The disclosure is directed to a method for reducing the chemical activity and photo activity of titanium dioxide nanoparticles comprising adding an amorphous alumina-stabilizing agent, specifically an anion, such as citric acid, to an aqueous slurry of the titanium dioxide nanoparticles; treating the aqueous slurry with a source of alumina, such as a solution of sodium aluminate, to form alumina-treated titanium dioxide nanoparticles; recovering the alumina-treated inorganic nanoparticles, wherein the nanoparticles have a particle size of about 50 to about 300 nm; and milling the nanoparticles to form nanoparticles having a particle size about 15% smaller than silica-treated inorganic nanoparticles milled under similar conditions. The titanium dioxide nanoparticles described herein are useful in cosmetic, coating and thermoplastic compositions.