Document ZKvgORn20Qv7wJ7w3zqnJkyZ

BRITISH OCCUPATIONAL HYCUHt SOCIETY COMMITTEE OH HYCIEHE STANDARDS 4-7 Izm REPORT Of TOE SUB-COMMITTEE ON ASBESTOS OH A STUDY OF THE HEALTH EXPERIENCE IN TVO UK ASBESTOS FACTORIES l Introduction Tha UK hygiene standard for chrysotile asbestos adopted for use with the Asbestos Regulations, 1969, woe based on a report published by the British Occupational Hygiene Society in 1968, describing a study on a cohort of toen in an esbeetoa textile factory (Factory A), first employed since 1st January 1933 and still employed on 30th June 1966, with at least 10 years' service in asbestos areas. The recomended standard was in the fora of e total integrated exposure (dust concentration in fibres per millilitre of air multiplied by the time of exposure in years) considered to carry a one per cent risk of a worker exhibiting the earliest signs of the possible effect of asbestos exposure. The figure arrived at was 100 fibrt-years/nl, giving an exposure of 2 fibre-years/ml over a working life of 50 years, and it la this limiting concentration which has been quoted in the literature of many countries. It is unusual, of course, for any individual to be exposed for so long a period in modern industrial practice. This 1968 standard has been the subject of a number of criticisms over the years, the main ones being the following:-* a) The cohort was biassed in that persons fulfilling the cohort definition, i.e. at least 10 years' service since 1933, were not included if their exposure had ceased prior to 30th June 1966 as a result of leaving asbestos areas or death. ` b) Similarly, persons reaching 10 years' service after 30th June 1966, even by as lictls as one month, were also excluded. c) The earliest sign of adverse effect was taken to be the presence of persistent basal rales, but this is not always so. There are other early signs which might well precede rales in certain cases. d) The data had been collected from one esbestos textile fectory only. ) During the period covered by the study, some crocldollte had been used in the fectory, exposure to which may have influenced the findings. Asbestos-associated cancer risks had not been considered. Cont'd.... Since 1968, a good deal of extra information has bccor* available on the adverse effects of exposure to asbestos. Diagnostic MthoJs have improved, in particular the introduction of lung (unction measurements end the adoption of an agreed classification for the assessment of asbestos workers X-ray fiLms (the ILO U/C I9?i Classifleacion). In addition, considerably more experience has been gained in dust sampling and analysis for the maaauramont of exposure end some data have become available, from a second factory (Factory B). In this factory also, however, while the flbrt used was principally chrysotile, some crocidoLite-besed yarn was processed at one stage during Che study. . The first attempt to up-date the 1968 data was made by a reconstituted Sub-Coanlttee between 1974 end 1976, using a group from Factory A extended to include men who Had completed 10 years' servlet between 30th June 1966 and Jlsc December 1972, Hen who had left the factory between those dates were also included. (j**\' The work of the B0HS Sub-Committee was to some degree overtaken by events. An Advisory Committee on Asbestos was sot up by the Health 6 Safety Commission in 1976, and a Medical Working Croup of this Committee was given the task of reviewing ell the available Information on asbestos and health end making appropriate recommendations,^Although the B0HS Sub-Committee's study was still at an early stage, it provided for the Medical Working Croup an inceria statement in April 1978, outlining the progress which had been siade up co that time and this was no doubt taken into account by the Kedical Working Croup who* submitting its report to the Advisory Committee on Asbeseosts When the Advisory Committee on Asbestos* issued its Final Report in October 1979, it rocoonended that the term "hygiene standard" should be replaced by "control limit" for the assessment of exposure to asbestos dust, because, in its view, there is no aonsrent threshold below which exposure entails no risk to human health. _They state thee .'.'this new concept is intended co represent a realistic level of airborne concentration bt dusfcY'crbsel'y'assoc*iited~with'the relevant legililation, above ""which no ~p5rou should^be occupationally "eyposed". "*** 2. The Work of the Present Sub-Comslttee It was decided in May, 1977 to further reconstitute the 8GHS Sub-Committee on Asbestos to revisw the Information available up to 3!st December 1976. The following seeps were taken;- a) In addition to the cohort from Factory A (all man), a second * cohort was studied comprising men end women from Factory B. Some members of the second cohort had worjeed on the production of textiles and others on friction materials. ` H b) 3och cohorts were confined to people who had not been exposed to asbestos before 