Document 85bMqpVyZGx34k77jNBeQM4Ek

Lung Content Analysis of Cases Occupationally Exposed to Chrysotile Asbestos jp( p. Nolan,' A. M. Langer/ and J. Addison* environmental Sciences Laboratory, Brooklyn College, Brooklyn, New York; institute of Occupational ll^jcine. Ltd, Edinburgh, Scotland, UK belong contents of six wockers who had been occupationally exposed to chrysotile asbestos vvere examined. Five were lung cancer cases from 1 ijjjgbec. Canada. The sixth, an /American worker who had developed pleural mesothelioma, was particularly interesting, with the lung content strifc- pgly distinct from the Canadian cases; chrysotile, the predominant fiber in his lung, was present at a concentration 300 times that of the average gl fiber content in the Canadian cases. The fiber length distribution of the chrysotile recovered from the U.S. mesothelioma case was indistinjuishabie from that of chrysotile specimens known to produce mesotheliomas in rats. It was also found that the characteristics of the caldum-mag^ ^urn-tron silicate fibers present in all six cases were not readily comparable to tremolite asbestos specimens known to induce mesotheliomas in ^rnals. -- Environ Health Perspect 102(Suppi 5):245-250 (1994) |r l tey words; chrysotile. lung content, mesothelioma Introduction ! -3 Human epidemiological studies and animal iperiments to determine the biological t :lFects of chrysotile present a complex pat:crn of results which are by no means consistent {1,2). Mortality due to meso thelioma in ciuysotile-exposed individuals is significantly lower than that for those !( exposed to croddolite, although the expertgfej mental animal studies do not dearly sup port that conclusion {3,4). Individuals with similar occupational exposure indices $ to chrysotile experience markedly different rates of developing lung cancer, depending on the industry in which the exposures occurred. Both Canadian chrysotile miners m and millers and American friction product manufacturing workers have a much lower risk than asbestos textile workers with simi lar levels of exposure (5,6). Such results indicate that the information required to predict the disease outcome is not provided by the exposure indices used to compare different groups. Even if exposure indices required to ^ compare the groups were known precisely, 'his paper was presented at the Workshop on Biopersistence of Respirable Synthetic Fibers and Minerals held 7-9 September 1992 n Lyon, France. This study was made possible by the support of Tie Sociele Natonale de TAmiante and the Asbestos nstitute of Canada. One of us (RPN) acknowledges support as a Fellow of the Stony WokFHerber. Fund af New York. Tne authors thank Or. J. S. Hanngton of Johannes burg. Republic of South Africa, for his excellent cnaque and comments. Address correspondence to Or. R. P. Nolan. Environ mental Sciences Laooratory. Brooklyn College. Brooxlyn. Telephone (718) 951-4491. Fax (718) 951 4438. other methods would still be needed where the exposure history was not available. Examination of the lung content of indi viduals exposed in chrysodle-related indus tries has provided a different index of exposure applicable to individual cases {7-9). In this report, the lung contents of five workers in Canada and one in the United States, who had been occupationally exposed to asbestos, were compared with those of some other industrially exposed groups reported elsewhere. In addition, several chrysotile specimens, the carcino genicity of which had been determined separately in experimental animals {10,11), were compared by size distribution with the chrysotile obtained from the lung con tents of a human case of mesothelioma. Origin of Lung Tissue Specimens and Rationale for Selection Lung specimens were obtained from six cases. Five comprised formalin-fixed bulk tissues taken ac autopsy from male lung cancer cases, who had been exposed occu pationally to chrysotile asbesros, alchough not exclusively, in Quebec, Canada. The sixth