Document M4qyp2YkJQO30m1Gjk1rXB7Ga

-L t-U IJ LI U- In: Bogovski, P. , Gilson, J.C., Timbrell, V. and Wagner, J.C. (ed.), Biological effects of asbestos. Proceedings of a Working Conference, Lyon, 2-6 October, 1972. IARC Scientific Publications Ho. 8. Lyon, IARC, 1973, pp. 295-303. 'ih viov Physical factors as etiological mechanisms 1*AR `. D'iV V. TIMBRELL1 -GC -'OB -.CM OLW WCF Prior to the 1964 New York meeting little atten probably back to the time it was first worked. tion had been given to the possible importance of Secondly, the epidemiological, pathological and ex physical factors in the etiology of lung diseases asso perimental evidence presents a scries of paradoxes ciated with exposure to asbestos dust. Animal ex which are useful in testing ideas concerning the periments had indicated that the fibrogcmcity of etiology. As an example, the major difference in the asbestos particles was probably related to fibre incidence of mesotheliomas in north-westerr. Cape length (Vorwald et ah, 1951: Wagner, 1963; Kolt Province compared with that in the Transvaal sug et at., 1964); but despite the fact that the relevant gests a need to search for some factor (or factors) in occupational exposures were by inhalation, a study the asbestos minerals that consistently have very described at the 1964 meeting (Timbrell, 1965) on the different magnitudes in the two area' In this aerodynamic properties of fibres seems to have been instance it seems likely that such a factor is not the first attempt to investigate these aspects. Sub chemical because, although there are variations in sequent inoculation experiments (Burger & Engel- mineral content of the fibres, these differences do not" brecht, 1970) have re-emphasised the importance of appear sufficient to.providc an explanation. In con fibre length in relation to fibrosis: while very recent trast, some of the physical properties differ so intrapleural inoculation studies (Stanton & Wrench. markedty that they impress Cas possible etiological 1972; Wagner et at., 1972*) have indicated a rela factors. tionship between fibre size and the development of mesotheliomas. In the light of this evidence it is not surprising that studies of the physical characteristics FIBRE IN LUNGS offibres in recent years have been profitable and now offer plausible explanations for some of the biologi cal observations. Studies ofpossible etiological factorsareassisted by .two features of asbestos minerals and of the associ ated lung diseases. First, asbestos minerals are crystals and as such exhibit certain constant physical properties: some of these features arc illustrated in Figure 1. It is no small advantage in analysis that the particles are consistently of fibrous shape, thus effecting a major reduction in the number of possible particle shapes compared to, for instance, coat; and that a few samples from an asbestos geological entity The aerodynamic study presented at the New York meeting provided explanations for some of the features of asbestos fibres detected in lung sections. The deposition of particles in lung airways is related to the aerodynamic diameter of the particles as measured by their free-falling speed. Most inhaled particles with falling speeds greater than that of a unit density sphere 10 pm in diameter are deposited in the upper bronchial tree by sedimentation or by impaction. So the demonstration that an asbestos fibre about 3 am in diameter has the same falling speed as that of a 10 um sphere of unit density in appear to be sufficient to give a good indication of the particle characteristics of the whole deposit. dicates that fibres of this diameter are the thickest that are likely to penetrate to the alveolar regions and to be eventually seen in lung sections. Tin's con 1MRC Pneumoconiosis Unit. LUndoujh Hospital, Penirth, UK. * Unpublished data. clusion is in good agreement with measurements made on fibres in lungs of city dwellers (Gross et at.. 1971) and with measurements made on fibres in -295 - '' %\ ASARCO ELP 0003626 296 BIOLOGICAL EFFECTS OF ASBESTOS CROC1EXOLITE . AMOSITE ANTHOPHYLLITE CHRYSOTILE l ASARCO ELP 0003627 PHYSICAL FACTORS AS ETIOLOGICAL MECHANISMS 297 lungs of rals exposed to airborne particles ofamphi* tration of fibres ofamphibolc asbestos and chrysolite, bole asbestos (Timbrcll et at., 1970). It may be noted In an experiment reported by Wagner (1972), in here that rat lungs ha%'e often provided data on which rats were exposed to dust clouds of the UICC particle deposition, retention and clearance which samples at equal concentrations, six times as much of arc comparable with those obtained from human each of the amphibolc samples was still retained after lungs. * several months compared to the amount of ehryso- Calculations made in this aerodynamic study in* tile. Timbrcll (1970; 1972a) has suggested that this dicated that interception is likely to prevent fibres may be partly due to the characteristic rectilinear longer than 200 pm from penetrating the nasal pas- shape