The Size Distribution of Airborne Bolivian Crocidolite Fibersof Cochabamba, Bolivia along Highway 4....
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San Lucas Medical, LLC The Size Distribution of Airborne Bolivian Crocidolite Fibers Drew R Van Orden, Richard J Lee, Matthew S Sanchez and Matthew D Zock Affiliation: RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, PA 15146, USA (Submission Date: 18 April 2012; Acceptance Date: 30 April 2012; Publicatixon Date: 26 July 2012) Correspondence: Drew R Van Orden, RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, Pennsylvania 15146, USA. e-mail: [email protected] INTRODUCTION Current Environmental Protection Agency (EPA) risk models and asbestos fiber counting techniques for monitor- ing airborne exposures treat all fibers as being equivalent and ignore the effect of dimension on toxicity. While this may have been appropriate when modern analytical methods were not available, the measurement world is now fully capable of making the appropriate distinctions. The importance of width as a determinant of carcinogenicity has been recog- nized for nearly 30 years within the published literature. Stanton, 1 in a landmark study of a suite of fibrous materials, found that durable mineral fibers with widths less than 0.25 mm and lengths longer than 8 mm were causally associated with the induction of tumors. Pott, 2 in a later study, postulated a fiber toxicity model that agreed reasonably well with the Stanton findings, but extended the concept to include varying toxicity as a function of length and width. Wylie et al. 3 reanalyzed samples used by Davis et al. 4 and found that tumor generation was strongly and inversely correlated with fiber width. Crump 5 testified increasing fiber length was correlated with disease occurrence. Lippman 6 reviewed a number of studies and suggested that mesothe- lioma was closely associated with long, thin fibers. Berman et al. 7 published a risk model, which found that disease was correlated almost exclusively with fibers longer than 40 mm and width less than 0.25 mm. Recently, Loomis et al. 8 reanalyzed textile mill samples and found that aspect ratios greater than 40:1 were highly correlated with disease. Incorporation of width into risk models has been limited by the availability of data from operating asbestos plants and, in particular, the observation of an asbestos plant for which there is no observable mesothelioma. There have been only limited studies published on the dimensions of asbestos fibers, with a few serving as the primary references in other literature. 913 A rare opportunity to collect detailed concentration and dimensional data of airborne crocidolite from an operating asbestos-cement plant using modern analytical procedures arose, providing a chance to supplement the existing literature on airborne fiber dimensions. The asbestos-cement plant is located in the eastern end of Cochabamba, Bolivia along Highway 4. Cochabamba is a relatively large city located in the Andes Mountains, approximately 250 km southeast of the capital La Paz. The city sits in a valley that runs east-west with mountains to the north and south. The crocidolite used in the products made at this facility is mined in central Bolivia, approximately 75 km northwest of Santa Cruz. The crocidolite is a fibrous magnesio- riebeckite 14 (blue asbestos). Crocidolite fibers from Bolivia were the subject of prior reports suggesting the fibers are generally longer and thicker than crocidolite from Australia or South Africa. 9 At the cement plant, the run-of-mine crocidolite was first pulverized using a hammer mill and then sieved on vibratory sieves to produce fibers of limited size ranges for use in the cement. The sieved crocidolite is mixed with other components, including recycled cement product, to produce a slurry that is vacuum filtered into sheet form. The sheets are then manually molded into a corrugated shape, air dried, and sawed (using circular saws) to accepted dimensions. The sawed sheets are cured in soaking pits for 28 days before being shipped to the consumer. Prior to shipping, the sheets are finished by a light hand sanding followed (at customer request) by applying one or more coats of paint. A detailed description of the cement plant and the industrial hygiene results can be found in Van Orden et al. 15 SAMPLE COLLECTION AND ANALYSIS Samples of airborne fibers were collected in different parts of the cement plant (sieving, mixing, forming, sawing, and finishing). Air sampling equipment adhered to the require- ments of the National Institute of Occupational Safety and Health (NIOSH) Method 7400. 16 Normally, pumps were operated at a flow rate of 23 liters per minute. Sample collection times were adjusted to minimize overloading of the filters. The samples were collected on cellulose ester mem- brane (0.8 mm pore size, 25-mm diameter) filters mounted in a new cassette with a 2-inch conductive extension. Air samples were collected on personnel operating sites in the sieving, mixing, forming, sawing, and finishing areas of the plant. Area samples were collected in all operations except the forming operation. The area samples were generally within 35 m from the person, with the ANNALS OF RESPIRATORY MEDICINE CASE REPORT www.slm-respiratory.com 1 AoRM 2012; 000:(000). Month 2012
Transcript of The Size Distribution of Airborne Bolivian Crocidolite Fibersof Cochabamba, Bolivia along Highway 4....
