Titania nanoparticles are a common ingredient in sunscreen with a high sun-protection factor (SPF). The small particles mean that such products are no longer the opaque white creams that were considered unattractive, and this has increased the uptake of these sunscreens. However, concerns have been raised about the safety of nanoparticles in such treatments. Although nanoparticles have been shown not to penetrate beyond the top layer of skin, titania nanoparticles react with UV light to produce potentially dangerous reactive oxygen species (ROS).

Now, Amanda Barnard from the Materials Science and Engineering division of CSIRO in Australia has made1 a theoretical comparison of the three key properties — SPF, transparency and ROS generation — of titania nanoparticles in sunscreen. The models show that both SPF and transparency increase as the nanoparticles become smaller. The two most common phases of titania — rutile and anatase — have different reactivity for the production of ROS, however, and the amount of each phase present is dependent on nanoparticle size. This interplay results in a peak in ROS production for nanoparticles at approximately 33 nm in size.

On balancing these factors, Barnard found that the most effective and least toxic nanoparticles would be those with an average diameter of less than 13 nm. The only toxicity taken into account in the study is that related to ROS generation, so further studies into risks associated with nanoparticles of this size remain important.