of amphibole fibres, compared to the curly sages. However, interception is not effective in morphology of chrysotile fibres, which could cause depositing fibres, not even long ones, in the trachea amphibole fibres to penetrate to the deeper parts of and other wide airways of the upper bronchial tree, the lung more efficiently than do. chrysotile fibres. The conclusion that long fibres which are thin enough The results of studies in lung" casts and narrow tubes, not to be deposited earlier by sedimentation and im- . which showed a relative penetration of amphibole paction can reach the alveolar regions explains satis- and chrysotile fibres'similar to that obtained in the factorily the numerous long asbestos fibres detected animal experiment, arc offered in support of this ex- `in lung sections. . planation. It is difficult to estimate what influence Further calculations indicated that long fibres the tendency of chrysotile fibres to break down which reach the respiratory bronchioles are effi- chemically and physically after prolonged residence ciently deposited in these fine tubes, and that bifurca- in the lung (Langcr et a/., 1971) may have had on the tions are the preferred deposition sites. It has been result of the animal experiment. The contribution suggested that long fibres are transported from these may not be substantial since the asbestos content of pulmonary regions less efficiently than are short the lungs was estimated from the silica present, and fibres because they find anchorage in these compli- it appears that whereas magnesium is rapidly leached catedanatomicalstructuresmorereadilvthandoshort from a chrysotile fibre in cico (Morgan &. Holmes, fibres (Timbrell & Skidmore, 196S), and that this 1970) a silicate skeleton remains, may explain why fibrosis tends to be associated first There is a lack of information on whether asbestos ' with the respiratory bronchioles (Wagner, 1963) and fibres can reach the pleura during inhalation or with long fibres. These conclusions are in good whether they migrate to this region. This question is agreement with the detection of accumulations of of particular interest in a consideration of the de- long fibres in small airways, especially at bifurca- 'velopment of mesotheliomas. In an experiment to tions; but the possibility of transportation of fibres compare the effects of long (5-200 ^m) fibres and to these regions from other deposition sites cannot be small particles of the UICC crocidolite sample in * . eliminated. Current studies on rats exposed by in- monkeys and baboons, Rcndall (1972 J) found that halation to radioactive crocidolite fibres (Morgan, small particles are not fibrogenic but that they can 1972 *) are likely to produce much needed data on reach the pleura in macrophages. Results available the pulmonary deposition pattern of fibres and their from a current electron microscope study on the subsequent clearance. lungs of rats exposed to dust clouds of the UICC - little information is available on the important samples at equal concentrations suggest that asbes- question of the relative efficiency of pulmonary pene- tos fibres can reach the alveoli near the pleura during * Personal communication. * Personal communication. Fig. 1. Eleetron micrographs of crocidolite (north-western Cape Province), amosite (Transvaal), anlhophyllite (Finland), chrysotile (Canada) at the same magnification (x 1700). Features to note ire: (i) the rectilinear shape of the amphibole fibres compared with the curved and twisted morphology of chrysotile fibres-; (it) the order of the drameters of the amphibole fibres. crocidolite < amosite < anthophyllite, which numerous e.m. examinations suggest is a characteristic of these fibres in the three geographical areas; (iii) the longitudinal fragmentation of the chrysotile fibres and tho small diameter (about 0.03 tin) of the ultimate fibrils. ASARCO ELP 0003628 % 298 BIOLOGICAL EFFECTS OF ASBESTOS inhalation. In animals killed immediately after a OCCUPATIONAL EXPOSURES short exposure to crocidolitc. small numbers of these fibres have been detected near the visceral pleura: Although a substantial amount of dust sampling some fibres have been detected in the pleura itself, -has been carried out to monitor occupational ex although this could be an artefact. posures to asbestos, very little sampling has been The numbers of amosite fibres observed near the done to provide basic information on the nature of pleura have been lower, as might be expected from the airborne particles. In particular, there has been the greater diameter and higher falling speed of this a lack of investigations designed to compare the type of fibre. nature of dust clouds in amphibole and in chrysotile The numbers of anthophyllite fibres detected near exposures, and to compare these with experimental the pleura have been still lower, as might be ex clouds. Major differences must be expected. pected from the stilt greater thickness of this fibre: Whereas the animal experiment reported