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The Size Distribution of Airborne BolivianCrocidolite Fibers
Drew R Van Orden, Richard J Lee, Matthew S Sanchez and Matthew D Zock
Affiliation: RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, PA 15146, USA
(Submission Date: 18 April 2012; Acceptance Date: 30 April 2012; Publicatixon Date: 26 July 2012)
Correspondence: Drew R Van Orden, RJ Lee Group, Inc., 350 Hochberg Road, Monroeville, Pennsylvania 15146, USA. e-mail: [email protected]
INTRODUCTION
Current Environmental Protection Agency (EPA) riskmodels and asbestos fiber counting techniques for monitor-ing airborne exposures treat all fibers as being equivalent andignore the effect of dimension on toxicity. While this mayhave been appropriate when modern analytical methods werenot available, the measurement world is now fully capableof making the appropriate distinctions. The importance ofwidth as a determinant of carcinogenicity has been recog-nized for nearly 30 years within the published literature.Stanton,1 in a landmark study of a suite of fibrous materials,found that durable mineral fibers with widths less than0.25 mm and lengths longer than 8 mm were causallyassociated with the induction of tumors. Pott,2 in a laterstudy, postulated a fiber toxicity model that agreed reasonablywell with the Stanton findings, but extended the conceptto include varying toxicity as a function of length andwidth. Wylie et al.3 reanalyzed samples used by Davis et al.4
and found that tumor generation was strongly and inverselycorrelated with fiber width. Crump5 testified increasing fiberlength was correlated with disease occurrence. Lippman6
reviewed a number of studies and suggested that mesothe-lioma was closely associated with long, thin fibers. Bermanet al.7 published a risk model, which found that diseasewas correlated almost exclusively with fibers longer than40 mm and width less than 0.25 mm. Recently, Loomis et al.8
reanalyzed textile mill samples and found that aspect ratiosgreater than 40:1 were highly correlated with disease.
Incorporation of width into risk models has been limitedby the availability of data from operating asbestos plantsand, in particular, the observation of an asbestos plant forwhich there is no observable mesothelioma. There havebeen only limited studies published on the dimensionsof asbestos fibers, with a few serving as the primaryreferences in other literature.9�13 A rare opportunity to collectdetailed concentration and dimensional data of airbornecrocidolite from an operating asbestos-cement plant usingmodern analytical procedures arose, providing a chanceto supplement the existing literature on airborne fiberdimensions.
The asbestos-cement plant is located in the eastern endof Cochabamba, Bolivia along Highway 4. Cochabambais a relatively large city located in the Andes Mountains,
approximately 250 km southeast of the capital La Paz.The city sits in a valley that runs east-west with mountainsto the north and south.