specimen, from the U.S. worker (Case 6), was taken during a surgical pro cedure. Two samples (called 6a and 6b) were fixed in paraffin wax. This U.S. case was a 59-year-old male wich an occupa tional history of asbesros exposure, who had been diagnosed as having mesothe lioma of the pleura. Tissue from a Canadian chrysotile miner presented as a grid preparation (provided by RF Dodson, University Health Center at Tyler, TX) was used to evaluate whether the calciummagnesium-iron silicate fibers, commonly referred to as tremolite, found in the five Canadian cases were comparable with sili cate fibers repotted by others. In this study, the [Ca.Mg.Fe] silicate fibers were not characterized in sufficient detail to jus tify identification as tremolite; this is to be done in further studies. Origin of the Chrysotile Reference Specimens Six mineral specimens were selected as standards fot comparison with the fibers found in the human lung contents; i) Chrysotile No. 1026, a Papetbestos No. 5, a Canadian fiber that was airfractionated with an Alpine fiberizer to Grade 7 (provided by J.-M. Lalancette, University of Sherbrooke, Canada). The carcinogenicity of a similar specimen was studied in an earlier animal experiment {10). ii) UICC chrysotile A (Zimbabwe) and B (Canada) (provided by the Pneu moconiosis Research Unit, Uandough Hospital, Penarth, Wales, UK). iii) Chrysotile, Calidria RG-144, chrysotile from New Idria, CA (commercially available from Union Carbide, New York, NY). The carcinogenicity of a similar specimen had been studied earlier in animal experiments (10). iv) Short Chrysotile IOM, a water-frac tionated Grade 4-T-30 Canadian chrysotile, the pathogenicity of which was evaluated in an experimental ani mal study (11). Environmental Health Perspectives ! NOLAN ETAL Table 1. Distribution of fiber lengths of four chrysolite specimens used as reference standards and of the lung content of U.S. Case $ ofmalignant mesothelioma of the pleura Percentage within each interval of fiber length Chrysotile specimen/final magnification No. fibers sized 1.0 pm 1.1-5.0 pm 5.1-10.0 pm >10 pm iia UICC A Zimbabwe/x iib UICC B. Canada/x 5000 iii Califomia/x 5000 iv Short lOM/x 15.000 Case 6. United States/x75Q0 3523 2903 2939 1523 442' ND, none detected. `Average for specimens 6a and 6b.4 None detected. 88.8% 84.9% 77.2% 72.8% 76.3 133% 10.6% 13.5% 20.5% 26.6% 22211.8% 0.4% 0.6% 13% 0.6% 1.51.4% 02% 04% 0.5% NO' ND Table 2. Comparison of the site distribution of two short chrysodle reference specimens and the residue of Canadian chrysofile sample 5. Specimen/ final magnification Average no. of fibers sized <0.25 pm 0.25-0.49 pm Percentage within each interval of fiber length 050-0.99 pm 1.00-139 pm 2.00-439 pm >5.00 pm i Chrysotife No. 1026' <15000/10,000/25,000 iv Shortchrysotile I0M 15o000 v Residue of Sample 5 x7500 Diameters (pm)4 5260 1020* 1727 843* 327 327 61.1 53 -- 21.4 3.5 -- 0,0 0.0 19.5 5.0 -- 27.4 29 -- 0.0 0.0 9.8 53 50.5 2751.4 56.6 15.6 0.1 006 6L8 1.6 35.1 16.83.4 34.4 39.5 0.17 0.12 260.6 13.4 631.6 12.9 333 0.34 023 02+01 1.0 0.5+01 1.0 110 0.43 29 *Average of three preparations by two analysts; six fibers showing diameters >0.25 pm (0.11%). `Calculated for fiber length distribution, notcourting <0.5 pm fibers. `Mean diameter with standard deviation foreach fiber length interval. Table 1 Weight of tissue and number offibers per gram of dry lung tissue'obtained from six occupationally exposed cases. Specimen Type of lung tissue Wet weight of tissue digested, mg No. of fields counted Counting method No of fibers/ grid opening' No of. fibers fm millions g of dry lung Casel Canadian Case 2 Canadian Case3 Canadian Case4 Canadian Case 5 Canadian Case6 'US 4 US Parenchyma Parenchyma with pleura4 Parenchyma with pleura Parenchyma with pleura Parenchyma with pleura Parenchyma Parenchyma 2700 1640 2060 2030 1990 12.3 123 15.0 15.0 5 Onscreen 21.5 52 23 + 6 5 Onscreen 4.2 22 74 5 Onscreen 27.0 4.5 43=8 5 Onscreen 30.2 + 8.1 43 = '3 