from skin cancer in certain demographics, so they do have an important function."
But, she says, questions have been raised about the safety of such sunscreens.
One particular concern is whether the nanoparticles interact with sunlight to produce free radicals that damage tissues or DNA.
Barnard's computer model examined titanium dioxide nanoparticles from 3 to 200 nanometres in size.
"This is the size range that would generally be used in different types of sunscreens," she says.
The model predicted the affect of nanoparticle size and concentration on sun-protection ability, transparency and potential to produce free radicals.
Barnard found that the size and concentrations of nanoparticles that gave the best transparency and sun protection also gave the highest potential for production of free radicals.
"Where we have the highest sun-protection factor - and it's pretty - it [the sunscreen] is also toxic, potentially," she says.
"Ultimately we have to trade off. We can't have our cake and eat it too."
Barnard found that only particles less than 13 nanometres in size would minimise free radical production while maximising transparency and sun protection.
But, she cautions against concluding that all particles under 13 nanometres in size would be free of potential risk.
"We only looked at free radical production and tiny particles like that could still be toxic for other reasons that still need to be tested," says Barnard.
She says further studies on the environmental impacts of nanoparticles in sunscreens are also required.
"All of these things wash off, when we're swimming, and we wash them off and flush them down the sink," Barnard says.
According to Georgia Miller from the Friends of the Earth Nanotechnology Project, most sunscreens containing titanium dioxide nanoparticles on the market would be using particles bigger than 13 nanometres.
If this is the case, says Miller, the research by Barnard suggests that the risk of free radical production in most is likely to cancel out the benefit of sun protection, and the Therapeutic Goods Administration (TGA) should not permit their use.
A spokeswoman for the TGA says the authority is examining the paper by Barnard, but was unable to provide further comment at the time of writing.
The TGA website states that to date, the current weight of evidence suggests that titanium dioxide and zinc oxide nanoparticles in sunscreens do not present a risk because they remain in the outer layer of the skin.
But environment and consumer groups are concerned about the lack of in-vivo studies of sunscreens and have been calling on the TGA to require nano-ingredients in sunscreen be tested, labelled and properly regulated.
Not all transparent or invisible sunscreens contain nanoparticles.