The crocidolite used in the products made at this facilityis mined in central Bolivia, approximately 75 km northwestof Santa Cruz. The crocidolite is a fibrous magnesio-riebeckite14 (blue asbestos). Crocidolite fibers from Boliviawere the subject of prior reports suggesting the fibers aregenerally longer and thicker than crocidolite from Australiaor South Africa.9
At the cement plant, the run-of-mine crocidolite wasfirst pulverized using a hammer mill and then sieved onvibratory sieves to produce fibers of limited size ranges foruse in the cement. The sieved crocidolite is mixed withother components, including recycled cement product, toproduce a slurry that is vacuum filtered into sheet form.The sheets are then manually molded into a corrugatedshape, air dried, and sawed (using circular saws) to accepteddimensions. The sawed sheets are cured in soaking pitsfor 28 days before being shipped to the consumer. Priorto shipping, the sheets are finished by a light hand sandingfollowed (at customer request) by applying one or morecoats of paint. A detailed description of the cementplant and the industrial hygiene results can be found inVan Orden et al.15
SAMPLE COLLECTION AND ANALYSISSamples of airborne fibers were collected in different parts of
the cement plant (sieving, mixing, forming, sawing, andfinishing). Air sampling equipment adhered to the require-ments of the National Institute of Occupational Safety andHealth (NIOSH) Method 7400.16 Normally, pumps wereoperated at a flow rate of 2�3 liters per minute. Samplecollection times were adjusted to minimize overloading of thefilters. The samples were collected on cellulose ester mem-brane (0.8 mm pore size, 25-mm diameter) filters mounted in anew cassette with a 2-inch conductive extension.
Air samples were collected on personnel operating sitesin the sieving, mixing, forming, sawing, and finishingareas of the plant. Area samples were collected in alloperations except the forming operation. The area sampleswere generally within 3�5 m from the person, with the
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exception of the sieving area samples. These samples werecollected 3�5 m outside of the building housing the sievingoperation. Area samples related to the forming operationwere not collected.
Each sample was analyzed using transmission electronmicroscopy (TEM) in general accordance with a modifiedNIOSH 7402 protocol.17 In these analyses, all fibers atleast 5 mm long with a minimum aspect ratio (length:width)of 3:1 were counted. Fibers were identified as crocidoliteon the basis of the elemental composition (energy dispersivex-ray spectroscopy) and the crystal structure (selected areaelectron diffraction).
STATISTICAL ANALYSIS
The data were evaluated to determine the averagedimensions and the distribution of fiber dimensions. Inaddition, both parametric (ANOVA)18 and nonparametric(Mann-Whitney and Kruskal-Wallis)19 analyses were per-formed to determine if there were any differences in fiberdimensions when comparing the various operations andpersonal/area samples.
RESULTS
A total of 4815 fibers were counted, with an averagelength of 10.990.2 mm, an average width of 0.4190.01 mm, and an average aspect ratio of 42.391.1. Of
these fibers, 4314 were from personal samples and 501from area samples. Fibers several hundred micrometers longwere observed, Figures 1 and 2, in the samples. These verylong fibers may be underrepresented in these statisticsbecause the TEM grid dimensions were only about 100�100 mm and only that portion of a fiber visible in a singlegrid opening was measured.
The summary statistics are shown in Table 1 (length),Table 2 (width), and Table 3 (aspect ratio) for the areaand personal samples for each operation. No area sampleswere collected in the vicinity of the forming operation.
The personal samples from the sieving operation aregenerally thinner (PB.0001) and have a higher aspect ratio(PB.0001) than the fibers from the other operations. Themixing samples have slightly different dimensions than dothe forming and sawing fibers. No single factor could beidentified for this difference other than the amount of waterused at this point in the process.
Overall, personal samples are generally longer than thearea samples, but not on a statistically significant basis.There is a significant difference in the fiber width in thefinishing samples (0.44 vs 0.28 mm, P� .0066 Mann-Whitney) when comparing personal and area samples, butnot in any other operation. There are no significantdifferences in fiber aspect ratio in any operation whencomparing personal and area samples.
Figure 1. Photograph of a sample of Bolivian crocidolite fibers as seen in a phase contrast microscope. The inscribed circle is a Walton-Becket graticulewith a diameter of 100 mm. Two fibers running vertically (red arrows) through the image are in excess of 185 mm long while the diagonally through theimage (white arrow) is 187 mm long.
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LCTable 4 shows the number of crocidolite fibers that fall into
several different proposed ‘‘risk’’ fibers. The proportion ofStanton fibers (longer than 8 mm and thinner than 0.25 mm) inthis data set was 22%. Phase contrast microscopy equivalent(PCME) fibers (5 mm longer and wider than 0.25 mm, with aminimum aspect ratio of 3:1) amounted to 62% of the fibers.Small percentages of the counted fibers fall into either theLippmann fibers range (10%, 5 mm longer and thinner than 0.1mm) or the Berman et al. range (0.5%, 40 mm longer andthinner than 0.5 mm).
