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Uncertainty over the best way to evaluate the risks of engineered nanomaterials (ENMs) is causing delay in regulation despite their rapidly growing use by industry and in consumer products. Because of their enormous diversity in size, composition, configurations and properties, it seems clear that ENMs cannot be regulated as a single class. Occupational exposure levels (OELs) for engineered nanomaterials may however be usefully set using proposed benchmark/safety factor approach. In the proposed approach, a benchmark OEL would be used for ENMs considered to be ‘relatively inert’ (such as TiO2). As a precaution, the recognised benchmark OEL would be based on an existing standard for fine particulate air pollution, which is highly biologically active and includes modes of particles in relevant size ranges. For ENMs which cannot be presumed to be relatively inert, additional uncertainty factors (e.g., 0.1 or 0.3) would be applied for toxicologically significant properties, such as metal or semimetal content, fibrous shape, biological activity, structure-activity relationships suggesting greater potency, and resemblance to known hazards. OELs set in this manner could be used as provisional regulatory standards until data become available to support more definitive standards. An illustration of the approach is given based on a the Canada-wide ambient air quality standard for fine particulate matter (PM2.5). The Canadian standard and the proposed benchmark OEL are mass-based, of necessity. While mass may not be the most relevant dose metric for evaluating the biological effects of ENMs, mass-based measurement techniques are the only practical means for routine workplace exposure monitoring at the present time.