A new Australian modeling study suggested that the most effective nanoparticles in some invisible sunscreens might be the most toxic. The study which is led by Dr Amanda Barnard of CSIRO Materials Science and Engineering in Melbourne is published today in the issue of Nature Nanotechnology.

The properties of the titanium nanoparticles which are used to make some sunscreen transparent are studied by using computer simulations. In certain demographics, the transparent sunscreens do increase usage and protection from skin cancer, according to Dr Barnard. But certain questions have been raised regarding the safety of such sunscreens.

One specific concern is whether these free radicals which are damaging to tissues or DNA, are released from the interaction between the nanoparticles and sunlight. In the study, titanium dioxide nanoparticles from three to 200 nanometers in size were examined.

The computer model predicted the affect of nanoparticle size and concentration of sun-protection ability, transparency and potential to produce free radicals.

The findings showed that the size and concentration of nanoparticles that provided the best transparency and sun protection have the highest potential for producing free radicals. Where we have the highest sun-protection factor - and it's pretty - it [the sunscreen] is also toxic, potentially, said Dr Barnard.

Minimum free radical potential that has maximum transparency and sun protection is found in particles less than 13 nanometers in size. The study warns against the conclusion that all particles less than 13 nanometers in size would be free of potential risk. Further studies need to be conducted in order to examine other environmental effects, and other risks.

Barnard reminded us that All of these things wash off, when we're swimming and we wash them off and flush them down the sink.

According to Georgia Miller from the Friends of the Earth Nanotechnology Project, the research by Dr Barnard suggests that the potential risk of free radical production in the sunscreen is likely to cancel out the sun protection benefit and Therapeutic Goods Administration (TGA) should not permit their use.

Currently, the TGA web site informs that the present weight of evidence does suggest that titanium oxide and zinc oxide nanoparticles in sunscreens do not pose a risk as they remain in the outer layer of the skin.

The TGA has been called by environmental and consumer groups to require nana ingredients present in sunscreens to be tested, labeled and properly regulated.