Table 5 shows the bivariate distributions for the fiberdimensions for all samples collected at the cement plant.The intervals shown (greater than or equal to lower size
and less than upper size) in the Table were chosen tocorrespond to those used by Shedd (Table B-6).13 Themajority of fibers fall into the range of 5�15 mm long and0.1�0.5 mm wide.
DISCUSSION
The fiber dimensions reported herein are from airbornefibers sampled at an operating asbestos-cement plant.Almost all of these fibers (99%) are thinner than 3 mmindicating that they are respirable fibers. A smaller percentage(22%) is Stanton-sized fibers, suggesting that a largeproportion of the fibers have a minimal carcinogenic effect.In contrast, as shown in Figure 3, the percentage of SouthAfrican crocidolite having a width less than 0.25 mm isnearly 80%,20 whereas the percentage of fibers in thiscase that had a width less than 0.25 mm was less than 50%.Furthermore, these data support the conclusions of Bermanand Crump that length is also a determinant. Fewer than1% of the fibers less than 0.25 mm width were longerthan 40 mm. Also of interest in Figure 3 is the compa-rison with non-asbestos tremolite populations. The Bolivianasbestos is thinner than non-asbestos tremolite but thickerthan the South African crocidolite. Thus, there is nowclear evidence that separates non-asbestos tremolite fromasbestos populations that produce asbestos disease andmesothelioma.
There were some differences in the size of fibers indifferent operations, but generally not a significant differ-ence. The air from the sieving operation was transportedaway from the cement plant (not toward it), so these fiberswould not be a basis for a background concentration inthe cement plant. Similarly, because of the building con-struction, the airborne fibers from the cement plant didnot affect the results from the sieving operation. Thereare no walls within the cement plant, rather it is an openfloor plan, so fibers from one operation form the backgroundfor the other operations. Airflow was broadly mixing toforming to sawing to finishing, though open walls alongthe finishing operation that diminished the backgroundimpact of the other operations on the samples collected inthe finishing area.
Figure 2. Photograph of a sample of Bolivian crocidolite as seen in ascanning electron microscope. The bright fiber seen running diagonally (redarrow) is approximately 185 mm long. The textured background is the mixedcellulose ester filter.
Table 1. Summary Statistics of the Length (mm) of Crocidolite Fibers 5 mm and Longer Collected at a Crocidolite-cement Plant in Cochabamba, Bolivia
Percentile
Operation Type Count Mean Standard deviation Geometric mean 10 25 50 75 90
Sieving Personal 2204 11.14 8.31 9.53 5.50 6.50 8.50 12.55 19.50
Area 237 10.08 5.19 9.09 5.56 6.16 8.33 11.76 17.60
Mixing Personal 664 9.27 6.40 8.27 5.28 6.00 7.70 10.00 14.56
Area 118 9.41 4.39 8.70 6.16 6.16 7.04 10.56 14.08
Forming Personal 319 11.70 9.91 9.87 5.81 6.43 8.80 13.20 20.00
Sawing Personal 663 11.93 8.65 10.14 5.72 7.00 8.80 13.64 20.77
Area 105 11.04 5.14 10.00 6.16 7.04 8.80 14.30 19.36
Finishing Personal 464 10.57 8.33 9.03 5.37 6.16 7.92 12.16 17.20
Area 41 8.22 3.60 7.78 6.16 6.16 7.04 9.68 10.56
