Document zd6bQ2NoyZ9gx87gN5xMjbNgB

Snvtrowiunlal BtaUk Ptrt Vol. M, pp. MS-MS, 1990 Human Disease Consequences of Fibe Exposures: A Review of Human Lung Pathology and Fiber Burden Data by Victor L. Roggli* Inhalation of aabcctoa 6ben result! in a variety of ocoplaitk and nonneopUatic diaraart of the respi ratory tract. Sorer of these diseases, inch as asbestooia, yenerally occur after prWonyed and Intensive exposure to asbestos, whereas other!, such as pleural mesothelioma, may occur foUowtny brief exposures. Inhalation of nonaabestiform mineral fibers can occur as well, and these libers can be recovered from human luny tissue. Thus, there has been considerable interest in the relationship between mineral fiber content of the luny and various petholoyic change* Techniques for fiber analysis of human tissues have not been standardised, and consequently results may differ appreciably from one laboratory to another. In all reported series, extremely hiyh fiber burdens are found in the lunys of individuals with asbestosis. Although there is s correlation between the tissue concentration of asbestos fiber* and the severity of pulmonary fibrosis, further studies of the mineraloyie correlates of fiber-induced pulmonary fibroois are needed. Mesothelioma may occur with fiber burdens considerably leas than those necessary to produce asbestoois. More information is needed reyarding the migration of fibers to the pleura and the numbers, types, and dimensions of fibers that accumulate at that site. Patients with asbestosis have a markedly increased risk for Iany cancer, but the risk of lung cancer attributable to asbestos in exposed worker* without asbestosis who also smoke is controversial. Combined epidemiclogic-mineralogk studies of a welldefined cohort are needed to resolve this Issue. In addition, more Information Is needed regarding the potential role of nonasbestos mineral fiber* in the pathogenesis of lung cancer. Introduction The development of techniques for assaying the min eral fiber content of tissues has provided researchers with the opportunity to correlate the occurrence of var ious fiber-related diseases with the cumulative fiber bur dens in the target organ. Exposure to asbestos gener ally occurs through the inhalation of airborne fibers, and thus the respiratory tract is the site of most asbestosrelated diseases. Consequently, most studies of tissue fiber burdens have concentrated on the analysis of lung parenchyma. Asbestos is both fibrogenic and carcino genic with respect to the respiratory system, with dis eases occurring in the pleura (pleural plaques, diffuse pleural fibrosis, malignant mesothelioma) and in the lung (asbestosis, bronchogenic carcinoma). The patho logic features of these diseases have been reviewed in detail elsewhere (1 -4) and will be discussed only insofar as they relate to lung fiber burdens. Asbestos is not a single mineralogic entity, but rather a group of mineralogic species that share the properties of high tensile strength, flexibility, and relative thermal and chemical resistance. Two mfijor groups of asbestiform minerals include serpentines and amphiboles. `Durham Veterans Administration and Duke University Medical Centers, Durham, NC 27710. Chrysotile asbestos is the sole representative of the serpentine group, and its structure, chemical compo sition, and persistence in biological systems differs ap preciably from those of the amphiboles. Commercially valuable forms of amphibole asbestos include amoote and croddohte. Other amphiboles, including actinolite, anthophyllite, and tremolite, have little or no commer cial value but may be found as contaminants of a variety of other mineral substances. Details of the physical and chemical properties of the various asbestiform minerals have been reviewed elsewhere (5). In addition, a variety of nonasbestiform fibrous minerals may be identified in human lung tissue samples, including talc and other silicates, silica, carbon, metal oxides (such as titanium, iron, or aluminum), zeolites, and man-made mineral fi bers (6-8). It is the purpose of this review to discuss various aspects of human lung fiber burden data as they relate to pulmonary disease. This will include a critical review of techniques for analyzing lung fiber burdens and the limitations of extrapolating results from one laboratory to another. The results of mineral fiber analysis in spe cific diseases, including asbeBtoeis, mesothelioma, be nign pleural diseases, and carcinoma of the lung, will also be evaluated. Finally, areas where there are gaps in our knowledge will be specifically identified, with suggestions for future research directions. ST0272702 | 296 V. L. ROGGLI !