Occupational Exposure to Carbon Nanotubes and Nanofibers

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Table 5–4 (Continued). Factors, assumptions, and options evaluated in the CNT risk assessment.Factor Assumption Options evaluatedHuman equivalent dose• Interspeciesnormalization• Duration of exposure• Clearance kinetics• Ventilation rate• Alveolar depositionfractionHumans would have an equal lungresponse (on average) to an estimatedequivalent alveolar doseWorking lifetime average cumulativeexposure would be associated with thehuman-equivalent lung responseTrue average working lifetime exposurelies between deposited and retained CNTdose estimates based on spherical particledosimetry modelWorkers on average breath the sameamount of air in a day; normal oronasalaugmenter breathing patternAirborne particle size distribution predictsdeposition fractionAlveolar surface areaAlveolar macrophage cell volume45-years (8-hr TWA, 40-hr workweek,50 wk/yr)Deposited dose (no clearance)Retained dose (normal clearance)Reference worker (9.6 m 3 /d)Spherical particle-based model(MPPD), incl. two versions• Density of 1• Density of
and analytical methods. NIOSH is recommendingthat NIOSH Method 5040 [NIOSH 1994; Birch2004a, b] be used to measure workplace airborneexposure to respirable CNT and CNF.An upper estimate of the LOQ of Method 5040(7 µg/m 3 ) was proposed as the draft REL (8-hr TWA)for CNT and CNF [NIOSH 2010]. This upper limitwas based on total carbon (TC) results found forfilter media from different vendors and lots, by differentlaboratories, and during a 6-month period.In practice, when elemental carbon (EC) results areused from media blanks submitted with the samplesto estimate the LOQ, a much lower value can beachieved (Section 6.1). It is important to note thatthe LOQ for NIOSH Method 5040 depends on themedia blank variability, filter area sampled, portionof filter analyzed, and the collected sample air volume.As discussed in Section 6.1, under optimumconditions an LOQ of 1 µg/m 3 can be obtained foran 8-hr respirable sample collected on a 25-mmfilter at a flow rate of 4 liters per minute (lpm).Considering the potential uncertainties in estimatingthe health risks and current limitations of analyticalmethodologies, NIOSH is recommending aREL of 1 µg/m 3 EC as an 8-hr TWA airborne respirablemass concentration for up to a 40-hr work week.For working lifetime exposures at 1 µg/m 3 , the MLErisk estimates for slight/mild level of lung effects(based on the rat subchronic inhalation studies ofMWCNT) range from 0.23% to 10%, and the 95%UCL estimates range from 0.53% to 16% dependingon the rat study and the assumptions used in estimatingthe CNT lung dose (Tables 5–2 and A–8).The more sensitive endpoint of minimal level oflung effects in the rat subchronic inhalation studieswere associated with MLE risk estimates of 2.4% to33% and 95% UCL estimates of 5.3% to 54% (Tables5–2 and A–7). Estimates of a 45-yr working lifetimeno-effect concentration (8-hr TWA) based onthe NOAEL or LOAEL estimates from the rat subchronicstudies are < 1 µg/m 3 (8-hr TWA) (SectionA.6.3). The estimates based on the short-termstudies of SWCNT and MWCNT in rats and miceare consistent with those from the subchronic studiesof MWCNT in rats (Section A.3).NIOSH recognizes that the REL may not be completelyhealth protective and that there is uncertaintyin these risk estimates, but maintaining exposuresbelow the REL should help to lower workers’ riskof developing occupational lung disease over aworking lifetime. The REL and other recommendationsin this CIB should also assist employers inestablishing an occupational health surveillanceprogram that includes elements of hazard andmedical surveillance. Until improved methods tomeasure airborne exposures to CNT and CNF areestablished, continued efforts should be made toreduce airborne concentrations to as low as possiblebelow the REL. Approaches to optimize thesampling and analysis of exposures are discussed inSection 6.1 and Appendix C. Examples of engineeringcontrols to reduce or eliminate workers’ exposuresto CNT and CNF are provided in Section 6.2.Additional guidance, including personal protectiveequipment, respirators, training, and medical surveillanceis provided in Sections 6.3 through 6.7.Based on available workplace exposure data, it isnot possible for NIOSH to determine whether theNIOSH REL can be achieved in all workplaceswhere exposure to CNT and CNF occur; however,exposure data that have been reported indicatethat implementing appropriate engineering controlmeasures (e.g., local exhaust ventilation, enclosures)can eliminate or greatly reduce worker exposures[Han et al. 2008a; Methner et al. 2008; Tsaiet al. 2009; Johnson et al. 2010; Lee et al. 2010; Cenaand Peters 2011; Dahm et al. 2011].5.2 Other DerivedOccupational ExposureLimits for CNTOne of the earliest OELs for CNT was proposedby the British Standards Institute [BSI 2007]—thebenchmark exposure limit (BEL) of 0.01 fiber/cm 3 ,or one-tenth of their asbestos exposure limit (Table5–5). Nanocyl [2009] derived an estimated OELof 2.5 µg/m 3 for an 8-hr TWA exposure based onapplying an overall assessment (a.k.a. uncertainty)factor of 40 to the LOAEL of 0.1 mg/m 3 in theNIOSH CIB 65 • Carbon Nanotubes and Nanofibers43
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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
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 and 26: AcknowledgementsThis Current Intell
Page 27 and 28: 1 IntroductionMany nanomaterial-bas
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: Table 5-4. Factors, assumptions, an
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
Page 78 and 79: tasks where worker exposures exceed
Page 80 and 81: As part of the evaluation of worker
Page 82 and 83: Table 6-1. EC LODs and LOQs for 25-
Page 84 and 85: 6.2 Engineering ControlsOne of the
Page 86 and 87: Table 6-6 (Continued). Examples of
Page 88 and 89: Table 6-7 (Continued). Engineering
Page 90 and 91: exposure estimates for SWCNT on ind
Page 92 and 93: Table 6-8. Respiratory protection f
Page 94 and 95: ••Workers in areas or in jobs w
Page 97 and 98: 7 Research NeedsAdditional data and
Page 99 and 100: 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
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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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