A combination approach may also be feasible, by using existing animal dose–response data for
well-characterized benchmark particles (for which risk has been quantified), along with short-ter min vivo or in vitro assays of an array of nanomaterials with similar chemical–physical properties and biological mode of action (e.g. carbonaceous particles; metal oxides) (Kuempel et al., 2007, in preparation). The comparative toxicity data could
then be used in a parallelogram type of analysis (Schoeny and Margosches, 1989; Sobels, 1993) to
infer the risk by the nanomaterial in comparison to the benchmark particle. Development and evaluation of categorical approaches to support OEL development is a priority area of the NIOSH NTRC strategic
plan (NIOSH, 2010b).