“A material with particles that are larger than 100 nm may have significant structuring on the nanometer size scale, thereby providing properties based upon this smaller structuring that may be toxicologically relevant (e.g., dramatically increased surface area as compared to the bulk). Nanoporous materials, such as zeolites, are a significant class of materials which have porosity in the sub-100 nm size range but whose primary particles may be large.”¹

1. G. Oberdörster, A. Maynard, K. Donaldson, et al, “Principles for characterizing the potential human health effects from exposure to nanomaterials: elements of a screening strategy,” Particle and Fibre Toxicology 2005, 2:8