Natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range.
Both nanoparticles and ultrafine particles may occur as either agglomerates or aggregates. In these cases, many much smaller particles are combined to varying degrees to form larger structures. The question of whether nanoparticles may be released from agglomerates or aggregates in the human body, and if so under what conditions, is the subject of intensive research. [source: www.dguv.de]