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Asbestos fiber content of lungs with diffuse interstitial fibrosis: an analytical scanning electron microscopic analysis of 249 cases.

Asbestosis is a fibrosing lung disease caused by exposure to asbestos. The degree of pulmonary fibrosis is related to the fiber burden in the lung and the type and duration of the exposure. The significance of individual host factors remains uncertain.

Criteria for the histologic diagnosis of asbestosis have been proposed by the Pneumoconiosis Committee of the College of American Pathologists (CAP) and the National Institute for Occupational Safety and Health (NIOSH). (1) The CAP criteria require "discrete foci of fibrosis in the walls of respiratory bronchioles associated with accumulations of asbestos bodies in histological sections.'' The former criterion may be difficult to assess in cases with diffuse fibrosis and could overlap with other diseases showing similar changes, for example, respiratory bronchiolitis-associated interstitial lung disease or exposure to various dusts other than asbestos. The latter criterion is vague by not stating a minimal number of asbestos bodies required. The light microscopic count of asbestos bodies may be a surrogate marker for an individual's exposure and has caused some degree of disagreement among experts. (2,3) Several studies have attempted to distinguish patients with background exposure from those whose exposure was at or above the threshold cumulative dose. They found that 2 asbestos bodies per square centimeter correlated with a fiber burden 40 times that found in a reference population and that more than 95% of asbestosis cases had more than 2 asbestos bodies per square centimeter. (3,4) The 1997 Helsinki criteria incorporated these findings into more evidence-based criteria for the diagnosis of asbestosis, requiring (1) diffuse interstitial fibrosis and (2) 2 or more asbestos bodies within a section area of 1 [cm.sup.2] or a count of uncoated asbestos fibers that falls into the range recorded by the same laboratory for asbestosis. (5)

Histologic grading of asbestosis can be performed according to a NIOSH scheme that includes scores for both severity and extent of disease. (1) An expert panel convened by the CAP is currently in the process of updating the asbestosis classification scheme and diagnostic criteria, with results expected to be published in 2009.

Occasionally, one encounters diffuse pulmonary fibrosis (DPF) in patients with a history of asbestos exposure, in whom a diagnosis of asbestosis cannot be made due to lack of the characteristic fibrosis pattern or, more commonly, due to the lack of asbestos bodies in histologic sections. Here we report the results of asbestos fiber analysis on 86 such cases. We determined the range of asbestos fiber concentrations seen in histologically confirmed asbestosis cases compared with that in DPF cases for which a histologic diagnosis of asbestosis could not be made. We assessed whether the fiber concentration correlates with the severity of fibrosis in asbestosis, and whether cases of DPF exist whose fiber concentrations overlap with those seen in asbestosis.

MATERIALS AND METHODS

Case Selection

The study group consisted of 163 cases of asbestosis and 86 DPF cases encountered between 1982 and 2007. Fiber burden analyses using scanning electron microscopy (SEM) with energy-dispersive x-ray analysis had been performed for all cases. The diagnosis of asbestosis was based on the criteria previously cited. (1,5) All patients reported a history of asbestos exposure, although the duration and specific source could not be ascertained in some. In one case of asbestosis showing an asbestos body count below the fifth percentile for asbestosis cases, fiber counts by electron microscopy were not performed. This case was excluded from the analysis. The diagnoses for the DPF cases were based on commonly accepted criteria such as gross and microscopic distribution of fibrosis, presence of fibroblastic foci, inflammation, honeycomb change, or alveolar filling. (6) Iron stains were performed on some cases of asbestosis and all cases of DPF to enhance the detection of asbestos bodies. No asbestos bodies were detected by light microscopy in the histologic slides of most DPF cases. Five cases showed rare equivocal fragments of asbestos bodies, and 3 additional cases showed rare asbestos bodies after extensive search. The latter 3 cases could arguably meet the criteria for a diagnosis of asbestosis. Approval from the institutional review board was obtained for this study.

Grading of Fibrosis

The severity of fibrosis in the asbestosis cases was graded according to a simplified version of the CAP-NIOSH grading scheme (Table 1). (7) If more than one slide was examined, an average score was obtained for an individual case by adding the scores for each slide and then dividing by the number of slides examined. Diffuse pulmonary fibrosis cases were assigned a fibrosis score of 3 in the presence of DPF and a fibrosis score of 4 if honeycomb changes were present. All DPF cases showed fibrosis at least as severe as a grade 3 asbestosis case. All grading was performed on thoracoscopically obtained or larger tissue samples (lobectomy, pneumonectomy, or autopsy).

