From Nature:
Tiny particles used in some sun creams have the potential to cause neurological damage, researchers in the United States have found. Bellina Veronesi of the US Environmental Protection Agency’s research laboratories in North Carolina and her co-workers have studied the effect of nanoparticles of titania (titanium oxide) on cultures of mice cells called microglia, which protect neurons in the brain from harm.
They find that the particles provoke the cells to manufacture chemicals that are protective in the short term but potentially damaging when released in the prolonged manner seen in the experiments. Scientists working with nanoparticles have known for a long time that size matters: at these very small scales, the properties of materials can change. For one thing, the chemical reactivity of powders depends on their surface area, which increases as the particles get smaller.
More here.