1st Jenuary 1951, because no dust exposure information was evaileble prior to that date, and who had at least 10 years accumulated exposure by 31se December 1976. Efforts were made to erece those who had left before that date so that they eould return for an X-ray and clinical examination, and In the case of those who had died, the cause of death was ascertained as far as possible. By doing so, it was hoped to meet some of rhe criticisms of the 1968 study. f3 c) Effort* were made to calculate ch* dust xpo*ur* for each individual In both cohorts* yr by yer, adjusted on an agreed basis to taka into account the changes In dust sampling and analysis which had taken pLace over the 23 year period of the study. d) At the outset, the objective of the Sub-Committee was to review the hygiene standarJ for chrysotilc asbestos dust* but the health experience over recent years has indicated that such a review would be of limited value if it did not, take some account of malignancy. Long-term* vide ranging prospective epidemiological studies of cohorts followed throughout their lives would be needed to support a hygiene stendsrd designed to reduce the incidence of malignancy attributed to asbestos exposure and* since malignancy has multifactorial causes, parameters other then asbestos exposure would need also to be recorded and considered. ^The outstanding other factor is cigsretce smoking. There is a wealth of epidemiological evidence chat smoking has a profound effect on the incidence of lung cancer in asbestos exposed workers, multiplying by many cimes^che^jrisjc run_by smokers in the general population.)/Tn the cohorts studied by the Sub-Committee detailed mortality records were not gene rally available. e) The Sub-Committee was therefore obliged to confine itself to a study of morbidity data with a view to obtaining a better understanding of the adverse effects of asbestos exposure during life. Tho Medical Data Medical information which was made available to the Sub-Committee is described in detail in Appendix 3. The mein features were:- e) Pairs of chest radiographs comprising the earliest pose - 1930 film end the latest pre - 1977 film for each person. These* along with "control" films were presented in turn to three physicians* who were asked to classify them independently in accordance with the ILO U/C (1971) International Classification of Radiographs of pneumoconioses, . b) All available information on medical examinations and lung function measurements carried out over the period of the study was provided by the medical officers of the respective a companies. At Factory A* workers had been given chese radiographs every three years from 1931 to 1967 end every ewo years thereafter. Lung function tests were'introduced in 1967 . end had been carried ouc every two years. At Factory B, chest radiographs had been carried ouc every two years from 1931 (unless `there was an indication to X-rey more often) and lung function tests had been carried out ovary two years slr.ee 1960. In order to arrive at an agreed interpretation of these date* the medical members of the Sub-Committee were asked to meet as a -forking Croup under the chairmanship of Dr. V.J. Smither "to determino the first medical Indication or Indicators of adverse effect of exposure to asbestos dust inhalation". A full report of their deliberations Is given in Appendix 1. Cont'd.... -A - Dust Concentration* a) Factory A The Casella Thermal Precipitator (T.P.) was used from 1931 to I960 end all monitoring was static* i. e. the sampling was conducted with a free-standing sampling instrument having the elr inlet at heed height end the instrument placed in e fixed position* nesr the operative* but without interfering with his movements in carrying out his task. The samples after "ashing" of the coverslips were counted at high magnification (600-1000 x) under dark ground reflected light using a 4 ms objective. Fibres were not specifically looked for and all particles judged to have an equivalent diameter of 0.5 urn end _ . above were included in the count. Between I960 end 1964* the Long^kunning Thermal Precipitator (LRTP) was used and again monitoring wet static. With this instrument it was possible to count fibres at about 300 x magnification end the results were expressed in fibres/ml using the convention that only fibres greeter than 3/um long end having an aspect ratio greater then 3 to l were counted. Later (circa 1970) when guidance notes on counting were published for outside bodies* e further convention limiting the count to fibres less then 3 .urn in diameter* was recommended. This is not considered to have affected the Factory A counts to any significant degree. From 1964 to 1974* static monitoring continued, but using a membrane filter sampling Instrument or a Royco Automatic Particle Counter (set to give similar numerical results). The membrane filter samples were counted at about 300 x magnification under phase'contrast conditions and the