> i u ' J / 2 / U J Techniques for Analysis of Pulmonary Fiber Burdens Several techniques have been devised for assaying mineral fiber content of human lung tissue (7.9-12). These techniques generally involve three basic steps: a) dissolution and removal of the organic matrix ma terial of the lung in which the fibers are embedded: 6) recovery and concentration of the mineral fibers; and c) analysis of the mineral fiber content by some form of microscopy. As summarized in Table 1, the actual an alytical result obtained on any one sample can be pro foundly influenced by the steps in the analytical pro cedure employed by the investigator. Indeed, inter laboratory comparison trials have shown that striking differences can occur between laboratories even when the same sample is analyzed (13). An ongoing interna tional interlaboratory comparison study is now drawing to its close, and should provide information regarding which steps in the analysis produce the greatest dis crepancies. In addition, intralaboratory variation can occur, which may be either due to changes in a labo ratory's procedures over time (14) or to variation in fiber content from one site to another within the lung (15,16). Such interlaboratory variation makes it difficult to compare results obtained by different laboratories. This does not necessarily invalidate the results of human fiber burden studies, since there is evidence for internal consistency within individual laboratories (13). Rather, one must cautiously compare results between labora tories, keeping in mind differences in analytical tech niques. Furthermore, when interpreting fiber burden data, one must realize that the analysis is occurring at a single point in time, usually when advanced disease is present. The fiber burden at that point may or may not relate to the tissue fiber content at the time when disease was actively evolving--a period of great inter- Table 1. Factors affecting fiber burden data. Digestion procedure Wet chemical digestion (alkali, enzymes) Low temperature plasma ashing Number of sites sampled Recovery procedure Use of a centrifugation step Use of a sonication step Filtration step (type of filter, pore size) Analytical procedure Microacopic technique (LM, PCM, TEM, SEM)* Magnification used Sizes of fibers counted and other counting rules Numbers of fibers or fields actually counted Reporting of results Asbestos bodies or fibers (or both combined) Sizes of fibers counted Concentration of fibers (per gram wet or dry lung or per cubic centimeter) *LM, light microscopy: PCM, phase contrast microscopy; TEM, transmission electron microscopy; SEM, scanning electron microscopy. est to investigative biologists. Nonetheless, there is a growing consensus that the fiber burdens which accu mulate in the lung are the primary determinant of sub sequent disease (17). Asbestosis Asbestosis is by definition pulmonary interstitial fi brosis developing in response to inhalation of asbestos fibers. The minimal histologic criteria for the diagnosis of asbestosis include peribronchiolar fibrosis associated with accumulations of asbestos bodies (2,18). Some in vestigators have challenged the requirement for finding asbestos bodies in histologic sections, since some work ers are exposed almost exclusively to chrysotile-containing products (19), and chrysotile forms asbestos bod ies less readily than amphibole asbestos fibers (20). However, studies of chrysotile miners and millers (21) and my own observations of textile workers exposed to chrysotile fiber show that asbestos Bodies are readily found in histologic sections of individuals with asbes tosis. Furthermore, these asbestos bodies can be shown by means of energy dispersive spectrometry to have chrysotile cores (21,22). Several studies have examined the asbestos content of lung tissue in series of patients with asbestosis (18,23-26). These data are summarized in Table 2. The values obtained are roughly similar among the reported series, with the exception of the unusually high median count for asbestos bodies in the study by Ashcroft and Heppleston (24) and the high mean count for uncoated fibers by electron microscopy in the stud\ !; Wagner et al. (26). Most of the discrepancies can be explained by methodologic differences. For example, Whitwell et ai. (23) used phase contrast light microscopy (PCLM) and courted all fibers greater than or equal to 6 |im in length, counting coated and uncoated fibers together. Ashcroft and Heppleston (24) used PCLM at a magni fication of 400 x and counted all visible fibers, enum erating coated and uncoated fibers separately. Warnock et al. (25) used transmission electron microscopy (TEM) and counted all fibers exceeding 0.25 pm in length. Wag ner et al. (26) used the PCLM method of Ashcroft and Heppleston (^4) as well as TEM. Roggli (18) used scan ning electron microscopy (SEM) at a magnification of 1000 x to count all fibers with length greater than or equal to 5 pm. Both Warnock et al. (25) and Roggli (18) counted asbestos