by the fact that anthophyllite fibres are capable at all of Wagner (1972), using the UICC samples, indicated a reaching the pleura during inhalation may be due to 6:1 retention of amphiiboles compared to chrysotile, their lath-like shape (Timbrel! et a!., 1970) which substantially larger ratios may be expected to occur gives them lower falling speeds and thus greater in occupational exposures. Both the amphibole and penetration than they would have if they were of chrysotile UICC samples were specially prepared so circular cross-section. as to be rich in fibres of respirable size. Measu red by Chrysotile fibres have very seldom been observed Hexhlet-type instruments, which take account of near the pleura in this study. This suggests support particle falling speed only, the proportions by weight for the conclusion, derived from considerations of of respirable particles are high (about 70%) and the morphology and the aerodynamic behaviour of approximately equal in all the UICC samples. chrysotile fibres, that this type of asbestos does not Furthermore, the samples contain only a small pro penetrate efficiently to the deeper parts of the lung. portion by number of long fibres (up to 200 pm); and But the fact that, in a current inhalation experiment, only these fibres, in conjunction with the difference UICC Canadian chrysotile has produced mesothe in fibre morphology, could contribute to a difference liomas, suggests that the present examination tech in particle deposition patterns between amphibole niques are not adequate to detect and quantify and chrysotile in this animal experiment. 1 n contrast, asbestos fibres in tissue, particularly very fine the industrial preparation of chrysotile is controlled chrysotile fibres. to avoid excessive opening of the fibres (Sinclair, In a recent study Pootey (1972) used the electron 1959) and the production of a high proportion of the microscope to examine chrysotile fibres in sections of fine fibrils; whereas milling experiments (Timbrell & 300 human lungs. The majority of the chrysotile Rendall, 1971 /72) have shown that it is not possible to particles were grouped together in large clusters of mill the amphibole asbestos types without producing strands (bundles of single chrysotile fibres), which extremely fine fibres: some crocidolites, for example were often detected at the bifurcation of small air that from north-western Cape Province, are parti ways. Very short individual fibres were also found cularly rich in fine fibre. The dense clouds of dust scattered throughout the lung sections. This dis and the layers of settled dust often seen in chrysotile tribution of short chrysotile fibres resembles that of mining areas consist largely of relatively thick strands amosite fibres in the lungs of rats used in an inhala and big flocks which settle rapidly and are available tion experiment (Timbrel! & Skidmore, 196S). Con for inhalation only for limited periods: some of siderations of fibre morphology suggest an explana these particles arc of such large aerodynamic size tion for these results: short and long chrysotile fibres that many types of dust sampling instruments do not characteristically have the same radii ofcurvature, so even aspirate them. Rendall & Timbrell using that short chrysotile fibres, being short arcs, are Hexhlet-type instruments observed some low values virtually straight and behave aerodynamically like (5%) for the proportion of respirable particles in air amphibole fibres. This indicates that chrysotile and samples collected in some South African chrysotile amphibole fibres will exhibit different aerodynamic mills, while Gibbs (1971) has reported low values behaviour if they are long, but similar behaviour if (range 14-25%) in Canadian mills; but there is a they are short. great need for more data on chrysotile and amphibole ASARCO ELP 0003629 PHYSICAL FACTORS AS ETIOLOGICAL'MECHANISMS 299 milling and other processes, for comparison pur Craltey (1971) has suggested that the difference is poses. A study with this objective is in progress. due to the electromotive influence of the excess ele These considerations suggest that even in occupa mental manganese in the respirable dust of north tional exposures to chrysolite enough fibre may reach western crocidolite over that in the respirable dust of the bronchial regions of the lung to produce fibrosis Transvaal crocidolite, whose parent rock is softer and carcinoma, but that the amphibole asbestos types and requires less harsh milling treatment to free the are more likely to produce these diseases because of fibre. The estimated three-fold excess of manganese higher effective exposures. Although the possibility does not, however, seem sufficient to explain the of some chrysolite fibres reaching the pleura clearly sharp disease pattern; but it is not possible to dis cannot be dismissed, especially the short fibres de- miss metals as the ultimate carcinogen, with the j tected by Pooley, this type of asbestos appears to be i fibres