5 Onscreen 82 23 41 H 3 Onscreen 121 17.2 13900 2900 1 Photomontage 240 27600 3 Onscreen 60.7 41.0 6620*1100 1 Photomontage 241 27600 * On screen courting and photomontages wore sized atX2D.OOO and x7500 final magnification, respectively. Onlyfibers 20.5 pm in length were counted.4Of visceral pleura. v) The residue from Canadian Chrysodle Sample No. 5, from which the chrysotile had been removed by chemical digesdon (12). vi) Tremolite asbestos from Jamestown, CA. Fiber Length Distribution of the Chrysotife Reference Specimens Only fiber length was determined, since the diameter has been found to be consis tently uniform at about 0.06 pm (8). The electron microscope grids used to deter mine the length chstribudons of the five reference chrysotile specimens were pre pared by two methods. In the first, the wipe-out method, the fibers were dispersed between two glass slides into a thin film of nitrocellulose and amylacetate. In the sec ond procedure specimens were prepared by dispersion in water with ultrasound, and 10 pi of the suspension were placed on a carbon-coated fotmvar grid. In ^ c4* 200-mesh nickel locator grids were uscFiber length distributions of spec'^f" prepared by either method yielded sim results. Electron microscopy grids of rerefCjj^ standards ii and iii were prepared by < wipe-out method. Fiber length distribu0 were detetmined by direct measurement photomontages of 110 pm x 110 pm openings, magnifiiti x5000. Reference 246 Environmental Health PersP&^y wipe-out technique. Each size determina tion was performed by two analysts and the data sets were averaged to reduce interob server error and variance. Enlargement of the photographic plates for the analysis of the fibers gave final magnifications of X5000, X10.000, and X25.000 (Table 2). Low magnification photographs were best for counting long fibers; higher mag nification was required for the resolution of fibrils with lengths <0.5 pm. The shortest chrysotile fibers detected were approxi mately 0.1 pm in length, with a unit fibril diameter of approximately 0.02 pm. Reference specimen iv, was similarly ana lyzed and counted at a final magnification of x 15,000. Specimen i contained a larger proportion of fibers <0.25 pm than speci men iv, bur if the fibers with lengchs <0.5 pm were excluded, fiber length of the two specimens were indistinguishable (Table 2). The length and diameter of the fibers in reference specimen v, predominantly ICa,Mg,Fe] silicates, were determined. None of the 327 fibers was <0.5 pm in length. The number of fibers >5 pm was approximately 13 times higher in this spec imen than in the other five. Fiber diame ters were determined for each length interval, with standard deviations. Diameters increased with increasing fiber length and were considerably larger than the diameters of 0.06 pm of chrysotile (Table 2). Hpra 1. Transmission electron photomicrographs of representative fibers found in the lung tissues of the jnadian cases. |A) A magnesium silicate liber found in Case 2, approximately 7.5 pm x 0.83 pm, morphologically imposed of individual 'fibrils'. Fibrils may represent mineral intergrowths. 18) A typical [Ca.Mg.Fel silicate fiber `ourtd in Case 4. approximately 2.4 pm x approximately 0.2 pm. The fiber is composed of individual 'fibrils' 3.03-0.04 m in diameter. (C) A typical [Ca.Mg.Fef silicate fiber found in a Canadian worker (grid provided by RF Dodson). The fiber is approximately 25 pm x 2.5 pm and composed of "fibrils'. (0) The selected area electron iffraction pattern of the fiber shown in (0. It contains Kikuchi lines indicating a thick fragment, characteristic of a deavage fragment, not an amphibole fiber asbestos (13.14). dard iv was prepared for electron microscopy by the water dispersion method. The length distribution was determined at magnification x 15,000. The four reference specimens were found to have very similar fiber length distributions (Table 1). Of the total 10,888 fibers, whose lengths were determined, 98.7 