I've pondered why the contaminates would need to be ferrous......it's so they react to magnetic fields because that's how you would be able to transfer power to things like nanites. I'm not talking the single use medical delivery nanites that attach themselves to your organs to deliver medication. I'm talking nanoscopic robotics. I don't actually believe this to be true, but it would seem to me that if it were true, then the materials would have to be ferrous.