acting as carriers. Suggestions that differences ; the least likely to be associated with pleural meso- j in oil content of the fibres in the two areas may be I theliomas from occupational exposures ifpenetration responsible are not supported by intrapleural inoeu- of fibre to the deeper alveoli adjacent to the pleura is ! Iation experiments in which, oil-extracted UICC an important factor in the production of these crocidolite from a north-western Cape mine pro tumours. duced a very similar mesothelioma rate to that with There is also a need for information on the nature untreated fibre (Wagner et a!. 1972 *). and concentration of airborne particles at various A recent suggestion is that differences in particle stages in the preparation of asbestos. It has been size may be important (Timbrell et at., 1971). Elec pointed out by Timbrell & Holmes (1970) that the tron microscope studies on fibres prepared by grind application of fiberising processes must progressively ing rock specimens from the two areas showed a increase the number of fibres and must make them consistent difference in fibre size: Transvaal fibres, more easily respirable because of the decrease in both crocidolite and amosite, were on average three their diameters. This suggests that there may be times the diameter, three times the length, and hence major differences in exposures at different points in 27 times the volume or mass of north-western Cape the industry, even when only one type of fibre is in fibres. Very recent studies on air samples have con volved. That such differences do occur is supported firmed these differences which, it has been suggested, by evidence that health risks have been high in the indicate for the Transvaal compared to the north asbestos textile trade (Merewether & Price, 1930; western Cape less liberation of respirable fibres; Doll, 1955; Enterline & Kendrick, 1967; Mancuso & higher fibre settling rates and shorter times available El-Attar, 1973; Newhouse, 1970), where the fibres for inhalation; comparable deposition rates in the are at their finest, shortest and cleanest. upper bronchial tree, but less efficient penetration to the deeper parts of the lung. The last comparison PLEURAL MESOTHELIOMAS IN THE NORTH-WESTERN CAPE AND THE TRANSVAAL may explain the apparent lack of association between mesotheliomas and asbestosis in the two areas. Electron microscope examinations of samples of Probably the greatest paradox presented by asbes Australian crocidolite have shown that the fibres arc tos diseases is the clear association between the de even smaller in diameter than those of north-western velopment of mesotheliomas and exposure to croci- Cape crocidolite (Timbrell et al,, 1970). This may be dolitc dust in the north-western Cape (Wagner et at., related to reports from Australia (Wagner et til.. 1960; Oettle, 1964), compared with the very rare 1971) that mesotheliomas have occurred following occurrence of these tumours in the Transvaal where exposure to crocidolite in the mining areas, and to both crocidolite and the closely related amosite are the fact that mesotheliomas have been observed in mined (Oettle, 1964; Harington et a!., 1971). This workers who were exposed to Australian crocidolite marked difference in mesothelioma risk is not satis when packing respirators during World War II factorily explained by differences in the commercial (Jones, 1968 a). production of fibre in the two areas (Keep, 1961; Harington et a!., 1971), nor by mineralogical factors, nor by the occurrence of asbestosis (Sluis-Cremer, 1965). 1 Unpublished data. * Personal communication. ASARCO ELP 0003630 300 BIOLOGICAL EFFECTS OF ASBESTOS INTRAPLEURAL INOCULATION EXPERIMENTS The development of mesotheliomas in rats follow ing intrapleural implantation or inoculation of fibrous materials has been studied in two recent ex periments by Stanton & Wrench (1972) and by Wagner et at., (1972 *). In these experiments chry solite exposure, which in general presents a smaller mesothelioma hazard for man than does exposure to crocidolite (Wagner et at., 1971), has produced me sotheliomas as readily as the latter. A probable ex planation for this paradox is that such inoculation experiments do not take account of the natural fac tors which operate during inhalation or, conse quently, of the important differences in the aero dynamic behaviour of fibres of the two main asbestos groups. In such inoculation experiments, sizing of the particles by electron microscopy is necessary, since some of the materials may contain a high proportion of particles that are not visible in the light micro scope. The data available on the physical and chemi cal characteristics of these materials are so complex that some indication of the important factors is essential for successful interpretation of biological results. Suggestions of which physical factors may be involved have been provided by recent studies on fibre-cell preparations. In investigations based on the working hypothesis that asbestos cancers may be related to the implantation of fibres in cells (which does not immediately kill the cells), the preferred orientations that fibres of amphibole asbestos types exhibit in a magnetic field (Timbrell. 