Occupational Exposure to Carbon Nanotubes and Nanofibers

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C.1 BackgroundNIOSH Method 5040 is based on a thermal-opticalanalysis technique [Birch and Cary 1996] fororganic and elemental carbon (OC and EC). Theanalysis quantifies total carbon (TC) in a sample asthe sum of OC and EC. The method was developedto measure diesel particulate matter (DPM) in occupationalsettings, but it can be applied to othertypes of carbonaceous aerosols. It is widely used forenvironmental and occupational monitoring.For the thermal-optical analysis, a portion (typicallya 1.5-cm 2 rectangular punch) of a quartz-fiberfilter sample is removed and placed on a smallquartz spatula. The spatula is inserted in the instrument’ssample oven, and the oven is tightly sealed.Quartz-fiber filters are required for sample collectionbecause temperatures of 850 °C and higher areemployed during the analysis. The thermal-opticalanalyzer is equipped with a pulsed diode laser andphoto detector that permit continuous monitoringof the filter transmittance. This optical feature correctsfor the “char” that forms during the analysisbecause of carbonization of some materials.Thermal-optical analysis proceeds in inert and oxidizingatmospheres. In both, the evolved carbon iscatalytically oxidized to carbon dioxide (CO 2). TheCO 2is then reduced to methane (CH 4), and CH 4isquantified with a flame ionization detector (FID).The OC (and carbonate, if present) is first removedin helium, as the temperature is increased to a presetmaximum. If sample charring occurs, the filtertransmittance decreases as the temperature isstepped to the maximum. After the OC is removedin helium, an oxygen-helium mix is introduced,and the temperature is again stepped to a maximum(850 ºC or higher, depending on the sample)to effect combustion of the remaining material. Asthe light-absorbing carbon (mainly EC and char)is oxidized from the filter, the filter transmittanceincreases. The split between the OC and ECis assigned when the initial (baseline) value of thefilter transmittance is reached. All carbon removedbefore the OC-EC split is considered organic, andthat removed after the split is considered elemental.If no charring occurs, the split is assigned beforeremoval of EC. If the sample chars, the split is notassigned until enough light-absorbing carbon isremoved to increase the transmittance to its initialvalue.OC and EC results are reported as microgramsper square centimeter (µg/cm 2 ) of sample deposit.The total OC and EC on the filter are calculated bymultiplying the reported values by the deposit area.Because only a portion of the sample is analyzed, itmust be representative of the entire deposit. Thus, ahomogeneous deposit is assumed. The entire filtermust be analyzed (in portions if a 37-mm filter isused) if the filter deposit is uneven.C.2 Method EvaluationThe reported accuracy of NIOSH 5040 is based onanalysis of TC in different sample types. Accuracywas based on TC, because there is no analyticalstandard for determining the OC-EC content of acomplex carbonaceous aerosol. In the method evaluation,five different organic compounds were analyzedto examine whether the instrument responseis compound dependent. Linear regression of thedata (43 analyses total) for all five compounds gavea slope and correlation coefficient (r) near unity[slope = 0.99 (± 0.01), r 2 = 0.999, n = 43], indicatinga compound-independent response. Eight differentcarbonaceous materials also were analyzed by threemethods, in-house by thermal-optical analysis andby two other methods used by two external laboratories.Sample materials included, DPM, coals, urbandust, and humic acid. Thermal-optical resultsagreed well with those reported by the two otherlaboratories. The variability of the TC results forthe three laboratories ranged from about 1%–7%.These findings [Birch and Cary 1996] demonstratethat carbon can be accurately quantified irrespectiveof the compound or sample type.In sampling DPM, different samplers gave comparableEC results because particles from combustionsources are generally less than one µm (diameter).As such, the particles are collected with high efficiency(near 100%) and evenly deposited on the150 NIOSH CIB 65 • Carbon Nanotubes and Nanofibers
