T. Long, et al., "Nanosize Titanium Dioxide Stimulates Reactive Oxygen Species in Brain Microglia and Damages Neurons in Vitro," Environmental Health Perspectives, Vol. 115, No. 11 (Nov. 2007).

The authors of this study note nanoscale titanium dioxide is used in a variety of applications that come into contact with people and the environment: toothpastes, sunscreens, cosmetics, food products, paints, surface coatings, and in environmental decontamination of air, soil, and water. 
The study was designed to test the response of human nerve cells to exposure to nanoscale titanium dioxide.  The authors theorized that the particles have an unusual size and unusual properties allowing them to enter body, cross biological barriers, enter systemic circulation, migrate to tissues and organs, bioaccumulate, and cause oxidative stress and cell damage.  The study used Degussa P25 which is an uncoated, photoactive, largely anatase form of nanoscale titanium dioxide — which is "not to be confused with nonphotoactive nanomaterial currently used in sunblocks and cosmetics."  The authors note that Degussa P25 is most currently used in waster treatment, self-cleaning windows, and antimicrobial coatings and paint.  The study found reactive oxygen species in brain microglia exposed to nanoscale titanium dioxide under special test conditions, as well as some neuron damage in another in vitro test.