Occupational Exposure to Carbon Nanotubes and Nanofibers

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particle in question. Conventionally, aerodynamicdiameter has been used as a reference diameter torepresent total particle deposition in the respiratorysystem over a wide particle size range. Models suchas MPPD [CIIT and RIVM 2006; ARA 2011] useparticle density (specified by the user), to convertaerodynamic to physical diameter and vice versa,and in this manner capture the key particle depositionmechanisms for spherical particles.However, for high-aspect ratio particles and particlesless than 500 nm diameter, including someindividual or airborne agglomerates of CNT, theaerodynamic diameters are much smaller thantheir diffusion-equivalent diameter (i.e., the measureof diameter that captures the diffusional depositionmechanism) [Baron et al. 2006; Kulkarni etal. 2009]. When the different equivalent diameterscould significantly differ, it is recommended to experimentallymeasure these property-equivalentdiameters, and subsequently use the measured diametersin the lung deposition models to provide areliable representation of each relevant depositionmechanism [Kulkarni et al. 2011].In the animal inhalation studies of CNT [Shvedovaet al. 2008; Ma-Hock et al. 2009; Pauluhn 2010a],the airborne particle sizes (MMAD) were in themicrometer size range (~1–3 µm) (Table A–2) andthe airborne CNT structures in those studies wereroughly spherical agglomerates—suggesting thatdeposition from diffusional mechanisms may benegligible and aerodynamic diameter may providea reasonable estimate of the deposition efficiency ofCNT in the respiratory tract. However, the densityof the airborne structures can affect the depositionefficiency predictions in MPPD [ARA 2011]. Anevaluation of the effect of the CNT density assumptionson the rat alveolar deposition fraction is providedin this section.In the rat model, MPPD version 2.1 (but not 2.0)accepts density values less than one. The MMAD(GSD) values reported in the subchronic rat inhalationstudies varied slightly with particle concentrationand sampling device [Ma-Hock et al. 2009;Pauluhn 2010a]. The central MMAD (GSD) valueswere used for the deposition fraction and lungburden estimates. The influence of the alternativeparticle size estimates was not fully evaluated but appearedto be minimal compared with other factors(MPPD rat model version and assumed density).In addition, the MPPD model estimates of CNT lungburden in rats are compared to the measured CNTlung burdens from two rat inhalation studies. Pauluhn[2010a] reported the amount of cobalt tracerin the rat lungs as well as the amount of Co that wasmatrix-bound to the CNT. The Ellinger-Ziegelbauerand Pauluhn [2009] 1-day inhalation study with 91-day post-exposure follow-up also reported Co data.These data provided a basis for comparison to lungburden estimates from the MPPD models.Results in Table A–9 show that the rat depositionestimates (at the same density) vary by a factor ofapproximately two depending on the version ofthe MPPD model (2.0 or 2.1). As discussed in SectionA.2.2, this is apparently because of a change inMPPD 2.1 in the deposition efficiency equationsfor the head region of the rat model, which reducesthe deposition efficiency of the alveolar region. Thelower density further reduces the alveolar depositionefficiency estimates. These findings suggest thatrat alveolar lung dose estimates based on MPPD2.1 (regardless of density assumption) would resultin greater estimated potency of the CNT (becausethe response proportions do not change) and thuslower BMD(L) estimates in rats and lower OEL estimates(by approximately a factor of two) than thoseshown in the main analyses. Table A–9 also showsthe human alveolar deposition fraction estimatesfrom MPPD 2.0 and 2.1 (Yeh and Schum depositionmodel). MPPD 2.0 and 2.1 provide similar depositionfraction estimates for particle density of 1 g/ml.Different density assumptions (within MPPD 2.1)also had less effect (up to approximately 20%).A.6.1.2 Cobalt Tracer vs. DosimetryModel Estimates of MWCNTLung DoseTable A–10 provides a comparison of the dose estimatesfrom either the MPPD 2.0 or 2.1 rat lung126 NIOSH CIB 65 • Carbon Nanotubes and Nanofibers
Table A–9. Comparison of rat or human alveolar deposition fraction of inhaled particles, by MPPDversion and density assumption *Rat subchronicinhalation studyMPPD 2.0 MPPD 2.1Density = 1 (g/ml) Density = 1 (g/ml) Density < 1 (g/ml) *Rat estimatesMa-Hock et al. [2009] 0.072 0.044 0.024Pauluhn [2010a] 0.046 0.027 0.023Human estimatesMa-Hock et al. [2009] 0.099 0.10 0.080Pauluhn [2010a] 0.086 0.090 0.084*Density: 0.043 g/ml [Ma-Hock et al. 2009]; 0.2 g/ml [Pauluhn 2010a]. Particle MMAD (GSD): 1.2 (2.7) [Ma-Hock et al. 2009];2.74 (2.11) [Pauluhn 2010]; tidal volume 2.1 ml [Ma-Hock et al. 2009]; 2.45 ml [Pauluhn 2010a]; inhalability adjustment (all).Table A–10. Comparison of MPPD model and cobalt-tracer based estimates of MWCNT lungburden—subchronic inhalation exposure in rats [Pauluhn 2010a]Exposureconcentration(mg/m 3 )Depositedlung dose(µg)Retainedlung dose(µg)Depositedlung dose(µg)Retainedlung dose(µg)MPPD 2.0 * MPPD 2.1 † MWCNT Retainedlung dose (µg)estimated fromcobalt-tracer ‡0.1 27 12 14 5.6 8.70.45 121 63 50 23 1091.62 436 271 222 125 3915.98 1,610 1,230 594 392 1,433*MMAD (GSD)—2.74 (2.11); assumed density of 1 g/ml; tidal volume—2.45 ml; alveolar deposition fraction estimate was 0.046.†Assumed density of 0.2 g/ml; tidal volume—2.45 ml; alveolar deposition fraction estimate was 0.023 for MMAD (GSD) of2.74 (2.11).‡Mass of cobalt was estimated from Figure 6 in Pauluhn [2010a] to be approximately 10, 125, 450, and 1,650 ng, respectively, byincreasing exposure concentration. The CNT amount in the lungs was estimated from the reported 0.115% Co that was matrixboundto the CNT [Pauluhn 2010a]; the remaining mass (99.885% was assumed to be CNT). The CNT mass was thus calculatedas CNT (ng) = [0.99885 × Co mass (ng)] / 0.00115. CNT (ng) × 0.001 µg/ng equals CNT (µg).NIOSH CIB 65 • Carbon Nanotubes and Nanofibers127
