While it's probably a negligible risk, we would breathe the dust from silk fibers if the stuff were produced in significant quantity.
We're already doing that; we produce nanoparticles of carbon from a number of industrial processes, most notably burning coal. IIRC there was an article on HN recently about finding some magnetic nanoparticles which had made their way into human brains.
Doesn't that make getting an MRI really really dangerous?
Actually IIRC the article described using MRI's to find those particles, but I don't exactly remember. Either way, no, they're just too small. What effect they have in the production of free radicals and the like though? Who knows yet.