Fiber Analysis

Fiber analysis was performed on lung tissue samples obtained either at the time of surgery (wedge biopsy, lobectomy, or pneumonectomy) or at autopsy. Most lung samples were formalin fixed, and a few were paraffin embedded. For the latter cases a correction factor (0.7) was applied to the final calculation so that the results were comparable to the formalin-fixed tissue results. (8) Lung tissue was digested using sodium hypochlorite and the residue collected on filters. (9) Filters were mounted on a glass slide (1-3 filters per case) and asbestos bodies counted by light microscopy at X200 magnification. Another filter was mounted on a carbon disk with colloidal graphite and sputter coated with gold or platinum for examination by SEM. Scanning electron microscopy was performed with JEOL JSM 840 (1980-1992) and JSM 6400 (1992-2005) scanning electron microscopes (JOEL, Peabody, Massachusetts) operated at an accelerating voltage of 20 kV, screen magnification of X1000, and a scan rate of 10 seconds per frame. Each field was scanned at least twice to search for fibers. Both coated (asbestos body) and uncoated fibers 5 [micro]m or more in length were counted, with a fiber defined as a mineral particle with an aspect (length to width) ratio of at least 3:1 and roughly parallel sides. Only fibers 5 [micro]m or longer were counted. A total of 100 fields (filter area of ~2.37 [mm.sup.2]) or 200 fibers, whichever occurred first, were counted for each sample. Blank filters were also examined and all reagents were prefiltered to avoid contamination with fibers. (9)

Statistical Analysis

Simple regression analysis was performed to correlate the grade of fibrosis of the asbestosis cases with the asbestos fiber or body count. Ninety-five percent prediction intervals were constructed for these linear models. Asbestos fiber concentrations of DPF cases were said to fall into the range of asbestosis cases if they fell into the 95% prediction interval at their respective fibrosis score. Analyses were performed using StatGraphics Centurion XV statistical software (Statistical Graphics Corp, Herndon, Virginia). Statistical significance was considered for P < .05.

RESULTS

This study expands the findings in 36 cases of asbestosis and 24 cases of idiopathic pulmonary fibrosis reported previously. (10)

The asbestosis group included 3 female and 160 male subjects, ranging in age from 44 to 91 years (median, 66 years). The duration of asbestos exposure was available in 110 cases and ranged from 4 to 49 years (median, 30 years). For 5 additional patients the duration of exposure provided was ''years.'' Of the 163 patients with asbestosis, 74 also had a malignant neoplasm of the lung, 40 had malignant pleural mesotheliomas, 6 had malignant peritoneal mesotheliomas, and 4 had malignant tumors of other than pulmonary or pleural origin.

The DPF group included 6 female and 80 male subjects, ranging in age from 20 to 91 years (median, 63 years). The duration of asbestos exposure was available in 31 cases and ranged from 2 to 39 years (median, 22 years). For 3 additional patients the duration of exposure provided was ''years.'' Twenty-three subjects were smokers, 8 were nonsmokers, and for 55 no smoking history was documented. The DPF cases fell into the following diagnostic categories: usual interstitial pneumonia (77%), nonspecific interstitial pneumonia (6%), desquamative interstitial pneumonia (4%), polymyositis-associated interstitial fibrosis (1%), and not further classified or unclassified fibrosis (12%). Of the 86 patients with DPF, 13 also had a malignant neoplasm of the lung, and 1 had metastatic disease to the lung from a primary tumor elsewhere.

The ranges of concentration of commercial and noncommercial amphibole fibers, chrysotile, nonasbestos mineral fibers, and asbestos bodies detected by light microscopy for the asbestosis and DPF cases are shown in Table 2. The predominant fiber types found in asbestosis were commercial amphiboles (amosite and crocidolite). In 5 asbestosis cases the noncommercial amphibole fiber burden was found to be markedly higher than the commercial amphibole fiber burden. These cases are summarized in Table 3. Most DPF cases showed commercial asbestos fiber concentrations below the detection limit. More DPF cases than asbestosis cases were found to contain noncommercial amphibole fibers (tremolite, actinolite, and anthophyllite), although the asbestosis cases that harbored noncommercial amphiboles did so in higher concentrations. Chrysotile was found in only 6 of the DPF and 17 of the asbestosis cases.

