Iron nanoparticles can be toxic to human lung cells.

Iron nanoparticles can be toxic to human lung cells.
http://www.environmentalhealthnews.org/ehs/newscience/iron-nanoparticles-toxic-to-human-lung-cells/

"Iron nanoparticles that are poised for use in large-scale pollution remediation can rapidly react with oxygen and cause lung cells to die."