results were again expressed in fibres/ml. After 1974* membrane filters were used exclusively end a change In technique was introduced in thatcounting was restricted to the area within a British Standard Graticule (B.S. 3623 - 1963) mounted in the microscope eyepiece* instedd of covering whole siicroecope fields of view* This had the effect of increasing the number of fibres counted per unit ares of the elide by a factor of about 2 (Beckett at el 1976)* so che fibre concentrations calculated in the 1960's (used In the 1966 BGKS study) waul/ used to be multiplied by 2 to compere them with counts taken t. `day. In en attempt to understand che early figures end relate them to 1977 membrane filter counts, en investigation wee carried ouc running all instruments side by side. The samples obtained from the older Instruments were counted using identical techniques to those employed in the factory when those.instruments were in u*. Bearing In mind that consldarablt extrapolation was necessary because conditions end processes in the factory are to-day vary different*, the following approximate' relationships were * derlvsd;- 1977 Membrane Filter - 0.07 TP 1977 Membrane Filter - 2.2 LRTP 1977 Membrane Filter - 2 x 1964-74 Membrane Filter b) Factory B The Owens Jet lopinger wee In use in 1951 end continued co be used routinely until 1939. Up so 1956, che "ashed" coverslips. were counted at a magnification of 900 - 1000 x and no, * differentiation was made between fibres end particles. After this date* the count . Coat'd..** was limited to particles.of size greater chan l ..urn and from January 1958, fibres longer then 5 ,um were recorded es well. Becween 1958 end 1961, the Long Running Thermal Precipitator (LRTP) was used on an experimental basis for comparison with the Owens Jet. After trials, the membrane filter sampler caste into use in July 1961 and in June 1963, phase contrast was introduced, increasing the counts by a factor of about 1.6. The use of an eyepiece graticule in the fora of a 7 no Miller Square began in 1971 and this was replaced by a circular graticule, 6 on in diameter, in 1974 . In contrast with the experience of Factory A, no Increase in count was observed as compered with counting full microscope fields. In addition to membrane flLters, the Royeo Instrument was widely used from 1963 onwards. It is claimad that by careful calibration of the instrument against membranes, the Royeo was more reliable than microscope counting. c) The Effect of Personal Sampling Towards the end of the period covered by the study, from about 1974 onwards, personal sampling was being increasingly used for biological monitoring. As experience was gained, it became apparent that che results of personal samples did not generally relate numerically to the results of static samples in the same working area, even for samples taken simultaneously. The Sub-Committee therefore made n study of the information available in this field as detailed in Appendix Z end accepted the following conclusions (i) When identical sampling methods end simultaneous sampling is undertaken and Che distances becween the static and personal sampling points are reasonably small, most of the personal sampling results obtained in a given working location are higher than those obtained from static tests. (ii) The differences between the two types of results tend to be greater when the static sampling points are relatively remote from the dust emission points (e.g. when background static sampling is adopted). (iii) In certain eases results from static sampling may be higher than those from personal sampling, due to factors such as the positioning of the stacie sampling * point with respect to air extraction systems. The Croups Studied a) Factory A 300 men met the criteria (or inclusion in the study, but no medical records were available for S of them. The present study is therefore concerned with 295 men. It was recognised that the 10 year restriction might have resulted in bias if f -6 - t many persona with less than IQ years exposure had suffered sdverse medical effaces because of such exposure. A random SZ sample was therefore taken of all those who had comeneed work in the factory during the 26 yearc covered by the study. This gave 9$0 names, but when women and chose who had never worked with atbestos were excluded, 487 men were left, 15 of whom had worked for more than 10 years. 