Occupational Exposure to Carbon Nanotubes and Nanofibers

As part of the evaluation of worker exposures toCNT and CNF, ‘background’ samples for EC determinationshould be collected outdoors (or atair intakes of the facility) and at indoor locationswhere exposure to CNT or CNF is unlikely. The ECconcentrations (using Method 5040) determinedfrom ‘background’ samples should be subtractedfrom the EC personal sample results to determinewhether worker exposures exceeded the REL. Initially,more samples may be required to characterizethe workplace thoroughly. This initial assessmentwill help refine the sampling approach and determinewhether EC interference is an issue. Carefulconsideration of environmental background is essential.For example, outdoor EC may sometimesbe higher than indoor background depending onthe facility’s air handling system. If so, the indoorEC background may be more representative of areaand worker samples.In workplaces where exposure to other types of EC(e.g., diesel soot, carbon black) may occur, the initialevaluation of a worker’s exposure should includethe simultaneous collection of a personal respirableEC sample and a personal sample for electron microscopyanalysis (e.g., TEM, SEM). Electron microscopyanalysis, in conjunction with energy dispersivex-ray spectroscopy (EDS), can be used forCNT and CNF identification. In addition, considerationshould be given to the sizing and counting ofCNT and CNF structures during electron microscopyanalysis should future efforts to control occupationalexposures be based on a different exposuremetric (e.g., number concentrations of airborneCNT and CNF structures in a given size bin). Whileno specific electron microscopy (e.g., TEM, SEM)method exists for the sizing and counting of CNTand CNF structures, methods used in the analysisof other ‘fibrous’ materials are available [NIOSH1994a; ISO 1999, 2002] and could be adapted in thecharacterization of exposures.NIOSH investigators have conducted a numberof surveys at CNT and CNF producers and/orsecondary users [Evans et al. 2010; Birch 2011a;Birch et al. 2011b; Dahm et al. 2011]. In manycases ‘background’ EC concentrations were