filter. In the method evaluation, different samplertypes (open-face 25-mm and 37-mm cassettes,298 personal cascade impactors, and four prototypeimpactors) were used to collect diesel exhaustaerosol at an express mail facility. The relative standarddeviation (RSD) for the mean EC concentrationwas 5.6% [Birch and Cary 1996]. Based on the95% confidence limit (19%; 13 degrees of freedom,n = 14) on the accuracy, the NIOSH accuracy criterion[Kennedy et al. 1995] was fulfilled. Variabilityfor the OC results was higher (RSD = 12.3%),which is to be expected when different samplers areused to collect aerosols that contain semi-volatile(and volatile) components, because these may havea filter face velocity dependence. The method precision(RSD) for triplicate analyses (1.5 cm 2 filterportions) of a 37-mm quartz-fiber filter sample ofDPM was normally better than 5%, and often 2%or less [NIOSH 1994a].In the method evaluation, the limit of detection(LOD) was estimated two ways: (1) through analysisof low-level calibration standards [Birch andCary 1996; NIOSH 1994], and (2) through analysisof pre-cleaned media blanks. In the first approach,OC standard solutions (sucrose and ethylene diaminetetraaceticacid [EDTA]) covering a range from0.23 to 2.82 µg C (or from 0.15 to 1.83 µg C per cm 2of filter) were analyzed. An aliquot (usually 10 µL)of the standard was applied to one end of a 1.5-cm 2rectangular filter portion that was pre-cleaned in thesample oven just before application of the aliquot.The filter portion was pre-cleaned to remove anyOC contamination, which can greatly increase theEC LOD when TC results are used for its estimation.After cleaning the filter portion, metal tweezers areused to remove from the sample oven the quartzspatula that holds the portion. External to the oven,the spatula is held in place by a metal bracket suchthat the standard can be applied without removingthe filter portion from the spatula. This avoids potentialcontamination from handling.Results of linear regression of the low-level calibrationdata were used to calculate the LOD as 3 σy/m,where σy is the standard error of the regression andm is the slope of the regression line. TC results wereused rather than OC because the pyrolysis correctionmay not account for all of the char formedduring analysis of the standard (because of lowsample loading and/or the position of the aliquotin the laser). If not, a small amount of the OC willbe reported as EC, introducing variability in theOC results and increasing the LOD. The LOD estimatedthrough the linear regression results was0.24 µg C per filter portion, or 0.15 µg/cm 2 .A simpler approach for LOD determination isthrough analysis of media blanks. In the methodevaluation, TC results for pre-cleaned, 1.5-cm 2portions of the filter media were used to calculatethe LOD estimate. The mean (n = 40) TC blankwas 0.03 ±0.1 µg TC. Thus, the LOD estimated asthree times the standard deviation for pre-cleanedmedia blanks (3 σ blank) was about 0.3 µg C. Thisresult agrees well with the value (0.24 µg C) estimatedthrough analysis of the standard solutions.Considering a 960-L air sample collected on a 37-mm filter and a 1.5-cm 2 sample portion, this LODtranslates to an air concentration of about 2 µg/m 3([0.3 µg TC/1.5 cm 2 ] [8.5 cm 2 ]/0.960 m 3 = 1.78 µg/m 3 ), corresponding to the reported upper LOQ ofabout 7 µg/m 3 (LOQ = 3.3 × LOD).As with all analytical methods, the LOD is a varyingnumber. However, the EC LOD (about 2 µg/m 3 ,or an LOQ of 7 µg/m 3 ) reported for NIOSH Method5040 is a high estimate. As discussed in Section6 of the CIB, it was based on analysis of precleanedmedia blanks from different filter lots, overa 6-month period, and by different analysts at twodifferent laboratories. Further, variability for theTC results, rather than the EC results, was used toestimate the LOD. These combined factors gave aconservative (high) estimate of the EC LOD. Moretypical values, under different sampling conditions,are discussed in Section 6.1 of the CIB.C.3 Inter-LaboratoryComparisonsWhen results of the initial method evaluationwere published [Birch and Cary 1996], an interlaboratorycomparison was not possible becauseNIOSH CIB 65 • Carbon Nanotubes and Nanofibers151
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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
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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
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 and 152: purified or unpurified (with differ
Page 153 and 154: Table A-9. Comparison of rat or hum
Page 155 and 156: A.6.1.3 Pulmonary Ventilation RateT
Page 157 and 158: used as the effect levels in evalua
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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: APPENDIX CNIOSH Method 5040
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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