0.8% were 5 pm. One of these, reference specimen iii, often referred to as a naturally occurring short fiber, had a fiber length distribution indistinguishable from the other reference specimens. Approximately 1% of the fibers in che four specimens were >5 pm in length. Specimens i and iv were selected to deter mine the fiber length distribution of two short reference samples of known patho genicity, and length intervals were selected to describe this fiber population. An average fiber length distribution of reference speci men i was determined from direct sizing on phorographic plares ofgrids prepared by the Preparation of the Lung Tissue Specimens Xylene was used to remove paraffin from the fixed tissues, which were then dried and weighed. After weighing, each tissue was added to 30 ml of 5% potassium hydroxide and digested for several hours at approximately 80C. The insoluble parti cles were then pelleted by centrifugation at 10,000gand the pellet was redispersed in a known volume of distilled water with a 10sec burst of ultrasound. A 10-pl drop of the dispersed suspension was placed on a carbon-coated foemvar 200-mesh nickel locator grid, two or three of which were prepared for each specimen. The type and wet weight of lung tissue are reported in Table 3. Characterization of Fiber Content of the Lung Tissues Each tissue preparation was examined by transmission electron microscope to deter mine the number of fibers per gram of dry lung tissue. Two criteria were used to define a fiber: the aspect ratio had to be M Volume 102. Supplement 5, October 1934 247 NOLAN ETAL Table l Fiber types present in lung tissue obtained from six cases of exposed woriters examined by analytic transmission electron microscopy. Casel Canadian Case 2 Canadian Case3 Canadian Case 4 Canadian Case 5 Canadian Case 6a U.S. Case 6b U.S. Jamestown, California (Ref. spec. vi| Residue chrysolite sample no. 5 (Ref spec, vl No (Mg.Ca.Fel silicate fibers/total nonchrysotile fibers Mg/Si Ca/Si Fe/Si Commercial asbestos Types present talc ' Mot detected 57/63 0134 0.03 0.4010.06 0.101094 Chrysotila 5/20 0.420.04 0.250.13 0.050.03 Chrysotile 56/76 0.3310.01 0.4510.16 0.1910.15 Chrysotile Anthophyllite- Amosite Amosite Crocidolite Anthophyllite talc 14/49 0.3210.01 0.3210.04 0.1310.03 Chrysotile Crocidolite 31/33 0.3210.05 0.3210.05 0.1110.13 Chrysotile -2/240 0.1610.08 0.3310.04 0.4310.13 Chrysotile -2/241 0.34+0.03 0.3810.03 0.0310.02 Chrysotile ND' 0.3210.06 013210.05 0.1410.06 NO NO Trace of chrysotile Table S. length distribution ofchrysotila fibers isolated from two lung parenchymal specimens from the U.S. pleural mesothelioma case (6afr). specimen No. fibets sized <0.50 pm 0.50-0.99 pm Percentage within each interval offfter length 190-19Spm 2.00-4.99 pm 590-7.99 pm Case 6a' Case 6b' 311 22.8 44.1 215 89 1.9 240* -- 57.1 292 10.4 2.5 572 57.9 27.8 10.1 3.7 0.5 241* -- 66.0 24.1 8.7 12 'Fiber lengths were determined at a magnification x 7500; all fibers and fibers > 0.50 pm counted, `length distribution notcounting <05 pm fibers. >800 pm 06 0.8 00 09 greater than 3:1, and the fiber length had magnification X20.000. Three to five fibers in a 110 pm X 110 pm grid opening to be >0.5 pm. The weight of wet tissue fields were randomly selected for counting. are known, as well as the total surface are> used prior to digestion ranged from 12.3 to The average number of fibers per grid of the grid, then the total number of fibers 2700 mg. opening varied among the six cases from present on the grid can be calculated. Since The fiber content of the tissue speci 4.2 2.2 to 121 17 with a precisian of this total is equivalent to the number of mens was determined by counting the 26 12% (Table 3). fibers in the 10 pi of suspension, and sitter number of fibers in a 110 pm x 110 pm Assuming the fibers are randomly the mass of the digested tissue and the vol grid opening on the fluorescent screen at a located on the grid, and the number of ume of water in which the insoluble