Revisiting an Old Friend Nano-TiO₂: A Crucial Assessment of Its Current Safety

The recent advances in nanoscience and nanotechnology have enhanced the synthesis of various forms of nanomaterials for practical applications. Undoubtedly, these nanomaterials are not without attendant toxicities to humans, environment, and other organisms. Moreover, the toxicity of nanomaterials dominates the landscape of current toxicity concerns highlighted by the FDA. Titanium dioxide nanoparticles (TiO2 NPs) contribute a large proportion of synthesized nanomaterials mainly due to their excellent photocatalytic activities, mechanical and chemical stability, bio- and chemical inertness, corrosion resistance, thin film transparency, and low production cost. These fascinating properties of TiO2 NPs have been extensively exploited and dramatically increased their utility for various applications such as in nanomedicine for cancer theranostics, nanobiotechnology, environment, pharmacy, energy, food, cosmetics, and paper industries.  Owing to the poor understanding of the impacts of NPs on humans, no clear regulation has been implemented for NPs among international authorities. Over time, the toxicity state of TiO2 NPs is typical of a double-edged sword. Hitherto, there is no restriction on the use of TiO2 NPs irrespective of the toxicity concerns raised by some researchers. This may have been dampened by the low-to-no toxicity reports from other researchers on these NPs. This review therefore looks into the recent toxicity reports from various studies conducted with/on TiO2 NPs as to ascertain their present-day safety. To elucidate this, we discussed the possible exposure routes to these NPs and their effects on the environment, plants, soil organisms, and aquatic species. We also provided insights on the toxicity mechanisms of TiO2 NPs and proposed future perspectives for improving their safe applications.