Tiny science, big risks.

So now that we know what nano is and why we should care, it’s important to consider that even with these striking benefits, there are also significant risks – for example, it is unknown how nanoparticles behave in the human body and in the environment. What's Hidden in Your Sunblock?: Although studies suggest that the sunblocks are safe for humans, some people worry that the tiny nanoparticles may impact the environment when they wash off our skin.
What's Hidden in Your Sunblock?: Although studies suggest that the sunblocks are safe for humans, some people worry that the tiny nanoparticles may impact the environment when they wash off our skin.Courtesy NISE Net - Creative Commons Attribution Non-Commercial Share Alike
Nanoparticles have been shown to penetrate the blood-brain barrier, so is it safe to use nano sunscreen? When you wash your nano-silver socks, are you sending those anti-bacterial nanoparticles out with your waste water, ultimately to your local lakes, rivers, and streams? What affect does that have on the food chain, starting at the bacterial level? We don’t know yet.

To add to the uncertainty, the products and production waste of nanotechnological endeavors are self-regulated by the companies who create them (not by the government). Some companies might not want you to know you're using nano-products, and since there are presently no nano-labeling laws they don't have to tell you. You might already be using nano-engineered items without your knowledge. And when you don't know, it's difficult to make an informed decision.