The fibrosis scores of the asbestosis cases correlated best with the number of uncoated commercial amphiboles (P = .01) but also with the total number of commercial amphibole fibers (coated and uncoated, P = .02), with the number of total uncoated fibers (asbestos and nonasbestos mineral fibers, P = .03), and with the number of asbestos bodies found in digestion samples by light microscopy (P = .03). No correlation was found between the fibrosis scores and the concentration of coated commercial amphiboles, noncommercial amphiboles, chrysotile, or nonasbestos mineral fibers.

Based on their fibrosis score, 7 of 82 (9%) DPF cases fell within the 95% prediction interval of asbestosis based on asbestos body counts by light microscopy (Figure 1). After excluding from analysis the 5 asbestosis cases containing predominantly noncommercial amphibole fibers (Table 3), 3 of 86 (3%) DPF cases fell within the 95% prediction interval of asbestosis based on the total commercial amphibole fiber count (Figure 2). In 42 of 86 DPF cases the commercial amphibole count was below the detection limit.

These cases were included in Figure 2 at half the detection limit. Twenty-five of 83 (30%) DPF cases fell within the 95% prediction interval of asbestosis based on the total uncoated fiber count (data not shown).

The characteristics of the 3 DPF patients whose commercial amphibole count showed overlap with the 95% prediction interval of asbestosis are shown in Table 4.

COMMENT

The current analysis is a follow-up of a previous study by Roggli. (10) The former study included 36 patients with asbestosis and 24 with DPF and examined the relationship between fibrosis and uncoated fibers detected by SEM, total fibers (coated and uncoated) by SEM, and asbestos body counts by light microscopy. In contrast, the current analysis included 163 patients with asbestosis and 86 with DPF and examined the relationship between fibrosis and 8 different parameters: total (coated plus uncoated) commercial amphibole fibers, uncoated commercial amphibole fibers, coated commercial amphibole fibers, total uncoated fibers, asbestos bodies (light microscopy), noncommercial amphibole fibers, chrysotile fibers, and nonasbestos mineral fibers.

Both studies showed high lung fiber burdens in patients with histologic asbestosis as defined by the 1982 CAP NIOSH study, thus confirming the utility of these histologic criteria. (1) In addition, both studies showed a strong and statistically significant correlation between the severity of fibrosis as assessed histologically using the CAP NIOSH criteria and the lung fiber burden as determined by SEM, consistent with observations from prior studies. (11-14) Our linear regression analyses in both studies showed a wide scatter of the data, consistent with individual variation in response to a given fiber load.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In our prior study, the best correlation of histologic fibrosis was with uncoated fibers detected by SEM. The present study further examines this relationship with respect to fiber types. The main fiber type in the population studied was commercial amphiboles (amosite or crocidolite), and the best correlation with severity of fibrosis was found for uncoated commercial amphibole fibers (P = .01). As with our prior study, there was also a significant correlation with total uncoated fibers. Unlike our prior study, there was a significant correlation between fibrosis and asbestos body counts by light microscopy (P > .05 versus P = .03). These results are not surprising because most of the uncoated fibers in our asbestosis population were commercial amphiboles, and commercial amphiboles constitute most of asbestos body cores. We found no correlation between fibrosis score and noncommercial amphiboles, chrysotile, or nonasbestos mineral fibers. These observations are similar to those reported by Churg and Vedal (15) in a study of workers with heavy mixed amosite and chrysotile exposure.

In 5 of our asbestosis cases, the noncommercial amphibole fiber count was 36 to 230 times (median, 38) greater than that of the commercial amphiboles (Table 3). Hence a small percentage of our cases of asbestosis are caused by exposures to noncommercial amphibole fibers. The sources of these noncommercial amphiboles varied and included tremolite contamination of chrysotile (cases 1 and 4, Table 3), tremolite and actinolite contamination of vermiculite (case 5, Table 3), environmental exposure to tremolite and actinolite (case 2, Table 3), and exposure to anthophyllite during the manufacture of cement pipes (case 3, Table 3). We excluded these cases from our analysis of the relationship between fibrosis score and commercial asbestos fiber count.

