Since the late 1980s, toxicological evidence has been emerging indicating that the health effects associated with inhaling nanoaerosols may not be closely associated with particle mass. Early studies with polytetrafluoroethene (PTFE) particles around 20 nm in diameter showed that airborne concentrations of a supposedly inert insoluble material lower than 50 μg/m3 could be fatal to rats [4][5][34]. Since then, a number of studies have indicated that the toxicity of insoluble materials increases with decreasing particle size on a mass for mass basis