pani cles were resuspended are known, then the number offibers per gram ofwet weight of Table 6. Mesotheliomas induced in rats by the intraperitoneal Injection of reference specimens of chrysotile asbestos'. Dose, mg Mesothelioma bearing animals No. % Average latency, weeks tissue can be calculated. The dry tissue weight is simply assumed to be 10 times less than the wet weight. By the above pro cedures, it was found that the fiber concen tration of the six tissue specimens varied lib. Canadian, UICC B iia, Zimbabwe, UICC A iii, New fcfria, California i. Short chrysotile. Grade 7* lib, Canada UICC 8 i. Short chrysotile. Grade 7* ift. Canadian. UICC 8 hr, Canadian chrysotile, Grade 4-T-30* hr. Shortchrysotile I0M 25 25 25 25 30 10 5 1 10 5 1 25 15 0.25 25 15 0JZ5 32 80.0 33 815 29 715 29 729 30 759 11 27.5 4 109 0-- 14 359 11 27.5 0 __ 24 95.8 24 91.7 24 66.7 24 91.7 24 33.3 24 0 92.2 89.7 85.3 76.4 76.1 57.0 59.7 -- 61.6 60.3 ~~ 51.6 73.0 105.1 710 96.4 -- 'Data from Matoni and Minardi 110} and Davis and Jones (/7J. `Water fractionated from Paperbestos 5 similar to specimen No. 1026. ` Results avaJaUe after76 weeks' study. `Start malarial for preparation ofShort Chrysotite K3M. from 7 4 x 10" to 13,900 2900 x 10' fibers ofdry lung (Table 3). _ In the five Canadian cases aniosite. anthophyllite-talc, chrysorile, crocidolite. and [Ca,Mg,Fe] silicate fibers were idenn- fied by energy dispersive X-ray spef' troscopy (EDS). Chiysorile was present m all five but was predominant in none. The nonchrysotile fibers were analyzed sequen tially by EDS. In Cases 1 and 5, where no commercial amphibole asbestos was p,e~ sent, over 90% of the fibers v/ete lCa,Mg,Fe] silicates (Figure 1). The anthophyUite-talc fibers in Cases and 3 were assumed to have original from a source other than exposure f0 anthophyllite asbestos. In the three case5 248 Environmental Health Perspective1 each of the two grid openings were deter mined from the photomontage (Table 3). The total number of fibers sized varied by almost 2-fold between the two tissue specimens. This difference was due princi pally co the large number of fibers <0.5 pra in length in Case 6b. If only fibers of lengths >0.5 pm are included, the numbers of fibers per grid opening are 240 for Case 6a and 241 for Case 6b, the two fiber length distributions are also quite similar (Tabic 5). No amosite or crocidolite fibers were found among 350 fibers scanned in Case 6a at a magnification X 20,000; nor among 180 fibers scanned in 6b; neither were they found in low magnification scans at x 5000 for 25 fields in each preparation. The ana lytical sensitivity indicates that the concen tration of amosite or crocidolite, if present, would be below approximately 4,600,000 fibers/g of dry lung. [Ca,Mg,Fe] silicate fibers were identified at an average concen tration of 2 0.71 in five fields examined. This corresponds to 223 90 X 10s fibers/g of dry lung. The average length, diamecec, and aspect ratio of 17 [Ca,Mg,Fe] silicate fibers sized from pho tographic plates was 6.7 4.3 pm, 0.36 0.2 pm, and 23.8 20 respectively. Fiber dimensions ranged from 15.0 pm x 0.2 pm to 2 pm X 0.17 pm. No fibers <2 pra were identified. Of the 11 nonfibrous particles analyzed, 10 had a mor phology and elemental composition consistent with that of quartz. Discussions and Conclusions Rfire Z Ml A typical tremolite asbestos from Jamestown. CA. The linear grid opening is approximately 110 pm. iatnpare this with (5), a residue from specimen 5. predominantly [Ca.Mg.Fe! silicates. The long fiber is oproximately 90 pm in length. The fibers in specimen no. 5 are of shorter length and greater diameter than those which ssracterize die tremolite asbestos specimen. 