bodies by conventional light micros copy. The median uncoated fiber count exceeds one mil lion (10s) fibers per gram of dried lung in all five studies. When these values are compared to the background levels of pulmonary fiber burden in the general popu lation as reported by various laboratories (18,23,26,27), it is observed that patients with asbestosis generally have extremely high fiber burdens. These observations fit well with epidemiologic data, which indicate that asbestosis generally occurs in in dividuals with prolonged and heavy exposure to air borne asbestos fibers (28). In addition, the data suggest that analysis of tissue asbestos burdens may be useful HUMAN FIBER BURDEN DATA 297 Table 2. Asbeetoe content of lung tiiiue in reported eerie* of patient! with aebeatoaie iIS).m No. of case* 23 Method'' PCLM Aibestos bodiei/g dried lung -- Uncoiled fiberag dried lung 8 (1.0-70) Reference <M) 12.2 32 22 PCLM (0 49-192) (1.3-493) m) 100 PCLM 1.5 -- (0.001-31.6) it5) 0.378" 3 3d 76 SEM' (0.006-16) (0.18-125) US) 0 123 6.68 22 TEM' (0.001-7.38) (1.6-121) its) 170 TEM _ 372 1.0-10,000) its) * Values reported are the median counts for millions (10*) of asbestos bodies or uncoated fibers per gram of dried lung tissue, with ranges indicated in parentheses, except for the study of Wagner et al. (2$), where only the mean value could be obtained from the data presented. ` PCLM, phase contrast light microscopy: SEM, scanning electron microscopy; TEM, transmission electron microscopy. ` In these two studies, asbestos bodies were counted by conventional light microscopy. "Values multiplied by a factor of 10 (approximate ratio of wet to dry lung weight) for purposes of comparison. ST0272704 for distinguishing asbestosis from other types of inter stitial fibrosis, including idiopathic varieties. Although this distinction can be made in the vast majority of cases by the identification of asbestos bodies in histologic sec tion (2,18), cases have been reported in which asbestos bodies could not be demonstrated histologically (29,30). The problem is to determine what level of fiber burden should be required to diagnose asbestosis in cases where asbestos bodies are not found in tissue sections. Most of the data for uncoated fibers (Table 2) relate to fibers which are 5 p.m or greater in length and suggest that at least a million fibers per gram of dry lung tissue should be present. This level is further supported by the study of Roggli (18) correlating the histologic se verity of asbestosis with the tissue burden of uncoated fibers 5 ixm or greater in length. The regression line in this study had an intercept of 100.000 fibers/g wet lung (approximately 106 fibers/g dry lung) at a histologic score of 0 (i.e., no fibrosis). Whereas the pathogenicity of asbestos fibers 5 p.m or greater in length is well established, that of shorter fibers remains unproven (31,32) and is an area requiring further investigation. The problem is compounded by the fact that short chrysotile fibers are a ubiquitous contaminant, they are difficult to count accurately (33), and they may be found in substantial numbers within lung tissues of individuals from the general population (27). There is insufficient data in the literature to sug gest any level of short fibers as a criterion for the di agnosis of asbestosis. Short fibers are usually present in lung tissue in substantially greater numbers than long fibers (i.e., * 5 pun in length) (11,27). Therefore, it is difficult to accurately assess the numbers of long fibers that are present in studies employing TEM to count all visible fibers at a high magnification (e.g., 20,000 x or greater). One suggestion for future correlative pathol ogy fiber burden studies employing TEM is the use of a stratified counting scheme that takes into account the numbers and dimensions of both short and long fibers. Although there is a good correlation between num bers of fibers per gram of lung tissue and severity of pulmonary interstitial fibrosis (18,23-26), there is a wide scatter in the data, indicating that factors other than tissue fiber burden are involved in determining the ultimate degree of fibrosis which develops (18,2i). In this regard, studies by Timbrell et al. (Si,35) have shown that in individuals exposed to the various types of amphibole asbestos, the severity of pulmonary fibro sis correlates better with the relative fiber surface area per unit weight of tissue than with the relative fiber number or mass, as determined by magnetic alignment and light scattering. These observations need to be con firmed by electron microscopic techniques that take into account not only fiber number, mass, and relative sur face area, abut also fiber dimensions and in particular absolute numbers of long fibers. In addition, it is nec essary to determine which if any of these factors apply to