There is growing interest in nanotechnology with a number of reports highlighting that there are some gaps in the knowledge of the risks posed by nanoparticles to human health and the environment. There is no official definition of the term “nanoparticle”, but generally this term is taken to mean particles with one or more dimensions of less than 100nm. By this definition, most inorganic sunscreen materials (fine particle titanium dioxide and zinc oxide) are regarded as nanomaterials.
HOWEVER, it is important to recognise the distinction between inorganic sunscreens and other types of nanoparticles. Trying to treat all nanomaterials as though they are the same is not a reasonable approach. Some nanoscale materials, for example quantum dots, have particle sizes as small as a few nanometres, and hence are orders of magnitude smaller than more conventional forms of the same materials. As such, these nanoparticles can exhibit properties which are fundamentally different from the larger particles. The same is not true of inorganic sunscreens. Materials such as TiO2 and ZnO exist in a continuum of particle sizes above and below 100nm and the chemical properties of the materials are fundamentally the same whatever their size. Even pigmentary forms of TiO2 and ZnO, which have mean particle sizes well outside the nanoscale range, include a proportion of particles less than 100nm. There is no step change in chemical properties between the “nano” forms and the larger TiO2 or ZnO particles.
The crystal size of inorganic sunscreens can be as small as 10-20nm. However, individual crystals are difficult to isolate and are not present in commercial forms of the materials. Rather, the crystals form aggregates, in which the crystals are tightly bound together. These aggregates are the smallest discrete particles which are actually present in dispersions of inorganic sunscreens, having a typical size of 30-150nm (equivalent sphere diameter). Such a particle size is necessary for the materials to function as sunscreens. In powder forms of inorganic sunscreens, these aggregates form loosely-bound agglomerates, with sizes in the range of 1-100microns (ie 1000-100,000nm). The UV attenuation from such large agglomerates will be minimal. In order to be effective the agglomerates are broken down during the dispersion and/or formulation process to particles with a distribution in the 100nm size range.
Unlike some of the more novel nanoparticles, inorganic sunscreens are not new; they have been in use for over 20 years, and the safety of these materials has been the subject of extensive and exhaustive study. This has shown that fine particle TiO2 and ZnO are safe for use in sunscreens. The current recognition of TiO2 as an UV filter on annex VII of the Cosmetics Directive is based on the extensive dossier on the various aspects of the consumer safety of TiO2, submitted by the industry and carefully evaluated by the SCCNFP (predecessor of SCCP). The opinion adopted by the SCCNFP in October 2000 is publicly available at: europa.eu.int/comm/health/ph_risk/committees/sccp/documents/out135_en.pdf.
Much of the concern around nanoparticles in cosmetics arises from speculation that the particles may penetrate through the skin. This specific aspect has been the subject of many studies, and the evidence shows that insoluble nanoparticles, such as those used in inorganic sunscreens, do not penetrate into or through human skin. Further, the industry submitted a review of recent literature on safety of nanomaterials in cosmetics with special references to skin absorption and resorption of ultrafine TiO2 to SCCP in 2005. Several new studies still indicated that ultrafine TiO2 does not penetrate the skin. The lack of penetration of UV attenuation forms of TiO2 used in sunscreens into or through human skin as well as absence of a health risk has been acknowledged by the Australian Therapeutic Goods Administration (TGA, 2006, www.tga.gov.au/npmeds/sunscreen-zotd.htm).
To Summarise:-
1. Inorganic sunscreens, because of their particle size, are generally classified as nanoparticles.
2. Powder forms of these materials consist of loosely-bound agglomerates which have a much larger size. However these agglomerates are broken down during dispersion or formulation; this is necessary for the particles to function effectively as sunscreens.
3. Nanoparticle forms of TiO2 and ZnO do not exhibit different chemical behaviour than the larger particle size versions of the same materials.
4. The safety of inorganic sunscreens has been proven by extensive and exhaustive studies, which have been subject to independent review by regulatory authorities and other bodies. In particular, it has been repeatedly shown that the particles do not penetrate the skin.
