Real Facts about Safety and Efficacy of Zinc Oxide and Titanium Dioxide in Solar Products

Abstract

Background: Titanium dioxide and zinc oxide were often criticized over the last decade because of their supposed noxious effects on human health. Moreover, these compounds which are frequently introduced in sunscreen products as UV filter, are sometimes associated with poor UVA protection factors. So, in order to clarify the real efficacy and safety status of these products, we provide here some bibliographic and experimental data regarding 1) their “real” protective effect against UVA rays and 2) their real harmful effects on human skin notably by studying their capability to penetrate through the human cutaneous tissue. Materials and Methods: We studied here 4 sunscreen products containing titanium dioxide and zinc oxide for 3 of them. First, because the UVA-PF values obtained for these compounds by using the “classical” in vitro ISO 24443 procedure seem to be significantly different from to those obtained by using the in vivo method ISO 24442, we chose to develop a new in vitro methodology in order to more precisely define the UVA-PF of titanium and zinc oxides. This new methodology was then used to lead UVA-PF studies with the 4 selected solar products. We also provide here an evaluation of the toxicological effects of titanium and zinc oxides on human skin based on the SCCS reports and analysis of recent and relevant bibliographic studies. Moreover, as the harmful effects of this type of products are closely linked to their ability to penetrate cutaneous tissue, we tested 7 sunscreen products to precise the skin penetration profiles of titanium dioxide and zinc oxide by using human skin explants mounted on Franz cells. Results: We here demonstrated that our new in vitro methodology gave some UVA-PF values very close to those obtained with in vivo methods and we took advantage of it to define more realistic UVA-PF for titanium dioxide and zinc oxide. Additionally, we here evaluated the human skin permeation and resorption capacities of titanium dioxide and zinc oxide incorporated in the 7 tested products. As it was defined by World Health Organization (WHO) in 2005, permeation consists in the ability for a compound to penetrate into different layers of a tissue, and the resorption consists in the absorption of this compound into the vascular system. In our experimental conditions, we showed 1) that zinc oxide and titanium dioxide permeations did not exceed 8.5 and 5.5 μg/cm2 of skin respectively (i.e. 0.89% and 0.26% of the applied product, respectively), and 2) that their resorptions were not significantly different from zero. As a consequence, we can assume that the supposed harmful effects of titanium dioxide and zinc oxide on cutaneous tissue could not be observed following the use of the tested solar products. Conclusion: Regarding their efficacy, we here provide, by using a new in vitro methodology for UVA-PF measurements (which is also very efficient to determine SPF), new evidence showing that titanium dioxide and zinc oxide could constitute “good” UV filters. In addition, our work with Franz cells reinforces the fact these compounds can be safely used for human skin solar protection.