asbestosis due to the inhalation of chrysotile. A recent study in this regard by Churg et al. (36) showed a direct correlation between fiber concentration and severity of fibrosis for both chrysotile and contaminating tremolite fibers, but no correlation of fibrosis with fiber size, sur face area, or mass for chrysotile and an inverse corre lation with fiber length, aspect ratio, and surface area for tremolite. Clearly, further studies of the mineralogic correlates of fiber-induced pulmonary fibrosis are needed. Mesothelioma Mesothelioma is a malignant tumor that derives from the serosal lining of the body cavities. The most common 298 V. L ROGGU 1 7 1 ?.O K^ site of origin is the pleura, followed by the peritoneum and pericardium. The pathologic features of these tu mors have been reviewed elsewhere {2.37,38). Mesothe lioma is a rare form of malignancy, and its occurrence is strongly associated with exposure to asbestos fibers decades prior to the development of clinical symptoms (1.28.39). Epidemiologic studies have shown that me sothelioma can develop years after brief or low level exposures, and recent studies have indicated that there are cases of mesothelioma for which no prior exposure to asbestos can be identified (37,iO.il). Therefore, there has been considerable interest in the mineral fiber con tent of the lung in patients with mesothelioma. Several studies have examined the asbestos content of lung tissue in series of patients with mesothelioma (23,42-4 7). These data are summarized in Table 3. Com parison with the data in Table 2 shows that there is considerable overlap of fiber content among patients with asbestosis and mesothelioma. This is not surpris ing, since according to Antman (1,8), about 20% of pa tients with pleural mesothelioma also meet criteria for asbestosis [26% in the series of Whit well et al. (23), 21% in the series of Roggli et al. (1,6)}. Studies examining the pulmonary fiber burdens in groups of patients with asbestosis versus mesothelioma have shown that me sothelioma may occur with fiber burdens considerably less than are required to produce asbestosis (23,46). This observation is in agreement with the epidemiologic findings noted above. One very important exception to this observation has been reported in miners and millers of chrysotile asbestos (49). In these patients, the total fiber burden in chrysotile workers with mesothelioma is considerably greater than the median fiber concen trations in workers with asbestosis. Furthermore, the ratio of tremolite (a contaminant of chrysotile ore) to chrysotile is considerably greater in the lungs of the workers with mesothelioma than in the workers with asbestosis (44 U9). These observations and scattered re ports of mesothelioma occurring in individuals exposed environmentally to tremolite have led some investiga tors to propose that it is the tremolite component of the chrysotile ore which is responsible for the development of mesothelioma in chrysotile mine workers (49). In consideration of Stanton's observations that fibers greater than 8.0 pm in length and less than 0.25 pm in diameter are the most efficient at producing mesothe lioma experimentally (50), it is of interest to examine fiber dimension data in studies of human lungs with regard to mesothelioma. The study by Churg and Wiggs (43) of amphibole-induced mesothelioma showed that 39% of amosite fibers and 23% of crocidolite fibers were 5 pm or greater in length. In contrast, the study by Churg et al. (44) of chrysotile-related mesotheliomas showed that only 11% of chrysotile fibers and 13% of tremolite fibers exceeded 5 pm in length. The vast ma jority of fibers in both studies were less than 0.25 pm in diameter (43,44). Lippmann (35) concluded in his re view of the human and animal data that it is primarily fibers greater than 5 pm in length and less than 0.1 pm in diameter that are responsible for the development of mesotheliomas. The fiber dimension and fiber burden data from chrysotile versus amphibole-induced meso thelioma in humans are consistent with either the hy pothesis that a) large numbers of short (<5 pm) asbes tos fibers are carcinogenic for the pleura in man, or that b) large burdens are necessary to provide sufficient Table 3. Asbestos content of lung tissue in reported series of patients with mesothelioma.