CONCLUDING REMARKS
Although there is uncertainty in the risk assessment of nanomaterials, it can be characterized to some extent. An evaluation of the main steps in the risk assessment process for inhaled particles shows which steps and assumptions have the largest influence on the risk estimates. This evaluation identifies the type of study data needed to reduce the uncertainty and the research priorities needed to obtain
those data. Some of these steps involve uncertainty that is specific to the nanomaterial (e.g. CNT lung clearance), and some are more broadly applicable
(e.g. extrapolating animal dose to humans; role of dose rate on the adverse effect). Other information is relatively well known (e.g. deposition fraction
of inhaled particles in the respiratory tract regions, given the breathing parameters and the airborne particle size). Targeted research using standardized test
methods and response endpoints would facilitate comparative toxicity assays and reduce uncertainty in risk assessment across nanomaterials. In addition to inhalation exposure (the focus of this article), dermal and other potential routes of exposure to nanomaterials in the workplace should be evaluated, as well as other possible effects beyond the lungs (NIOSH, 2009a; 2010b).