As in our prior study, the current analysis shows that DPF cases that do not meet histologic criteria for the diagnosis of asbestosis have in the vast majority fiber burdens below the 95% prediction interval for asbestosis, despite exposure history. Thus, the fiber burden analysis supports the interpretation that these cases are not asbestosis. Although one might argue that chrysotile may be the culprit in some of these cases, this is unlikely because bona fide cases of asbestosis caused by chrysotile have readily detectable chrysotile (and tremolite) fibers by electron microscopy and also meet histologic criteria for asbestosis in finding asbestos bodies in histologic sections. (16) Similarly, it is also unlikely that the fibrosis was caused by fibers shorter than 5 |xm in length, as fibers in this size range have not been demonstrated to be fibrogenic. (17)

Contrary to our prior study, there was some overlap between the DPF group and the asbestosis group in the current analysis. This is of particular interest for the 3 cases with commercial amphibole fiber burdens overlapping with the 95% prediction interval for asbestosis. Such rare cases probably are true examples of occult asbestosis as originally proposed by Churg. (18) Possible explanations for such cases not meeting histologic criteria for asbestosis include poor coating efficiency (ie, asbestos body formation) for some individuals and sampling error related to variability of asbestos bodies in histologic sections. (19,20) None of the DPF cases in the present study had noncommercial amphibole fiber levels approaching those seen in noncommercial amphibole-related asbestosis (Tables 2 and 3).

In addition, there were 7 cases with asbestos body counts (as determined by light microscopy) overlapping with the 95% prediction interval for asbestosis. None of these 7 cases had commercial amphibole fiber levels within the 95% prediction interval for asbestosis. Some individuals are particularly heavy coaters (eg, welders with a heavy lung burden of iron oxides) so that a high percentage of their commercial amphibole burden consists of asbestos bodies. (19) We do not accept such DPF cases in which only the asbestos body counts (but not commercial amphibole fiber counts) overlap with the asbestosis group as true examples of asbestosis. Indeed, false-positive diagnoses of asbestosis may result from laboratories using light microscopy asbestos body counts alone in the analysis of cases that do not otherwise meet histologic criteria for a diagnosis of asbestosis.

In summary, most DPF cases studied here did not contain asbestos fibers in numbers within the range typically observed in cases of bona fide asbestosis. We conclude that strict histologic criteria such as the Helsinki criteria are useful for the positive identification of asbestosis among cases of advanced pulmonary fibrosis. A history of asbestos exposure alone is not sufficient for a diagnosis of asbestosis in patients with DPF. (21) If asbestos bodies cannot be detected by light microscopy in cases with a compelling exposure history, fiber analysis by electron microscopy and energy-dispersive x-ray analysis may help avoid false-negative diagnoses, and its use should be considered in these uncommon cases. Tissue asbestos analysis is not necessary in cases with classic histologic features of usual interstitial pneumonia (ie, honeycomb changes, fibroblastic foci, and absence of asbestos bodies).

References

(1.) Craighead JE, Abraham JL, Churg A, et al. The pathology of asbestos associated diseases of the lungs and pleural cavities: diagnostic criteria and proposed grading schema: report of the Pneumoconiosis Committee of the College of American Pathologists and the National Institute of Occupational Safety and Health. Arch Pathol Lab Med. 1982;106(11):544-596.

(2.) Warnock ML, Wolery G. Asbestos bodies or fibers in the diagnosis of asbestosis. Environ Res. 1 987;44(1):29-44.

(3.) Roggli VL, Pratt PC. Numbers of asbestos bodies on iron-stained tissue sections in relation to asbestos body counts in lung tissue digests. Hum Pathol.1983; 14(4):355-361.

(4.) Roggli VL, Sharma A. Analysis of tissue mineral fiber content. In: Roggli VL, Oury TD, Sporn TA, eds. Pathology of Asbestos-Associated Diseases. 2nd ed. New York, NY: Springer Science and Business Media; 2004:309-354.

(5.) Asbestos, asbestosis, and cancer: the Helsinki criteria for diagnosis and attribution. Scand J Work Environ Health. 1997;23(4):311-316.

(6.) Travis WD, King TE, Bateman ED, et al. American Thoracic Society/European Respiratory Society International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias. Am J Respir Crit Care Med. 2002;165(2):277 304.

(7.) Sporn TA, Roggli VL. Asbestosis. In: Roggli VL, Oury TD, Sporn TA, eds. Pathology of Asbestos-Associated Diseases. 2nd ed. New York, NY: Springer Science and Business Media; 2004:97.