1972b) have been used to apply the fibres er.d-on to the membranes to impale the cells. These studies have indicated that if fibres are thinner than a threshold diameter they may be thus implanted without destroying the mor phology of the cells: the threshold diameter and its range remain to be determined, a tentative value being set at 0.5 pm. Similar experiments have em phasised that fibres are of an ideal shape to transmit force and motion over considerable distances in cell agglomerations, and that these features are functions of fibre length; also, that whereas fibres which are short compared with cell diameters are able to move comparatively freely, longer fibres readily find an chorage, giving rise to the possibility of relative motion between fibre ends and cells and of irritation and penetration of membranes. These observations suggest that if the carcinogenesis associated with asbestos is related to such mechanical action there may be a threshold Ienglh, of the order of the size of the cell diameters, that the fibres must exceed in order to produce this biological response. A further factor that must be taken into account in examining the results of intrapleural inoculation experiments is the tendency of chrysotilc fibres in vivo to fragment longitudinally into fine fibrils (Suzuki & Churg, 1969); thus the degree of frag mentation and hence the number of fibres produced can be expected to depend on variations in physical and physiological conditions (Timbrell, 1972a). Fibres of the amphibole types of asbestos, on the other hand, appear to retain their morphology and characteristic diameter distributions (Timbrell et at., 1970). Although a direct relationship between these physical factors and the development of mesothelio mas has not been demonstrated, they are the relevant factors in the Stanton & Wrench (1972) and the Wagner et at. (1972l) experiments, if it can be con sidered that consistency in the interpretation of the biological observations made in these two studies is a satisfactory test. In examining the results, fibre dia- : meter is once more found to be a central factor, 1 largely because the number of fibres in a given quantity of material placed in the pleural cavity (in each study a standard weight of each material was inoculated or implanted) is determined more by fibre diameter (inversely proportional to diameter1) than by fibre length. When the non-chrysotile materials (samples ofcrocidolite, amosite, chrysotilc, brucite, synthetic aluminium silicate fibre and aluminium oxide par ticles) used in the Wagner et at. (1972 a) experiment are classified according to fibre diameter, the order obtained is in good agreement with the order in terms of the number of mesotheliomas produced: the finer the fibre the greater its carcinogenicity, j The in vivo dispersion of chrysotile materials and their fibre diameters are difficult to quantify, but their position in the classification by carcinogenicity is understandable from their fragmentation and agglomeration characteristics. In the Stanton & Wrench (1972) experiment a total of 17 samples, in cluding samples ofseveral types ofasbestos as well as of fibrous glass and metallic nickel fragments, were used. When the results reported in that study are 1 Unpublished data. * Unpublished dau. ASARCO ELP 0003631 x ...L PHYSICAL FACTORS AS ETIOLOGICAL MECHANISMS 301 examined using the light microscope data provided by those authors for the size of the coarser materials and the electron microscope data collected by Timbrell for the size of the amphibolc asbestos types, there is again good agreement between the order of the materials in terms of fibre diameter and their classification according to the number of mesothe liomas produced. The formation of mesothelio mas by fine glass fibres in this study, in contrast to the absence of these tumours in an earlier Wagner1 experiment using coarser glass fibre, is consistent with the concept of a threshold fibre diameter. Furthermore, the observation made in the Stanton &. Wrench experiment that the number of mesothelio mas produced decreased with degree of pulverisation of the materials is consistent with the concept of a threshold fibre length. Further experiments are needed using additional types of fibrous materials, and preferably employing fibres in narrow ranges of diameter and length. POSSIBLE FACTORS IN CARCINOGENESIS The fact that the fibrous form of a range of materials (including several types of asbestos,' brucite, synthetic aluminium silicate and glass) produces* * Unpublished data. mesotheliomas suggests that particle morphology may be an essential factor. The fact that librous materials differing greatly in chemical composition produce mesotheliomas suggests that the initiation of a tumour is unlikely to be