* No. of cases 100 15 14 19 10 Method" PCLM SEM SEM SEM' TEM Asbestos bodies/g dried lung -- -- -- 48* (0.002- 9770) -- Uncoated fibers/g dried lung 0.75 (0-70) 11 (2-490) 2.4 (0.4-37) 0.81* (0.012-28.9) 3.5 (0.1-85.2) Reference (13) Ul) US) US) US) 238 6 TEM ___ (52 -2190) U4) 3.2 20 TEM' (0.04-460) 18 U7) * Values reported are the median counts for thousands (10*) of asbestos bodies or millions (10*1 of uncoated fibers per gram of dried lung tissue, with ranges indicated in parentheses, except for the study of Gaudichet et. al. U7), where only the mean value for total fibers per gm dried lung could be obtained from the data presented. bPCLM, phase contrast light microscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy. ' In these two studies, asbestos bodies were counted by conventional light microscopy. * Values multiplied by a factor of 10 (approximate ratio of wet to dry lung weight) for purposes of comparison. HUMAN FIBER BURDEN DATA m ST0272706 numbers of "Stanton-sized" tremolite fibers. However, the data are also consistent with a third possibility: due to chrysotile's fragility and tendency to break into small individual fibrils, very high exposure levels are neces sary to maintain a sufficient level of "Stanton-Bized" chrysotile fibers in contact with the pleura. This latter hypothesis may be applicable to possible mesothelioma risks from nonasbestos mineral fibers, such as manmade mineral fibers, many of which are either soluble in vivo (51) or tend to fracture transversely, resulting in shorter fibers that may be more readily cleared from the lung (52). An area requiring further study is the migration and distribution of amphibole versus chrysotile fibers to the visceral and parietal pleura. It is reasonable to assume that fibers actually reaching the pleura are the ones responsible for diseases of the pleura, and the dimen sions and types of fibers accumulating in the pleura are not necessarily similar to those accumulating in the lung parenchyma. In this regard, Sebastien et al. (53) re ported that in individuals exposed to mixtures of fibers, there was a relative accumulation of longer amphibole fibers in the lung parenchyma, whereas short du^sotile fibers accumulate in the pleura. However, Churg et al. (55) were unable to find a difference in the size or type of fibers isolated from peripheral versus central lung parenchyma in Canadian chrysotile workers. One prob lem with such studies is that samples of peripheral lung necessarily include a large proportion of lung paren chyma, so that any differences between fiber content of lung parenchyma and visceral pleura per se would be masked or minimized. Although there are substantial data now available on the fiber content of lung paren chyma from the general population with various ana lytical techniques (23,274647), no comparable data ex ist for the visceral and parietal pleura. Furthermore, the migration of fibers from the lungs to the peritonea] cavity needs to be further clarified. Benign Pleural Disease The most common pathologic abnormality related to the inhalation of asbestos fibers is the parietal pleural plaque. These lesions occur as circumscribed, elevated areas of pleural thickening with a cartilaginous consis tency, located most often over the domes of the dia phragm or along the posterolateral chest wall overlying the ribs (2). They are ivory colored with a smooth or knobby surface (the latter resembling candle wax drip pings), and may be calcified. Other pleural abnormalities related to asbestos exposure include diffuse visceral pleural fibrosis, rounded atelectasis, and benign asbes tos pleural effusion. These abnormalities apparently re sult from inflammation and repair stimulated by asbes tos fibers reaching the pleural surface. Epidemiologic studies indicate that benign asbestos-related pleural diseases may develop following brief or low level ex posures (1,28). Several studies have examined the asbestos content of lung tissue in series of patients with benign asbestos- related pleural disease 42,55-57). Most of these have dealt with parietal pleural plaques, and the studies are summarized in Table 4. These data show that patients with parietal pleural plaques have tissue fiber burdens that are on the average substantially lower than those of patients with asbestosis (Table 2 vs Table 4) and are somewhat lower than but of about the same order of magnitude as patients with mesothelioma (Table 3 vs Table 4). The studies by Wamock et al. (55) and Churg (55) both showed a significant increase in the concen trations of commercial amphiboles (amosite or crotidolite) in the lungs of patients with plaques as compared to a reference population, but no significant differences for chrysotile or noncommercial amphiboles. Whitwell et al. (23) included 21 patients with pleural plaques in their normal control series of 100 cases, and found that 65% of the cases with more than 20,000 flbers/g by PCLM but only 5.5% of cases with fewer than 20,000 fibera/g had plaques. All of these observations support a role for asbestos fibers in the production of pleural plaques and confirm the epidemiologic findings that plaques may develop following brief or low-level ex posures. The study of patients with diffuse pleural fi brosis by Stephens et al. (57) indicates that these pa tients have on the average a greater fiber burden than