Occupational Exposure to Carbon Nanotubes and Nanofibers

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Philip Sayre, Ph.DAssociate Director, Risk Assessment DivisionOffice of Pollution Prevention & Toxics, U.S. EPARonald H. White, M.S.T.Deputy Director, Risk Sciences and Public Policy InstituteJohns Hopkins UniversityBloomberg School of Public HealthPublic ReviewNIOSH greatly appreciates the public comments on the December 2010 draft documentthat were submitted to the NIOSH docket. The comments and responses to them can beseen at: www.cdc.gov/niosh/docket/archive/docket161A.htmlxxivNIOSH CIB 65 • Carbon Nanotubes and Nanofibers
1 IntroductionMany nanomaterial-based products are now commerciallyavailable. These include nanoscale powders,solutions, and suspensions of nanoscale materials,as well as composite materials and devicesincorporating nanomaterials. The International Organizationfor Standardization (ISO) has developednomenclature and terminology for nanomaterials[ISO/TS 2008]. According to ISO 27687:2008, anano-object is material with one, two, or three externaldimensions in the size range from approximately1–100 nanometers (nm). Sub-categories ofa nano-object are (1) nanoplate, a nano-object withone external dimension at the nanoscale (i.e., 1–100nm); (2) nanofiber, a nano-object with two externaldimensions at the nanoscale, with a nanotubedefined as a hollow nanofiber and a nanorod as asolid nanofiber; and (3) nanoparticle, a nano-objectwith all three external dimensions at the nanoscale.Nano-objects are commonly incorporated in a largermatrix or substrate called a nanomaterial. Thistaxonomy differs slightly from that suggested by thetitle of this CIB “Occupational Exposure to CarbonNanotubes and Nanofibers.” By this title, NIOSH isnot suggesting an alternative taxonomy, but ratheridentifying the nano-objects (nanoscale carbon fiberand tube structures) that have been evaluatedto date in toxicology and workplace exposure measurementstudies.Carbon nanotubes (CNT) are nanoscale cylindersof carbon (essentially consisting of seamlessly“rolled” sheets of graphene) that can be producedwith very large aspect ratios. There is no single typeof carbon nanotube. They may differ in shape, dimension,physical characteristics, surface coatings,chemical composition, or surface functionalization.This includes “raw” CNT, which contain residualmetal catalysts vs. “purified” CNT, from whichmost of the metal catalysts have been removed.Single-walled carbon nanotubes (SWCNT) consistof a single rolled graphene sheet and have a typicaldiameter of approximately 1–2 nm. Multi-walledcarbon nanotubes (MWCNT) consist of manysingle-walled tubes stacked one inside the otherwith diameters in the range of 2–100 nm, dependingon the number of encapsulated tubes formingthe CNT structure. SWCNT and MWCNT canvary in length, with some being up to many tens ofmicrometers long [Thostenson et al. 2001]. Carbonnanofibers (CNF), which are structurally similar toMWCNT, have typical diameters approximately 40to 200 nm [Ku et al. [2006]. CNF have lengths rangingfrom tens of micrometers to several centimeters,average aspect ratios (length to diameter ratio)of > 100, and they display various morphologies,including cupped or stacked graphene structures.The primary characteristic that distinguishes CNFfrom CNT resides in graphene plane alignment. Ifthe graphene plane and fiber axis do not align, thestructure is defined as CNF, but when parallel, thestructure is considered a CNT [ISO/TS 2008].The synthesis of CNT and CNF requires a carbonsource and an energy source [Sanchez et al. 2009].CNT and CNF are synthesized by several distinctmethods, including chemical vapor deposition(CVD), arc discharge, laser ablation, and highpressureCO conversion (HiPco). Depending onmaterial and method of synthesis, a metal catalystmaybe used to increase yield and sample homogeneity,and to reduce the synthesis temperature.The diameter of the fibers depends on the dimensionsof the metal nanoparticle used as a catalyst;the shape, symmetry, dimensions, growth rate, andcrystallinity of the materials are influenced by theselection of the catalyst, carbon source, temperature,and time of the reaction. Different amountsof residual catalyst often exist following synthesis;consequently, post-synthesis treatments are usedto increase the purity of the product. The mostNIOSH CIB 65 • Carbon Nanotubes and Nanofibers1
Page 1 and 2: CURRENT INTELLIGENCE BULLETIN 65Occ
Page 3 and 4: Current Intelligence Bulletin 65Occ
Page 5 and 6: ForewordThe Occupational Safety and
Page 7 and 8: Executive SummaryOverviewCarbon nan
Page 9 and 10: 2009; Pauluhn 2010a; Porter et al.
