Over the last several years, nanoparticles have come under scrutiny for adverse health effects. Nanoparticles are ultrafine particles between 1 to 100 nanometers in diameter. (To put this in perspective, the average human hair is around 80,000 nanometers thick.) Because of their size, which can be engineered and manipulated at the atomic or molecular level, nanoparticles exhibit unique physical, chemical, and biological properties. Titanium dioxide is one of the most commonly produced nanoparticles in the world.

A 2022 review on past studies of titanium dioxide and rat lung cancer, for instance, said the original study was under extreme conditions and its effects were not replicated in other animal species. Additionally, the review concluded that the few studies which did directly focus on titanium dioxide's impact on humans did not end up showing any increased cancer risk.