patients with plaques alone, but less than patients with asbestosis (Table 2 vs Table 4). Many of the questions raised in the previous section with respect to mesothelioma also apply to benign as bestos-related pleura] disease, particularly in regard to migration of fibers to the pleura. The mechanism of formation of pleural plaques and their peculiar locali zation to the parietal pleura is poorty understood, es pecially in terms of the dimensions and types of fibers that gain access to this compartment. Carcinoma of the Lung The association between asbestos exposure and an increased risk for lung cancer has been well-established epidemiologically (1,28,32), and cigarette smoking and asbestos appear to act in a synergistic fashion to in crease this risk (58). The pathologic features of lung cancer among asbestos workers are similar to those of nonexposed cigarette smokers, showing the same dis tribution of histologic patterns Ufl,59). There is an in creased predominance of lower lobe cancers among as bestos workers in contrast to the upper lobe pre dominance in nonexposed cigarette smokers (60). The association between asbestos-exposure and lung cancer is widely accepted lor patients with asbestosis, and some investigators have proposed the concept that these tumors are scar cancers. However, only a minor ity of cases fit the classic concept of a peripheral scar carcinoma, and most are the usual bronchogenic carci noma. Whether lung cancers occurring in cigarette smoking asbestos workers without asbestosis can be partly attributed to the asbestos exposure is a highly controversial issue (61-66). It is therefore of interest 900 V. L. ROGGU Table I. Aabeetoe content of lung tissue in reported series of patient! with benign asbestos-related pleural diieaae.* No. of cases 14 Method" SEM Asbestos bodies/g dned lung -- Uncoated fibers/g dried lung 2.2 (0.1-13) Reference Ut) 48.7* 0.5' 17 SEM (0-408) (0.007-1.74) (56) 7.8" 0.54' 20 TEM* (0.3-9.600) (0.018-71) (54) 17.3* 1.14' 29 TEM* (0-194) (ND) (55) 0.131 r PCLM -- (0.029-0.378) (57) TEM 28.9 ___ (9.2-83,5) * Values reported are the median counts for thousands (10*) of asbestos bodies or millions (10*) of uncoated fibers per gram of dried lung tissue, with ranges indicated in parentheses, except for the study of Churg {55), where only the mean value for total fibers per gram was given and a range could not be determined (ND). "PCLM, phase contrast light microscopy; SEM, scanning electron microscopy; TEM. transmission electron microscopy. 'Values multiplied by a factor of 10 (approximate ratio of wet to dry lung weight) for purp ses of comparison. *In these two studies, asbestos bodies were counted by conventional light microscopy. 'Cases in series of Stephens et al. (57) are diffuse pleural fibrosis. All others are parietal pleural plaques. b lU Z /Z /U / to review what has been learned from fiber burden analysis in this regard. Several studies have examined the asbestos content of lung tissue in series of patients with lung cancer (23,25,47,56,67) and these data are summarized in Table 5. The values reported depend not only on the analytical techniques employed by the various authors, but on the way the cases were selected as well. Whitwell et al. (23) examined 100 consecutive cases of lung cancer and found very similar results between cancer cases and controls. Gaudichet et al. (47) included 20 patients with squamous carcinoma and 20 with adenocarcinoma of the lung and found similar asbestos body and fiber counts in these two groups as compared to 20 patients with pulmonary metastases and 20 with cardiovascular dis ease. Roggli (56) studied 30 selected cases of lung cancer with some history of asbestos exposure, but without asbestosis or pleural plaques. The series of Wamock et al. (25) included 7 of9 cases with histologically confirmed asbestosis, and the series of Wamock and Isenberg (67) included 12 of 62 cases with asbestosis. These studies indicate that in populations with no appreciable occu pational exposure to asbestos and with substantial ex posure to cigarette smoke, there is little evidence for a No. of cases 100 30 40 Table 5. Asbestos content of lung tissue in reported series of patients with lung cancer.* Method* PCLM Asbestos bodies'g dried lung -- Uncoated fibers/g dried lung 0.009 (0-0.115) SEM 11.8* (0-510) 0.25' (0.007-1.74) TEM* 0.16 (0-290) 16 Reference (25) (56) (47) 35.6 5.83 9 TEM* (0.41-840) (3.10-73.3) C5) 3.75 75 TEM* (0-1000) (0.OT7-97) (67) `Values reported are the median counts for thousands (10*1 of asbestos bodies or millions (10*) of uncoated fibers per gram of dried lung tissue, with ranges indicated in parentheses, except for the study of Gaudichet et al. (if), where only the mean value for total fibers per gram dried hutg could be obtained from the data presented. "PCLM, phase contrast light microscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy. 