Occupational Exposure to Carbon Nanotubes and Nanofibers

CNMs, with MWCNT agglomerates observed tobe 500 to 1,000 nm in diameter.The National Institute for Occupational Safety andHealth (NIOSH) conducted emission and exposureassessment studies at 12 sites where engineerednanomaterials were produced or used [Methner etal. 2010a]. Studies were conducted in research anddevelopment laboratories, pilot plants, and smallscalemanufacturing facilities handling SWCNT,MWCNT, CNF, fullerenes, carbon nanopearls,metal oxides, electrospun nylon, and quantumdots. Airborne exposures were characterized usinga variety of measurement techniques (e.g., CPC,OPC, TEM) [Methner et al. 2010b]. The purposeof the studies was to determine whether airborneexposures to these engineered nanomaterials occurand to assess the capabilities of various measurementtechniques in quantifying exposures. Ina research and development laboratory handlingCNF, airborne particle number concentrations (determinedby CPC) were reported as 4000 particles/cm 3 during weighing/mixing and 5000 particles/cm 3 during wet sawing. These concentrations weresubstantially less than the reported backgroundparticle concentration of 19,500 particles/cm 3 .Samples collected for TEM particle characterizationindicated the aerosol release of some CNF. Allhandling of CNF was in a laboratory hood (withHEPA filtered vacuum) for the weighing/mixingand wet saw cutting of CNF composite materials.In a facility making CNF in a chemical vapor phasereactor, OPC particle count concentrations rangedfrom 5,400 particles/cm 3 (300–500 nm particlesize) to a high of 139,500 particles/cm 3 (500–100nm particle size). Higher airborne particle concentrationswere found during the manual scooping ofCNF in the absence of exposure control measures.Samples collected for TEM particle characterizationindicated the aerosol release of some CNF. Inanother research and development laboratory, thepotential for airborne exposure to MWCNT wasevaluated during weighing, mixing, and sonication.All handling of MWCNT was performed ina laboratory hood (without HEPA filtered vacuum).Particle concentrations were determined byCPC (particle size 10–1000nm) and OPC (particlesize 300–500nm, 500–100nm). CPC particle concentrationsranged from 1480–1580 particles/cm 3(weighing MWCNT in hood) to 2200–2800 particles/cm3 (sonication of MWCNT). The backgroundparticle concentration determined by CPC was 700particles/cm 3 . Airborne particle concentrationsdetermined by OPC ranged from 3,900–123,400particles/cm 3 (weighing) to 6,500–42,800 particles/cm 3 (sonication). Background particle concentrationsdetermined by OPC ranged from 700particles/cm 3 (1–10 µm particle size) to 13,700particles/cm 3 (300–500 nm particle size). The higherparticle concentrations determined with the OPCindicated the presence of larger, possibly agglomeratedparticles. Samples collected for TEM particlecharacterization indicated the aerosol release of agglomeratedMWCNT.Subsequent studies conducted by NIOSH at sixprimary and secondary pilot or small-scale manufacturingfacilities (SWCNT, MWCNT, CNF)employed a combination of filter-based samplesto evaluate PBZ and area respirable and inhalablemass concentrations of EC as well as concentrationsof CNT and CNF structures determined byTEM analysis [Dahm et al. 2011]. A total of 83filter-based samples (30 samples at primary and22 at secondary manufacturers) were collected forEC determination (NIOSH Method 5040) and 31samples for TEM analysis (NIOSH Method 7402).Similar processes and tasks were reported in thethree primary and three secondary manufacturersof CNT. These processes and tasks consisted of:(1) similar production and harvesting methods forCNT, and common cleaning/housekeeping proceduresin primary manufacturers, and (2) commonCNT handling practices such as weighing, mixing,sonication, manual transfer, cleaning, and spraycoating operations in secondary manufacturers.Worker PBZ inhalable mass concentrations foundat primary manufacturers ranged from 0.68 to5.25 µg/m 3 EC with an average concentration of2.42 µg/m 3 . Area samples for EC determinationfrom these samples ranged from non-detectableto 4.62 µg/m 3 while outdoor background samples6 NIOSH CIB 65 • Carbon Nanotubes and Nanofibers