1C) fiber is a high aspect ratio (Ca.Mg.Fe! silicate fiber found in Case 4 is j anuareo with tremolite asbestos from Jamestown,,CA ID). Arrow (0| points to fiber of 2 pm diameter. where amosite or crocidolite were found, only approximately 42% of the tionchrysotile fibers had [Ca,Mg,Fe] sili cate compositions. The ratio of the EDS counts for calcium, magnesium and. iron to olicon are given in Table 4. For comparison, the analyses of the tibcrs present in a tremolite asbestos, refer ence specimen vi and in reference specimen ire included. The analysis of the two parenchymal lung tissue specimens in the T.S. pleural mesothelioma. Case 6, showed ;nat the predominant fiber type was chrysotile, and the fiber concentration averaged approximate// 10,000 x 10( fibers/g of dry lung. This is approximately 300 times greater than the average total fiber concentration in the Canadian cases. To determine the fiber length distribu tion and to quantify the fiber number per field more accurately in each of the two specimens, the U.S. 6a and 6b, a series of overlapping photographs of a complete grid opening was taken and assembled into a photomontage. The length of each fiber present and the total number of fibers in The lung content of Case 6, who devel oped a pleural mesothelioma, is of particu lar interest because of the extremely high content of chrysotile found. Approximately 27,600 X 10` fibers >0.5 pm in lengtb/g of dry lung tissue were found in each of the two parenchymal tissue specimens and more than 99% of the fibers identified were chrysotile. Using a protocol with an analytical sensitivity of approximately 4,600,000 fibers/g of dry lung, no com mercial amphibole asbestos minerals were detected in either specimen of Case 6. About 0.8% of the fibers present were cal cium-magnesium-iron silicates. The fiber length distribution in each of the rwo specimens in Case 6 was deter mined. Although long chrysotile fibers were identified--the longest fiber found being 33 pm--on average, only 1.5% of the 883 fibers sized were S5 pm in length (Table 5). This fiber length distribution is comparable with that obtained from work ers in a Swedish cement plant where 1.42% ''ofurrje 102. Supplements, October 1994 249 NOLAN ETAL of the 7849 fibers sized were 26 [im (8) but considerably shorter than the fiber length distribution obtained in an American textile plant where 7.7% of 1459 fibers sized were 26 pm (8). The average concentration in Case 6, counting fibers of all lengths, was approximately 50,600 X 106 fibers/g of dry lung. This concentration is approximately 670 times higher than the mean concentration deter mined for asbestos cement workers and 880 times higher than that for asbestos textile workers. The length distribution of the fibers found in Case 6 was compared to the five reference chrysotile mineral specimens, the carcinogenicity of which had been deter mined earlier in experimental animal stud ies {10,11). On average, 1.3 0.9% of the fibers were 25 pm in length in the two UICC reference specimens (iia and iib) and the commercial specimen (iii) (Table 1). For the intervals chosen, the fiber length distribution of these three reference speci mens and that of Case 6 were similar. A 25 mg dose of each of the three reference spec imens, when administered to rats by intraperitoneal injection, produced a simi lar incidence ofmesothelioma (Table 6). The length distribution of the fibers found in Case 6 was also compared to two reference specimens. Smaller intervals were selected to better define those fiber lengths predominantly <5 pm (Table 2). If the <0.5 pm fibers are ignored, the fiber length distributions in reference specimens i and iv and Case 6 are indistinguishable. By intraperitoneal injection at high dose (25 mg), both i and iv produced high inci dences of mesotheliomas in rats (72.5% and 91.7% respectively) {10,11). As doses were reduced, the two short chrysotile specimens (i and iv) produced fewer mesotheliomas than che longer reference specimens. Although this decrease in the production of mesotheliomas in the rats often is attributed to reduction in fiber length, it is important to recognize that long chrysotile fibers are generally bundles of individual fibers. Once these bundles are dispersed, the