Page 11 and 12: neurogenic sig nals from sensory ir
Page 13 and 14: possible. Until the results from an
Page 15 and 16: ••Follow exposure and hazard as
Page 17 and 18: Periodic Evaluations••Evaluatio
Page 19 and 20: ContentsForeword ..................
Page 21 and 22: A.3.2 Comparison of Short-term and
Page 23 and 24: ESPFeFMPSFPSSgGMGSDHCLHECHEPAhrISOI
Page 25: AcknowledgementsThis Current Intell
Page 29: 2 Potential for ExposureThe novel a
Page 32 and 33: CNMs, with MWCNT agglomerates obser
Page 34 and 35: composite materials with local exha
Page 36 and 37: information on air contaminants. Sa
Page 39 and 40: 3 Evidence for Potential Adverse He
Page 41 and 42: decreasing agglomerate size increas
Page 43 and 44: examined up to 60 days post-exposur
Page 45 and 46: 3.3 SWCNT and MWCNTIntraperitoneal
Page 47 and 48: The same potency sequence was obser
Page 49 and 50: Table 3-3. Findings from published
Page 55 and 56: Table 3-7 (Continued). Findings fro
Page 57: Table 3-8. Findings from published
Page 60 and 61: length, respectively) [Muller et al
Page 63 and 64: 5 CNT Risk Assessment and Recommend
Page 65 and 66: A-6). Risk estimates derived from o
Page 67 and 68: Table 5-4. Factors, assumptions, an
Page 69 and 70: and analytical methods. NIOSH is re
Page 71 and 72: Table 5-5. Recommended occupational
Page 73 and 74: deficits in animals or clinically s
Page 75: (3) Rat lung dose estimationIn the
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tasks where worker exposures exceed
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As part of the evaluation of worker
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Table 6-1. EC LODs and LOQs for 25-
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6.2 Engineering ControlsOne of the
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Table 6-6 (Continued). Examples of
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Table 6-7 (Continued). Engineering
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exposure estimates for SWCNT on ind
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Table 6-8. Respiratory protection f
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••Workers in areas or in jobs w
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7 Research NeedsAdditional data and
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ReferencesACGIH [1984]. Particle si
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Bolton RE, Vincent HJ, Jones AD, Ad
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eport issued on July 22, 2011. NEDO
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Kobayashi N, Naya M, Mizuno K, Yama
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Methner M, Hodson L, Geraci C [2010
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Human Services, Centers for Disease
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Piegorsch WW, Bailer AF [2005]. Qua
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AD, Baron PA [2003]. Exposure to ca
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Varga C, Szendi K [2010]. Carbon na
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ContentsA.1 Introduction ..........
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A.1 IntroductionThe increasing prod
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provide an informal check on the es
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these same dose groups; this effect
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Table A-1. Rodent study information
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the deposited (no clearance) and th
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The other BMDS models failed to con
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Figure A-2. Benchmark dose model (m
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Figure A-3 (continued). Benchmark d
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Table A-3. Benchmark dose estimates
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Table A-5. Benchmark dose estimates
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histopathology grade 2 or higher lu
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Table A-8. Working lifetime percent
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developing early-stage adverse lung
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Figure A-4. Dose-response relations
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cell surface area). However, the wo
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purified or unpurified (with differ
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Table A-9. Comparison of rat or hum
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A.6.1.3 Pulmonary Ventilation RateT
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used as the effect levels in evalua
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the DF estimate, although a larger
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or overloading, of particle clearan
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Table A-13. Human-equivalent retain
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A.7.1 Particle CharacteristicsBoth
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and density. The following MMAD and
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Table A-15. CNT lung dose normalize
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B.1 Key Terms Related toMedical Sur
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APPENDIX CNIOSH Method 5040
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filter. In the method evaluation, d
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Most of the studies on sampling art
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e analyzed to determine the onset o
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Delivering on the Nation’s promis
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