Magnetic nano-particles seem to pose a serious urban public health risk so I am searching ways to remove them from the air around cities. Obviously, electromagnets would work but that is not what hospitals or underground transport systems use. Any time a vehicle brakes (especially when on metal tracks) these nanoparticles are produced so the scope is large.

http://www.newsweek.com/potentially-toxic-magnetic-nanoparticle-pollution-human-brains-495950