14 of these were in the study group, one having been inadvertently omitted in che original record search. Of the remaining 472, ?IX had lata than one year's exposure up to the end of 1976 and only about 6X had worked for between 5 and 10 years. Of 16 men who had left che factory for medical reasons, one had worked with asbestos for 3 years and the remaining IS had less than one year's exposure. The conclusion reached from this sampling exercise, . therefore, was that it seemed unlikaly that the criteria for admission to che study had excluded many who had already Suffered serious health effects arising from their exposure. b) Factory B Although at least 499 persons met the criteria for inclusion in the study, some had to be excluded because of incomplete medical and radiological data. In the end, only 351 persons for whom two serial chest radiographs were available were considered, comprising 323 men and 28 women. Information was provided for 3361 persons who had been exposed since January 1951, but for lees than 10 years. The pattern of exposure in this group was different from chat of Factory A, 22Z having been exposed for leee then one year, ee agalnet 71Z. end 72Z for 1 to 5 years, as against 22Z. Thera was also a considerable contrast becween the factories in the sub-groups who hsd left asbestos areas on medical advice. In Factory D, of 171 such persons, 106 (62Z) had only been exposed for from l to S years. One reason for this difference was that a number of people were advised to leave asbestos areas in the early 1960s on the appearance of early aigns of lung function abnormality. Theae persona were lost to the study, although some stay have developed these abnormalities as a result of their exposure. \ . was also possible thee some had completed more than 10 yea exposure at the time of withdrawal. The Radiological Data factory A and Factory 8 , The earliast (poat 1950) and latest (pre 1977) chbse radiographs for ths 295 man from Factory A and the 351 persons from Factory 8 were made available for the study. In addition, 160 chest radiographs from 160 persons employed at the factories, but with no known direct exposure to asbestos were also provided. These, together with the 1292 films from the exposed groups, were pooled and arranged in random order. ` Coot'd.... 7 The Clinictl Data a) factory A ' Available lung function and anthropometric measurements, information on smoking habits and records of whether or not chest sounds had been heard were taken from the modlcal records of the 293 men. There were no lung function data available for 56 of them but ell of these had attended for medical examination at least once during the study period. At least two sets of lung function measurements were available for 184 men in the group. b) factory 6 Similar information to the above was made available for the 351 persons in this group. No lung function data were aveilable for 38 of them, the recorda indicating cha praaence or abeanea of chest sounds at the lateet medical examination ware included for study, but earlier assessments of whether ehis condition had or had not occurred were not studied. 8. The Dust Exposure Data n) factory A An estimate of exposure to asbestos dust year by year was provided for each man. without knowledge of hla medical history. Using the conversion factors given in Paragraph 4 (a) these exposures were expressed as fibre-years per cubie centimetre, assuming ststie sampling by membrane filter and ^ 'counting to the 1977 standard method. No attempt was made .to . convert to personal sampling, because of the uncertainties attending such conversion (see Paragraph 4 (c) and Appendix 2). b) factory 8 Information was given on the time worked by the persona concerned at the various factory locations and measurements of dust concentrations at some of these location* were provided. There were, however, many gaps in the records and examination . showed thet cho information available might provide reasonably reliable estimates of cumulative dust exposure for only 16Z of the group. As will be seen, this put some restriction on the anelysis of the factory 8 data. ' 9. Analysis of the Data # a) Radiological Results (i) Parenchymal Abnormalities That distributions of film classifications by factory, reader end type of opacity for both the earlier and later films of the pairs revealed systematic differences between the three readers in their interpretation of the distinction between small rounded and small Irregular opacities. Less variability was found for "combined opacities" and this parameter was therefore used in the analysis. One deviation from the general 2 1 i i j o q a i :i v ....... 8- pattern was the relatively frequent assessment of abnormality by Reader 3 among the later film* from Factory 8. Overall, however, parenchymal abnormalities were noted in a higher proportion of the factory A group than for Factory 8. (il) Pleural Abnormalities . Taking an average of the observations of the three readers, 4.4x of the-films showed obliteration of the costophrenic angle, 2.II ahower pleural thickening and 0.4J pleural calcification. More abnormalities were seen on films from Factory A than from Factory 8, but it is important to note that the differences between readers were larger than the differences between factories. (ili) Technical Quality of Radiographs ` There were large differences between readers in their judgement of the quality of the radiographs, but all three were less satisfied with the films from Factory A than with chose from Factory 8. That the