due to special characteris tics of the materials, for examptc, a certain type of molecular structure. It may be significant that fibres appear to be more carcinogenic the closer their diameters approach the thickness of cell membranes and the size of cell organelles. CONCLUSIONS Extracellular processes and the physical features of fibres thus appear to constitute a major part of the etiology of the lung diseases associated vs ith exposure to. asbestos dust, including the cancers. The bio logical responses which different types of asbestos fibres produce when inhaled seem to be governed largely by the aerodynamic properties of the fibres. This apparent deep involvement of physical factors suggests that enough basic information is already available to indicate the engineering measures neeJed to minimise the health risks, and that knowledge of the cellular processes may contribute little to the achievement of this objective. However, further in vestigations ofasbestos cancers may provide valuable clues for the elucidation of mechanisms of carcino genesis. SUMMARY Studies in recent years have indicated that physical factors are deeply involved in the etiology of lung diseases associated with exposure to asbestos dust, and these studies now offer explanations for some of the biological observations. The physical characteristics of the fibres which form the basis of these suggested explanations are fibre diameter, fibre length and fibre morphology, the central parameter being fibre diameter. In this paper, physical factors arc discussed as possible etiologica! mechanisms, and the apparent marked influence of fibre size in two very recent intrapleural inoculation experi ments is described. ACKNOWLEDGMENTS I am grateful to my colleagues Mr G. Berry, Dr J. C. Gilson, Dr F. D. Pooley and Dr J. C. Wagner for many valu able discussions and to Dr J. S. P. Jones, Mr A. Morgan, Mr R. E. G. Rendail and Dr M. F. Stanton for access to un- published material. - REFERENCES Burger, B. F. & En&clbrecht. F. 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(1969) Structure and development of the asbestos body. American Journal of Pathology, 55,79-107 Timbrell, V. (1965) The inhalation of fibrous dusts. Annals ofthe New York Academy ofSciences, 132,255273 Timbrel!, V.'(1970) The inhalation of fibres. In: Shapiro, H. A., cd., Pneumoconiosis. Proceedings of the Inter national Conference, Johannesburg, 1969, Cape Town, Oxford University Press, pp. 3-9 Timbrell, V. (1972a) Inhalation and biological effects of asbestos. In: Mercer, T. T.,-Morrow, P. E. & Stober, W., cds.. Assessment of Airborne Particles. Proceed ings ofthe Third Rochester International Conference on Environmental Toxicity, Rochester, 1970, Springfield, Illinois, pp. 429-445 Timbrell, V. (1972b) Alignment of amphibole asbestos fibres by magnetic fields. Microscope, 20, 365-368 Timbrell. V., Griffiths, D. M. & Poolcy, F. D. (1971) Possible biological importance of fibre diameters of South African amphiboles. Nature (London), 232, 55-56 Timbrell, V. &. Holmes, S. (1970) Suggestions on criteria for sampling asbestos dust. In: Shapiro, H. A., cd.. Pneumoconiosis. Proceedings of the International Con ference, Johannesburg, 1969, Cape Town, Oxford Uni versity Press, pp. 610-612 Timbrell. Y,, Pooley. F. & Wagner, J. C. (1970) Charac teristics of respirable asbestos fibres. In: Shapiro, H. A., ed.. Pneumoconiosis. Proceedings of the Inter national Conference, Johannesburg, 1969, Cape Town, Oxford University Press, pp. 120-125 ASARCO ELP 0003633 PHYSICAL FACTORS AS ETIOLOGICAL MECHANISMS 303 Timbrcll, V. & Rcndall, R. H. G. (1971/72) Preparation of the U1CC standard reference samples of asbestos. Powder Technology, 5,219-til Timbrcll, V. & Skidmore, J. W. (1968) Significance of fibre length in experimental asbestosis. In: Holstein & Anspach. eds., Internationale Konferenz uber die btologhehen Wirkungen dcs Asbestes, Dresden, 19SS, pp. 52-56 Vorwaid, A. J., Ourkan, T. M. & Pratt, P. C. (1951) Ex perimental studies of asbestosis. Archives ofIndustrial Hygiene, 3, 1-43 Wagner,J. C. (1963) Asbestosis in experimental animals. British Journal ofIndustrial Medicine, 20, 1-12 Wagner, 3. C. (1972) The significance of asbestos in tissue. In: Recent Results in Cancer Research, 39, Grundmann, E. A Tulinus, H.. eds.. Current Problems in the Epidemiology of Cancer and Lymphomas. New York, Springer Vcrlag, p. 37 Wagner, J. C.. Berry. G. & Timbrcll. V. (1970) Mesothe liomas in rats following the intra-pleura! inoculation of . asbestos. In: Shapiro. II. A., ed., Pncutnnconinus. Proceedings of the Internationa! Conference. Johanna.burg, 1969, Cape Town, Oxford University Press pp. 216-219 Wagner, J. C.. Gilson. J. C., Berry, G. & Timbrcll. V. (1971) Epidemiology of asbestos cancers. British Medical Bulletin, 21, 71-76 Wagner, J. C.. SIcggs. C. A. & Marchand. P. (Nut) Diffuse pleural mesothelioma and asbestos exposure m the North Western Cape Province. Brithh Journal of Industrial Medicine, 17, 260-271 ASARCO ELP 0003634