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The fibrosity index21 for the Bolivian crocidolite is 0.323which is statistically different to that reported by Shedd(0.406). The lower the fibrosity index, the more uniform arethe widths of the fibers as length increases. For example,other asbestos fibers have fibrosity indices of 0.184 (amosite),0.142 (South African crocidolite), 0.015�0.237 (chrysotile),21
0.294 (Australian crocidolite), and 0.330�0.478 (South Afri-can crocidolite).9 In contrast, non-asbestos amphibole miner-als have fibrosity indices ranging from 0.564 (riebeckite),0.666 (tremolite),21 0.68 (grunerite), 0.76 (ferroactinolite),and 0.78 (actinolite).22 This indicates that, in non-asbestosamphibole particles, as the particle length increases, there is
Table 2. Summary Statistics of the Width (mm) of Crocidolite Fibers 5 mm and Longer Collected at a Crocidolite-cement Plant in Cochabamba, Bolivia
Percentile
Operation Type Count Mean Standard deviation Geometric mean 10 25 50 75 90
Sieving Personal 2204 0.38 0.48 0.28 0.10 0.15 0.25 0.50 0.75
Area 237 0.35 0.26 0.29 0.18 0.18 0.26 0.46 0.74
Mixing Personal 664 0.41 0.39 0.31 0.18 0.18 0.26 0.45 0.88
Area 118 0.46 0.50 0.33 0.18 0.18 0.26 0.44 0.88
Forming Personal 319 0.45 0.50 0.32 0.18 0.18 0.26 0.51 1.00
Sawing Personal 663 0.50 0.59 0.34 0.10 0.18 0.30 0.53 1.00
Area 105 0.41 0.40 0.31 0.18 0.18 0.26 0.44 0.88
Finishing Personal 464 0.44 0.38 0.32 0.09 0.18 0.35 0.53 0.90
Area 41 0.28 0.20 0.24 0.18 0.18 0.18 0.35 0.44
Table 3. Summary Statistics of the Aspect Ratio of Crocidolite Fibers 5 mm and Longer Collected at a Crocidolite-cement Plant in Cochabamba, Bolivia
Percentile
Operation Type Count Mean Standard Deviation Geometric mean 10 25 50 75 90
Sieving Personal 2204 44.6 36.5 33.7 12.3 20.0 35.0 56.7 87.5
Area 237 38.9 25.4 31.6 13.8 20.0 32.4 55.0 75.0
Mixing Personal 664 36.8 32.5 27.0 9.2 15.9 30.0 44.4 70.0
Area 118 34.3 24.1 26.5 8.8 16.0 35.0 40.0 73.5
Forming Personal 319 41.8 37.3 30.8 10.9 19.6 32.5 50.0 75.0
Sawing Personal 663 43.0 44.4 29.7 8.8 17.5 32.5 50.0 90.0
Area 105 40.3 26.5 32.1 14.0 20.8 35.0 50.0 75.0
Finishing Personal 464 43.4 54.0 28.3 8.8 14.2 27.5 55.0 85.0
Area 41 38.6 25.7 32.3 15.2 22.0 35.0 50.0 62.0
Table 4. The Number of Fibers Counted in Various Risk Fiber Classes
Operation Type Total count PCM equivalenta Stantonb Lippmanc Berman et al.d
Sieving Personal 2204 1349 585 230 15
Area 237 130 50 13 0
Mixing Personal 664 393 115 54 3
Area 118 69 17 4 0
Forming Personal 319 200 69 20 2
Sawing Personal 663 451 122 70 4
Area 105 60 21 0 0
Finishing Personal 464 318 69 79 3
Area 41 15 10 1 0
Total 4815 2985 1058 471 27
a5 mm and longer, 0.25 mm and wider, minimum aspect ratio 3:1.b8 mm and longer, 0.25 mm and thinner.c5 mm and longer, 0.1 mm and thinner.d40 mm and longer, 0.4 mm and thinner.
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a comparable increase in particle width. In asbestos fibers,the fiber (or fibril) width is nearly constant for all fiberlengths. Compared to Shedd, the fiber widths are in anarrower size range (less variability), which may be relatedto the samples coming from a different mine or it may reflectdifferences in processing the samples.