more frequent assessment of parenchymal change in the Faccory A films was not due to this cause alone waa confirmed by assessing separately a sub-group of film pairs of good quality. These variations in film quality did not affect the pattern of assessment of pleural abnormalities. (iv) Small Opacities and Smoking Habits All readers detected saal'. opacities more frequently in Factory A among smokers and ex-smokers chan among noiv* smokers (about III higher). Nevertheless, small opacities ware also detected on about 121 of the notvfmokara' films, indicating that the changes seen on the later films were not wholly attributable to smoking. No such* Information could be gleaned from the Faccory B films, becaus Readers t and 2 judged only a marginally higher propoi*`.on of combined opacities in - the later films than in the controls. It should be noted, however, that Reader 3 classified 18Z of these films into cecegory 0/1 or higher, compared with 6X of the controls. . b) Radiological Changes and Dust Exposure (l) Factory A In an effort to relate profusion of combined small opacities with cumulated dust exposure, four measures of exposure were examined for each individual. The first (E ) was the cumulative exposure from commencing work in asbestos areas up to the time of the earlier of the two films and the second (^) was the exposure incurred between films. The third measure (E ) was the cumulative exposure from commencement up to the later ^ film i.e. Ea Eb.. The fourt.h_ measure (Eu.) was an Cont'd.... 9 attempt to approximate the exposure up to the point whan cht observed change occurred and was taken to be E 1/2E^. Ho aaaoclotion could be found between radiological change and E^ or E^ separately. There was a weak association with E , but the atrongesc was with E. and it waa thla measure of exposure which was finally adopted. ,' (it) Factory B As referred to in Paragraph 5 (d) (11), the way in which the dust data had been collected foe Factory B, together with the number of change* which had been bade over the period of the atudy, precluded any attempt to estimate emulative duat exposures for Individuals. Reliable Information on time worked, however, over the period of the study, was available for 18? persons who had definitely worked previously in occupations with potential exposure to dust other than asbestos and for 128 persons with no such prior exposure. An assessment was made of changes in profusion of combined email opacities for both of these sub-groups, related to time in years spent working with asbestos. Reeders 1 end 3 recorded more radiological change between pairs of films for persons with longer periods of exposure to . asbestos, but Reader 2 did not show this trend. c) . Lung Function Chances and Oust Exposure (i) Factory A ' . ' Of the 295 men in the group, one set of lung function measurements was available for 239 and two sets or mors for 184 of them. An attempt was made to correlate the latest measurement recorded with the copulative duat exposure up to that time, using e linear multiple regression model to take account of age, height, weight and smoking habits. The lung function criteria considered were FEV., FVC, CTf and the ratio fEV./FVC, Only for FEV. and FVC did the dust exposure correlate significantly at the 6X level; neither with CTF nor with FZVj/FVC was any correlation observed. (it) Factory B Despite the difficulty associated with*the dust data, the information from Factory B was examined further in the hope that it might be possible to proceed to e case-control study. Unfortunately, this analysis had to be abandoned because the sub-sot of date suitable Cor detailed study was seriously biassed epideaiologically (see also 5 (e) (ii) above). The Criteria for Adverse Effect Further analyses were made in response to advice from the medical members of the sub-committee on what constitutes the earliest Cont'd,... " /C= medical indication of adverse effects of exposure to ecbestos dust .(Appendixl), Despite certain difficulties In applying 'thdse '* ' `recommendations, seven statistical criteria were defined. Six of them reflect approximately some of the suggestions from the Hedieal Working Croup. The scvtnth (rate of reduction in FEV^) was Included because earlier analyses indicated chat this measure*of Iuny fun,*p tnn ! Mrt closely related to thpJguit exposure data than the other indices of function tinder consideration. II. The Occurrence of Adverse Effects e) factory A t Of the 295 men In the group, the resultk from 183 of them met one or more of the seven criteria. The total number of occurrences was 274. The most frequently occurring effect was F (FEV /FVC < 0.70), being present In 111 men and in 52 of them it was aceoapsnled by at least one of the ocher categories of adverse effect. The distribution of men with i ' one, two and with three or more effects was 58, 26 and 16 * per cent respectively. *** factory 8 \ ~ 02 the 351 parsons in the group, the results from 163 (154 men, 9 women) met one or more of the seven criteria. The total number of occurrences was 257. As in Factory A, the , moat frequently occurring effect was f, being present In 83 members of the group. Because chare were fewer radiological changes recorded for Factory B, there waa a smaller number with effects A and B, but the distribution of porxons with one, two end with three or mate effects was not dissimilar from Factory A, being 66, 19 and IS per cent respectively. |2. Interpretation of the Data on Adverse F.ffects This was only possible for Factory A, for which cumulative dust exposures were available. The probability of the occurrence of an adverse effect before ehe correspond!