'Values multiplied by a factor of 10 (approximate ratio of wet to dry lung weight) for purposes of comparison. * In these three studies, asbestos bodies were counted by conventional light microscopy. HUMAN FIBER BURDEN DATA 101 Table 6. Aibntoi content of lung tiuue in reference or control population*.* No. of eases 100 Method" PCLM Asbestos bodies/g dried lung -- Uncosted fibers/g dried lung 0.007 (0-0.521) Reference (2J) 0.020J 0.034'' 10 SEM' (0-0.22) (0.016-0.056) (**> 0.25 28 SEM -- (0-4.8) 05) 0.28* 1.29* 20 TEM' (0.02-0.84) (0.260- 7.56) (tn 0.18 20 TEM' (0-3.2) 11.2 U7) 23 TEM -- 0.62 (71) `Values reported are the median counts for thousand* (10*) of asbestos bodies or millions (10*) of uncoated fibers per gram of dried hing tissue, with ranges indicated in parentheses, except for the study of Gaudkhet et al. U7), where only the mean value for bAal fibers per gram dried lung could be obtained from the data presented. "PCLM, phase contrast light microscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy. 'lr nesc three studies, asbestos '<odies were counted by conventional light microscopy. * Values multiplied by a factor of 10 (approximate ratio of wet to dry lung weight) for purposes of comparison. ST0272708 contributing role for asbestos in these cancers (S3,17) and that in populations with some occupational exposure to asbestos but without asbestosis, the tissue asbestos burden is greater than that of the general, nonexposed population (25,56,67). These studies do not prove whether asbestos is a substantial contributing factor to the lung cancers in those patients exposed to asbestos who do not have asbestosis. These observations are not surprising when one con siders that 85 to 90% of lung cancers occurring annually in the United States are attributable to cigarette smok ing, whereas as few as 2% of cases may be related to asbestos exposure (M). However, the association of lung cancer with asbestosis is truly astounding, with somewhere between 40 and 65% of individuals with as bestosis ultimately succumbing to carcinoma of the lung (1,28,69). Indeed, it is quite possible that the excess numbers of lung cancers occurring in asbestos-exposed populations are entirely attributable to those occurring in individuals with asbestosis (61), although others have argued that it is the amount of asbestos exposure rather than the fibrotic reaction that is the determining factor (67). In order to resolve this issue, it will be necessary to study tissue fiber burdens in cohorts of asbestos workers who do not have asbestosis. One could then use logistic analysis to compare fiber burden levels and smoking history in individuals dying from lung cancer versus other causes of death. In this manner, it could be determined whether differences are explainable by smoking habit alone or if fiber burden is a separate contributing factor. Fiber dimensions are probably im portant as weU, and in this regard, Lippmann (35) con cluded in his review of the human and animal data that it is primarily fibers greater than 10 pm in length and greater than 0.15 pm in diameter that are responsible for development of lung cancer. F urther study of the possible role of nonasbestos min eral fibers and nonfibrous mineral particles in the path ogenesis of lung cancer is also needed. It has been sug gested that the increased numbers of mineral fibers and particles found in the lungs of smokers with lung cancer as compared to a group of age-matched smokers without lung cancer may play a pathogenetic role (70). Alter natively, smokers who develop lung cancer may simply have genetically determined less efficient clearance mechanisms for fibers, particles, tars, and associated carcinogens that may find their way into the respiratory tract (71). Normal Lungs Determination of background levels of fibers to be expected in the general population is an extraordinarily difficult task because it is no simple matter to define what is normal or to exclude unknown exposures. Sev eral investigators have established ranges of fiber bur dens identified in control or reference populations (22,23,2745,i7,72), and some of these are summarized in Table 6. The variations in reported values can largely be accounted for by methodologic differences and pa tient selection criteria. In any analysis of fiber burden data in a population with a given disease, it is of critical importance to compare the findings with those of an appropriate reference or control population for which the same analytical technique was employed. REFERENCES 1. SelikofT, I. J.. and Lee, D. H. Asbestos snd Disease. Academic Press, New York. 1978. 2. Craighead, J. E., Abraham. J. L., Churg. A., Green, F. H. Y., Kleinerman, J., Pratt, P. C., Seemayer, T. 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