Ilgren et al.23 have reported that there is no evidenceof elevated mesothelioma in the region of Cochabambaand there have been no reported mesotheliomas in thework force at this cement plant. The cement plant has beenin operation for more than 50 years, longer than the
generally accepted lag time for the development of mesothe-lioma. As noted in the industrial hygiene data from thisplant,15 the airborne fiber concentrations ranged from 0.5to 1000 f/mL and are certainly high enough to elevate thedisease risk in any worker and high enough to producewhat have been described as ‘‘mesothelioma hot spots’’.Had these exposures been found in Australian or SouthAfrican mines, they would have implied that one in threeworkers would contract mesothelioma. The absence of anyrecognizable disease confirms that the fiber dimensions havea much larger impact on disease causation than previously
Table 5. Size Distribution for Fibers 5 mm and Longer for all Samples Collected at a Crocidolite-cement Plant in Cochabamba, Bolivia.
Width
Length B0.025 0.025�0.05 0.05�0.075 0.075�0.1 0.1�0.2 0.2�0.5 0.5�1 1�5 ]5 mm Total
5�10 0 23 15 170 957 1476 518 166 1 3326
10�15 0 2 2 33 274 523 205 121 2 1162
15�20 0 1 1 11 76 183 104 54 1 431
20�25 0 0 0 4 33 82 48 40 0 207
25�30 0 0 0 1 15 39 33 19 0 107
30�35 0 0 0 1 7 27 14 7 0 56
35�40 0 0 0 0 2 14 11 5 0 32
40�45 0 0 0 0 2 10 9 10 0 31
45�50 0 0 0 1 0 7 4 4 0 16
]50 mm 0 0 0 0 3 12 23 16 2 56
Total 0 26 18 221 1369 2373 969 442 6 5424
Figure 3. Comparison of the percentage of fibers 5 mm and longer that are also 0.25 mm and thinner among various amphibole mineral standards. While theBolivian crocidolite contains a large amount of long, thin fibers, it does not contain as high a percentage as other asbestos amphibole samples. See Van Ordenet al.20
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accepted. This should prompt a reevaluation of the currentlyaccepted risk models.
The currently accepted risk model24 assumes a linear doseresponse using the PCME concentration as the exposuremetric. Using the same model but the different exposuremetrics suggested in Table 4, these data can be used todetermine the relative ratio of the slope factor in order toretain the same risk for the Bolivian crocidolite. The averageconcentrations for the PCME fibers were 116.4 f/cc, Stanton52 f/cc, Lippmann 20.2 f/cc, and Berman 1.5 f/cc. To calculatethe same risk, the slope factors would have the ratioof PCME: Stanton: Lippmann: Berman of 1:2.2:5.8:77. Theseratios suggest an upper limit to the slope factors for eachrisk metric.
No study has employed modern analytical techniques toassess airborne asbestos in settings comparable to historicalworkplaces. This study provides fundamental data that canbe used to update the currently accepted risk models. Thesefindings are significant to both public health and publicpolicy, especially in terms of potential exposure to mineralfibers or elongated mineral particles found in contemporaryraw material operations, such as in mines and quarries.
Acknowledgements: The authors would like to thank Sr. JavierTejada and Sra. Ana Inturias Tapia for their kindness in hosting usduring our visit to the plant. Their openness and sincere desire toimprove their operation led to this report.
Disclosure: This study was partially supported by RJ Lee Group.
REFERENCES1. Stanton MF, Layard M, Tegeris A, et al. Relation of particle dimension
to carcinogenicity in amphibole asbestoses and other fibrous minerals.J Natl Cancer Inst. 1981;67:965�975.
2. Pott F. Some aspects on the dosimetry of the carcinogenic potency ofasbestos and other fibrous dusts. Staub*Reinhalt Luft. 1978;38:486�490.
3. Wylie A, Bailey KF, Kelse J, Lee R. The importance of width in fibercarcinogenicity and its implications for public policy. Am Ind Hyg Assoc J.1993;54:239�252.
4. Davis JMG, Addison J, McIntosh C, Miller BG, Niven K. Variations inthe carcinogenicity of tremolite dust samples of differing morphology.Ann N Y Acad Sci. 1991;643:473�489.
5. Crump KS. 1986. Asbestos potency assessment for EPA hearing. Preparedfor Asbestos Information Association, presented at U.S. EPA hearings,June 26, 1986.