fiber length distribution of the six chrysotile specimens was very similar. The fiber content of Case 6 differed markedly from the five Canadian cases. The mean total fiber concentration of the Canadian cases was 300 rimes less than in Case 6, and the [Ca,Mg,FeJ silicate fiber concentration was 6 times less. In none of the Canadian cases was chrysotile the pre dominant fiber type, whereas in Case 6. approximately 99% of the fibers detected were chrysotile and approximately 0.8% were [Ca,Mg,Fe] silicates. Conversely, in the Canadian cases, on a fiber number and mass basis, [Ca.Mg.Fe] silicate fibers exceeded the chrysotile present. These [Ca,Mg,Fe] silicate fibers, commoniy referred to as tremolite, frequently have been found in the lungs of Quebec miners and millers, particularly in Thetford (7,9). When reviewing the results of studies where oniy fibers 25 pm in length are counted, however, a bias towards rremolite may be introduced since the fiber length distributions of chrysotile and tremolite arc different and thus contribute to the out come: 11% of the [Ca,Mg,Fe] silicates in reference specimen v were found 25 pm, while the. mean of the three reference chrysotile specimens was 1.3%. REFERENCES 1. Wagner JC, ed. The Biological Effects of Chrysotile. Philadelphia: Lippincott, 1987. 2. Harington JS. The carcinogenicity of chrysotile asbestos. Ann NY Acad Si 643:465-472 (1991). 3. Wagner JC Berry G, Skidmore JW, Ttmbrell V. The effects of the inhalation ofasbestos in rats. Br J Cancer 29:252-269 (1974). 4. Davis JMG, Bolton RE, Miller BG, Niven K. Mesothelioma dose ' response following intra-pcritoncal injection of mineral fibers. Inc J Exp Pathol 72:263-274 (1991). 5. McDonald AD, Fry JS, Woolley AJ, McDonald JC. Dust exposure and mortality in an American chrysotile asbestos friction products in plant. Bt J Ind Med 41:151-157 (1984). 6. McDonald JC, McDonald AD. Epidemiology of asbestos-related lung cancer. In: Asbestos Related Malignancy (Antman K, Aisner J, eds). Orlando, FLGrune and Stratton, 1987:57-79. 7. Pooley FD. An examination of the fibrous mineral content of asbestos lung tissue from the Canadian chrysotile mining industry. Env Res 12:281-298 (1976). 8. Pooler FD, Mitha R. Fiber types, concentrations and characteristics found in lung tissues of chry^orile-exposcd cases and controls. In: The Biological Effects of Chrysotile, (Wagner JC, ed). PhiladelphiatLippincott, 1987:1-11. 9. Sdbastien P, McDonald JC, McDonald AD, Case B, Harley R, Respiratory cancer in chtysotiie textile and mining industries: expo sure inferences from lung analysis. Br J Ind Med 46: 180-187 (1989). 10. Maltoni C, Minardi F. Recent results of carcinogenicity bioassayt of fibres and other particulate materials. In: Non-occupacionai Exposure to Mineral Fibers (Bignon J, Peto J, Saracci R, eds! IARC Scientific Publications No. 90, Lyondnternational Agency for Research on Cancer, 1989:46-53. 11. Davis JMG, Jones AD. Comparisons of the pathogenicity of lonf and shore fibres of chrysotile asbestos in rats. Br J Exp PaI" 69:717-737(1988). . 12. Addison J, Davies LST. Analysts ofamphibolc asbestos in chtysoolc and other minerals. Ann OccupHyg34:159-175 (1990). 13. Langcr AM, Nolan RP, Addison J. Distinguishing between amp"" bole asbestos fibers and donate cleavage fragments of their non asbestos analogues. In: NATO Advanced Research Workshop on Mechanisms in Fiber Carcinogenesis, (Brown RC, Hoskins JA. Johnson NF, eds). New YoriePlenum Press, 1991,253-267. 14. Nolan RP, Langer AM, Oechsle GW, Addison J, Colfiesh DE- Assodadon of tremolite habit with bioiogicai potential: PrelimtjwO' report. In: NATO Advanced Research workshop on Mechanism* of Fibre Carcinogenesis (Brown RC, Hosldns JA, Johnson Ne eds). New YorlcPfemim Press, 1991,231--251- 250 EnvironmentalHealth PerspecV**.: Biopersistence of Respirable Synthetic Fibers and Minerals Volume 102, Supplement 5 October 1994 NATIONAL INSTITUTES OF HEALTH National Institute of Environmental Health Sciences