(8.) Roggli VL, Pratt PC, Brody AR. Asbestos content of lung tissue in asbestos- associated diseases: a study of 110 cases. Br J ind Med. 1986;43(1):1 8-28.

(9.) Roggli VL. Tissue digestion techniques. In: Roggli VL, Oury TD, Sporn TA, eds. Pathology of Asbestos-Associated Diseases. 2nd ed. New York, NY: Springer Science and Business Media; 2004:402-410.

(10.) Roggli VL. Scanning electron microscopic analysis of mineral fiber content of lung tissue in the evaluation of diffuse pulmonary fibrosis. Scanning Microsc. 1991;5(1):71-83.

(11.) Wagner JC, Moncrieff CB, Coles R, Griffiths DM, Munday DE. Correlation between fibre content of the lungs and disease in naval dockyard workers. Br) Ind Med. 1 986;43(6):391-395.

(12.) Whitwell F, Scott J, Grimshaw M. Relationship between occupations and asbestos fibre content of the lungs in patients with pleural mesothelioma, lung cancer, and other diseases. Thorax. 1977;32(4):377-386.

(13.) Warnock ML, Kuwahara TJ, Wolery G. The relation of asbestos burden to asbestosis and lung cancer. Pathol Annu. 1 983;18(2):109-145.

(14.) Ashcroft T, Heppleston AG. The optical and electron microscopic determination of pulmonary asbestos fiber concentration and its relation to the human pathological reaction. J Clin Pathol. 1973;26(3):224-234.

(15.) Churg A, Vedal S. Fiber burden and patterns of asbestos-related disease in workers with heavy mixed amosite and chrysotile exposure. Am J Respir Crit Care Med. 1994;150(3);663-669.

(16.) Holden J, Churg A. Asbestos bodies and the diagnosis of asbestosis in chrysotile workers. Environ Res. 1986;39(1):232-236.

(17.) Agency for Toxic Substances and Disease Registry. Report on the expert panel on health effects of asbestos and synthetic vitreous fibers: the influence of fiber length. October 29-30, 2002; New York, NY.

(18.) Churg A. Occult asbestosis. Lab Invest. 1982;46:13A.

(19.) Roggli VL. Asbestos bodies and nonasbestos ferruginous bodies. In: Roggli VL, Oury TD, Sporn TA, eds. Pathology of Asbestos-Associated Diseases. 2nd ed. New York, NY: Springer Science and Business Media; 2004:34-44.

(20.) Morgan A, Holmes A. Distribution and characteristics of amphibole asbestos fibres, measured with the light microscope, in the left lung of an insulation worker. Br J Ind Med. 1983;40(1):45-50.

(21.) Jones RN. The diagnosis of asbestosis. Am Rev Respir Dis. 1991;144(3, pt 1):477-478.

(22.) Srebro SH, Roggli VL. Asbestos-related disease associated with exposure to asbestiform tremolite. Am J Ind Med. 1994;26(6):809-819.

Frank Schneider, MD; Thomas A. Sporn, MD; Victor L. Roggli, MD

Accepted for publication May 22, 2009.

From the Department of Pathology, Duke University Medical Center, Durham, North Carolina.

The authors have no relevant financial interest in the products or companies described in this article.

Presented in part at the Congress of the International Academy of Pathology, Athens, Greece, October 16, 2008.

Reprints: Victor L. Roggli, MD, Department of Pathology, Duke University Medical Center, DUMC 3712, Durham, NC 27710 (e-mail: roggl002@mc.duke.edu).
Table 1. Grading of Severity of Fibrosis for Asbestosis Cases

Grade 0    No appreciable peribronchiolar fibrosis, or less
             than half of bronchioles involved
Grade 1    Fibrosis confined to the walls of respiratory
             bronchioles and the first adjacent tier of adjacent
             alveoli, with involvement of more than half of all
             bronchioles on a slide
Grade 2    Extension of fibrosis to involve alveolar ducts
             and/or 2 or more tiers of alveoli adjacent to the
             respiratory bronchiole, with sparing of at least
             some alveoli between adjacent bronchioles
Grade 3    Fibrotic thickening of the walls of all alveoli
             between at least 2 adjacent respiratory
             bronchioles
Grade 4    Honeycomb changes

