Nanoparticles have to be used within tolerable or prescribed limits to prevent ill effects on health. Titanium dioxide is used in sunscreen lotion while toothpaste contains hydroxyl appetite to seal the tubuli in dentin connected to nerve cells and thereby prevent pain in teeth.
As far as we know, there is no threshold concentration for living organism and even soil and water environment. Still long way to go in this contest. I am not sure EPA/WHO is working on this or not. I heard NMs is banned in the UK and some other countries.