6. Lippmann M. Asbestos exposure indices. Environ Res. 1988;46:86�106.7. Berman D, Crump K, Chatfield E, Davis J, Jones A. The sizes, shapes,
and mineralogy of asbestos structures that induce lung tumors ormesothelioma in AF/HAN rats following inhalation. Risk Anal. 1995;15:181�195.
8. Loomis D, Dement J, Richardson D, Wolf S. Asbestos fibre dimensionsand lung cancer mortality among workers exposed to chrysotile. OccupEnv Med. 2010;67:580�584.
9. Gibbs GW, Hwang CY. Physical parameters of airborne asbestos fibers invarious work environments*preliminary findings. Am Ind Hyg Assoc J.1975;36:459�466.
10. Dement JM, Harris RL. Estimates of pulmonary and gastrointestinal depositionfor occupational fiber exposures. Cincinnati, OH: National Institute ofOccupational Safety and Health, DHEW publication No. 79�135; 1979.
11. Gibbs GW, Hwang CY. Dimensions of airborne asbestos fibers. In:Wagner JC, ed. Biological Effects of Mineral Fibers (IARC Scientific PublicationsNo. 82. Lyon, France: International Agency for Research on Cancer;1980:69�78.
12. Hwang CY. Size and shape of airborne asbestos fibres in mines andmills. Br J Ind Med. 1983;40:273�279.
13. Shedd KB Fiber Dimensions of Crocidolites From Western Australia, Bolivia, andthe Cape and Transvaal Provinces of South Africa. Washington, DC: US Bureauof Mines, RI 8998; 1985.
14. Leake B, Woolley AR, Arps CES, et al. Nomenclature of amphiboles:report of the subcommittee on amphiboles of the International Miner-alogical Association, Commission of New Minerals and Mineral Names.Canadian Mineralogist. 1997;35:219�246.
15. Van Orden D, Lee R, Sanchez M, Zock M, Ilgren E, Kamiya Y. Evaluationof airborne crocidolite fibers at an asbestos-cement plant. Ann Respir Med.(in press).
16. NIOSH Manual of Analytical Methods. Asbestos and other fibers by PCM.NIOSH Manual of Analytical Methods, Method 7400; 1994. http://www.cdc.gov/niosh/docs/2003-154/pdfs/7400.pdf. Accessed [2 Septem-ber 2011].
17. NIOSH Manual of Analytical Methods. Asbestos by TEM. NIOSH Manualof Analytical Methods, Method 7402; 1994. http://www.cdc.gov/niosh/docs/2003-154/pdfs/7402.pdf. Accessed [2 September 2011].
18. Neter J, Waserman W, Kutner M. Applied Linear Statistical Models: Regression,Analysis of Variance and Experimental Design, 3rd Edition, Richard D. Irwin.1990.
19. Daniel W. Applied Nonparametric Statistics. Boston, MA: Houghton Mifflin;1978.
20. Van Orden D, Lee R, Allison K, Addison J. Width distributions ofasbestos and non-asbestos amphibole minerals. Indoor Built Environ.2009;18:531�540.
21. Siegrest H, Wylie A. Characterizing and discriminating the shapeof asbestos particles. Environ Res. 1980;23:348�361.
22. Virta R, Shedd K, Wylie A, Snyder J. Size and shape characteristicsof amphibole asbestos (amosite) and amphibole cleavage fragments(actinolite, cummingtonite) collected on occupational air monitoringfilters. In: V.A. Marple, Benjamin Y.H. Liu eds. Aerosols in the Mining andIndustrial Work Environment, Vol. 2, 1983:633�643.
23. Ilgren E, Ramirez R, Claros E. et al (in press). Fiber width as adeterminant of mesothelioma induction and threshold*Bolivian croci-dolite: epidemiological evidence from Bolivia*mesothelioma demogra-phy and exposure pathways. Ann Respir Med. (in press).
24. Nicholson WJ, ed. Airborne Asbestos Health Assessment Update. Washington,DC: U.S. Environmental Protection Agency, Environmental Criteria andAssessment Office, EPA Report No. EPA/600/8-84/003F; June 1986.
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