  Categorization approaches have been effectively applied to chemicals, and
many have tried to apply variations of these approaches to nanomaterials. Given
the added complexities of nanomaterials, this has been challenging.
International cooperation on categorization approaches has been made, primarily
through the Organization for Economic Cooperation and Development. Progress has
been limited given the complexities of nanomaterials, especially given the need
to consider not only the intrinsic properties of the materials but also
properties dependent on the system into which it is introduced. Consideration
must also be given to the different purposes and contexts, in particular
regulatory contexts, to which the categorization schemes would be applied. More
progress can be anticipated by focusing on areas of overlap among countries
such as physicochemical properties.
