In his 2002 novel, "Prey", the late Michael Crichton advanced a fictional scenario in which nanoparticles escaped from a lab and formed swarms in the desert with the drive and ability to kill humans and other animals whom they could use as feeding-templates on which bacteria could grow to replicate more of their kind. While such a scenario does appear alarmist and unlikely in reality, there remains nonetheless a sense of disquiet over the safety of nanomaterials, and their potential toxicity should they be released into the environment.
In 2008, manufacturers of skin "care-products" decided to avoid them in their formulations and now EU ministers have decided that nanosilver particles (also used e.g. in washing machines and in shoes to get rid of nasty smells) and multiwalled carbon nanotubes should be banned in electronic and electrical products. Members of the EU Environment Committee made this call during their vote on possible amendments to the "Restriction of Hazardous Substances Directive".
In addition, the Committee has recommended that all electrical and electronic products (including fast-computers and solar cells) that contained "nanomaterials of any nature" should be so labelled as containing them. Hence an onus would be on manufacturers to provide safety hazard information on any nanomaterials that their products may contain.
This appears quite tricky for example given the putative application where carbon nanotubes could be used as "synthetic nerves" in limb prosthetics, by acting as template around which neural tissue might grow. As introduced into the human body directly, any potential toxicity might prove rather difficult.
My awareness of the toxicity of carbon nanotubes is that the jury is out. I know of one study of them designed to search for the formation of free-radicals (toxic, short-lived molecules derived from oxygen) which seemed to indicate that the nanotubes actually soaked-up these species, leaving less of them than would be the case in their absence. That said, there are other studies that support a toxic role for carbon nanotubes. Since silver nanoparticles act in cleaning biological stains and smells by producing hydroxyl and other oxygen radicals, which are toxic in vivo, then if they were ingested the consequences could be dire.
The vote on the proposals is due in October, as reported in the July 2010 edition of Chemistry World, published by The Royal Society of Chemistry.