Because of their extremely small size and large surface area to volume ratio, nanoparticles can easily cross the biological cells and membranes. Once inside the body, they can enter the bloodstream and hence reach different organs.
Titanium dioxide nanoparticles are one of the largest manufactured nanoparticles in the world. They are used in variety of applications as paints, printing ink, rubber, paper, cosmetics, pharmaceuticals, sunscreens, and car materials.
Zinc oxide nanoparticles are also employed in cosmetics and modern sunscreens, in coatings (UV protection) and electronic device.
A study by Khan et al clearly suggest that both zinc oxide and titanium dioxide in nanoform are significantly cytotoxic (toxic to cells) as well as genotoxic (damaging to genetic information within cells) at all concentrations. However, nanoparticles of zinc oxide were significantly more genotoxic than those of titanium dioxide at higher concentrations.