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CURRENT INTELLIGENCE BULLETIN 65Occ
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Current Intelligence Bulletin 65Occ
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ForewordThe Occupational Safety and
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Executive SummaryOverviewCarbon nan
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2009; Pauluhn 2010a; Porter et al.
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neurogenic sig nals from sensory ir
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possible. Until the results from an
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••Follow exposure and hazard as
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Periodic Evaluations••Evaluatio
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ContentsForeword ..................
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A.3.2 Comparison of Short-term and
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ESPFeFMPSFPSSgGMGSDHCLHECHEPAhrISOI
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AcknowledgementsThis Current Intell
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1 IntroductionMany nanomaterial-bas
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2 Potential for ExposureThe novel a
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CNMs, with MWCNT agglomerates obser
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composite materials with local exha
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information on air contaminants. Sa
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3 Evidence for Potential Adverse He
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decreasing agglomerate size increas
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examined up to 60 days post-exposur
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3.3 SWCNT and MWCNTIntraperitoneal
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The same potency sequence was obser
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Table 3-3. Findings from published
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Table 3-7 (Continued). Findings fro
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length, respectively) [Muller et al
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5 CNT Risk Assessment and Recommend
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A-6). Risk estimates derived from o
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Table 5-4. Factors, assumptions, an
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and analytical methods. NIOSH is re
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Table 5-5. Recommended occupational
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deficits in animals or clinically s
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(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
Page 101 and 102: Bolton RE, Vincent HJ, Jones AD, Ad
Page 103 and 104: eport issued on July 22, 2011. NEDO
Page 105 and 106: Kobayashi N, Naya M, Mizuno K, Yama
Page 107 and 108: Methner M, Hodson L, Geraci C [2010
Page 109 and 110: Human Services, Centers for Disease
Page 111 and 112: Piegorsch WW, Bailer AF [2005]. Qua
Page 113 and 114: AD, Baron PA [2003]. Exposure to ca
Page 115: Varga C, Szendi K [2010]. Carbon na
Page 119 and 120: ContentsA.1 Introduction ..........
Page 121 and 122: A.1 IntroductionThe increasing prod
Page 123 and 124: provide an informal check on the es
Page 125 and 126: these same dose groups; this effect
Page 127 and 128: Table A-1. Rodent study information
Page 129 and 130: the deposited (no clearance) and th
Page 131 and 132: The other BMDS models failed to con
Page 133 and 134: Figure A-2. Benchmark dose model (m
Page 135 and 136: Figure A-3 (continued). Benchmark d
Page 137 and 138: Table A-3. Benchmark dose estimates
Page 139 and 140: Table A-5. Benchmark dose estimates
Page 141 and 142: histopathology grade 2 or higher lu
Page 143 and 144: Table A-8. Working lifetime percent
Page 145 and 146: developing early-stage adverse lung
Page 147 and 148: Figure A-4. Dose-response relations
Page 149 and 150: cell surface area). However, the wo
Page 151: purified or unpurified (with differ
Page 155 and 156: A.6.1.3 Pulmonary Ventilation RateT
Page 157 and 158: used as the effect levels in evalua
Page 159 and 160: the DF estimate, although a larger
Page 161 and 162: or overloading, of particle clearan
Page 163 and 164: Table A-13. Human-equivalent retain
Page 165 and 166: A.7.1 Particle CharacteristicsBoth
Page 167 and 168: and density. The following MMAD and
Page 169: Table A-15. CNT lung dose normalize
Page 172 and 173: B.1 Key Terms Related toMedical Sur
Page 175 and 176: APPENDIX CNIOSH Method 5040
Page 177 and 178: filter. In the method evaluation, d
Page 179 and 180: Most of the studies on sampling art
Page 181 and 182: e analyzed to determine the onset o
Page 184: Delivering on the Nation’s promis
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