Table 2. Asbestos and Nonasbestos Fiber Concentrations (a)
Detected in Asbestosis and Diffuse Pulmonary Fibrosis Cases

                      Commercial
                    Amphiboles (b)       Commercial
                      (Coated and      Amphiboles (b)    Noncommercial
      Cases            Uncoated)         (Uncoated)      Amphiboles (c)

Asbestosis         <520-8 540 000     <660-7 800 000     <920-780 000
                   (median, 170 000)  (median, 110 000)  (median, <920)
Diffuse pulmonary  <330-39 000        <80-39 000         <100-14 000
  fibrosis         (median, <330)     (median, <490)     (median, 490)

                                                   Asbestos Bodies
                                                      (by Light
      Cases          Chrysotile         NAMF       Microscopy) (d)

Asbestosis         <910-1 220 000  <490-240 000    230-1 600 000
                   (median, <910)  (median, 2500)  (median, 23 000)
Diffuse pulmonary  <690-26 000     <120-120 000      0-7700
  fibrosis         (median, <690)  (median, 4400)  (median, 16)

Abbreviation: NAMF, nonasbestos mineral fibers.

(a) Reported as asbestos bodies or uncoated fibers 5 [micro]m
or greater in length per gram of wet lung tissue.

(b) Commercial amphiboles include amosite and crocidolite.

(c) Noncommercial amphiboles include tremolite, actinolite,
and anthophyllite.

(d) Asbestos bodies detected on the filter preparation
after digestion of the lung sample.

Table 3. Fiber Burden (a) of the 5 Asbestosis Patients Whose
Noncommercial Amphibole Fiber Count Was Higher Than the Commercial
Amphibole Fiber Count

Case    Age,             Diagnosis               Asbestos Exposure,
No.     y/Sex   (in Addition to Asbestosis)           Duration

1 (b)   74/M    Unilateral diffuse pleural    Manufactured asbestos
                  fibrosis                      blankets and gaskets,
                                                7 y
2       ND/M    Necrotizing granulomatous     Lived in Turkey, ND
                  inflammation
3       75/M    Centrilobular emphysema       Anthophyllite cement pipe
                                                plant, 2 y; automobile
                                                industry, 25 y
4       66/M    Malignant pleural             Plasterer, dry wall, 11 y
                  mesothelioma
5 (b)   44/M    Lung adenocarcinoma           Lived near vermiculite
                                                processing plant, 20 y

          Coated      Uncoated
Case    Commercial   Commercial   Noncommercial
No.     Amphiboles   Amphiboles    Amphiboles     Chrysotile    NAMF

1 (b)      2100       <29 000        471 000        68 000      21 300

2       <10 700       <37 200        499 000       <22 000      43 500

3         <3500       <37 200        779 000       <37 200     <37 200

4         <2400       <24 200        483 000       <24 200      24 200

5 (b)     <3000         <8200        150 000         <8200       <8200

Abbreviations: NAMF, nonasbestos mineral fibers; ND, not documented.

(a) Fibers 5 [micro]m or greater in length per gram of wet lung
tissue.

(b) Previously published. (22)

Table 4. Characteristics of the 3 Male Patients Whose Commercial
Amphibole Fiber Count Fell Into the 95% Range of 1 That Seen in
Asbestosis

Patient
No.       Age, y               Diagnosis

1           75     Pulmonary fibrosis, lung carcinoma,
                     status post radiation therapy
2           59     Diffuse interstitial fibrosis
3           ND     Diffuse interstitial fibrosis

Patient                          Smoking     Pleural   Fibrosis
No.       Exposure, Duration     History     Plaques   Grade (a)

1         Sheet metal worker,     60-70        Yes        4
            shipyard 24 y       pack-years
2         Foundry worker, ND        ND         No         3
3         ND, ND                    ND         ND         3

              Total           Uncoated
Patient     Commercial       Commercial
No.       Amphiboles (b)   Amphiboles (b)

1             38 800           38 800
2             13 200           13 200
3             16 100           13 300

Abbreviation: ND, not documented.

(a) Fibrosis grade according to Table 1.

(b) Fibers 5 [micro]m or greater in length per gram
of wet lung tissue.
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Author:Schneider, Frank; Sporn, Thomas A.; Roggli, Victor L.
Publication:Archives of Pathology & Laboratory Medicine
Article Type:Report
Geographic Code:1USA
Date:Mar 1, 2010
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