I think that all kinds of nanoparticles are appearing naturally in the form of smoke and dust. It is known to be unhealthy, but nothing like existential risk. Smoking cigarets is known to damage brain, but it is not a grey goo.
I included prions in another my map biological risks, here: http://lesswrong.com/r/discussion/lw/nbw/the_map_of_global_catastrophic_risks_connected/
I think that all kinds of nanoparticles are appearing naturally in the form of smoke and dust. It is known to be unhealthy, but nothing like existential risk. Smoking cigarets is known to damage brain, but it is not a grey goo.
I included prions in another my map biological risks, here: http://lesswrong.com/r/discussion/lw/nbw/the_map_of_global_catastrophic_risks_connected/