TiO2-NPs Toxicity and Safety: An Update of the Findings Published over the Last Six Years

Nanotechnology has greatly impacted our daily life and has certainly yielded many promising benefits. Titanium dioxide nanoparticles (TiO2-NPs) are among those produced on a large industrial scale that have found many practical applications in industry and daily life. Due to their presence in products such as food, cosmetics, sunscreens, medications, paints or textiles, contact with TiO2-NPs in our daily life is inevitable. The small size, together with the corresponding large specific surface area, make nanoparticles able to penetrate through cellular barriers and reach various parts of the body through different routes of exposure, including inhalation, injection, dermal penetration, and gastrointestinal tract absorption. Furthermore, after long-term exposure, the TiO2-NPs could accumulate in tissues leading to chronic diseases. This raises serious doubts about their potentially harmful effects on human health. In the past, TiO2-NPs have been considered inert, however, many in vitro studies have shown that they were cyto- and genotoxic, leading to the production of reactive oxygen species (ROS) and to the activation of signaling pathways involved in inflammation and cell death. Several in vivo studies have also demonstrated that TiO2-NPs, once in the bloodstream, could reach and accumulate in important organs causing toxic effects. Very recently, the International Agency for Research on Cancer (IARC) has classified these nanoparticles as possibly carcinogenic to humans. In this survey, we summarize the latest advances in acknowledging the toxicity and safety of TiO2-NPs. Since the literature is often controversial, further studies are still needed to define the risk/benefit ratio of using these nanoparticles. Overall, the data herein reported are critical for assessing human risk after exposure to TiO2-NPs.