** dust exposure had been aecmulated was estimated using the "1 iduct-Limit" (or "Life-Table") method. The probability/expoeute results were plotted on logistic/log scales end the patterns for adverse effects A, B, c, 0, E. F and G ere shown in Flgotes 4, 6, 7, 8, 9 end 10 of Appendix 3. The results ere summarised in Figure M in the fora of a plot of * estimated probabilities that at least one of thi seven adverse effects may occur by the exposure shown. When more than one effect * was present in tho same man, that oeeurrlng earliest was used, ft will be seen that five persons showed an adverse effect at axposures less than 25 fibre-years/ml. In two cates cha effece concerned was E (unusual race of change in Cas Transfer Factor) with on* each for C (unusual rate'of change in FEV.), F (FEV./FVC < 0.70) . and C (chest sounds). At first sight, the graphs suggest that it might be appropriate to fit straight lines to the injiviJuai observations, thus postulating a logistic model for the various adverse effecc/exposure relationships, but the deviations below about 100 fibre-years/ml (50 fibra-years/mi by 1968 standards) preclude this. It would therefor* be unwise to use sueh fitted lines to make predictions where exposures have been below this level. t,r - * i, u Conclusions (1) As far aa Factory A is concarneO, it appear* unlikely that the criteria for admission to the study (not less than 10 year* exposure) excluded Aany who had auffered serious health effects arising from chair exposure. (il) The data from factory 8 must be ^regarded1 as biassed for epidemiological purposes beeause the medical care policy vaa to remove persons from exposure whenever examination indicated eerly slgne of lung function abnormality. These persons were not included in the etudy of Factory ft, yet some of them may have developed the signe as a result of their exposure to asbestos. (ill) The radiological results for Factory A showed an association between the occurrence of perenchymal changes and estimates of cumulative exposure to asbestos dust up to the approximate point in doe at which the changee were likely eo have occurred. Although it was not possible to estimate cumulative dust exposures for most persons from Factory 8, there wee some evidence that longer periods of exposure were associated with higher chances of developing small opacities on Chest radiographs. (iv) Small opacities were detected more frequently among smokers and ex-smokers, but the perenchymal changes obsarved in tha group could not be attributed wholly to smoking habits. (v) Overall, the threo readers judged 5.JZ of films from 'Factory A and 1.6% of thoso from Factory & as showing obliteration of the costophrenic angle, resulte which were not significantly dlffsrent from observations on tho 160 radiographs of persons who had not been exposed to asbestos. The tame wee true of other pleural abnormalities which were recorded even less frequently. . (vi) Standardised levels of FtV. and FVC among men who had accumulated relatival/ exposures in Factory A ware lower then the levels among those who had received only low exposures and the correlations with dust exposure were statistically significant at the 6% level. Any euch . apparent correlations with Gas Trans far Factor and with V&V./fVC could easily have arisen by chance In view of the residual variability in the data. . (ell) The results Suggest that for cumulative exposures, based on _ static tests, (see Paragraph 4 (c)> up to about 25 fibre-years/ml, * ' tiie probability that any one of the seven defined events ' occurs is lass than 2z. For axposures lesa than 50 fibre-years/ol, the estimated probability is less than 71 end for exposures up to 100 /ibre-yeare/al tho probability inersases to i f 17 - 202. tc must be recognised, however, that the \ i ftetistlcal definitions of the events concerned ere based 1 | broadly on guidelines suggested by the Sub-Con&lttee's (i Medical Advisers regarding "the earliest index that the . 1/ chest of an asbestos worker was adversely effected from \j whatever cause'*} they do not constitute clinieel diagnoses *t * ]/ of asbestos-related disease. * Cont'd.... W R6FEREHCES %_____ ___ ,, mi v-nrysociie-Asoescos Oust. Annals of Occupational Hygiene, 11, 47-69. 2. Berry, G., Gilson, J.C., Holmes, Lewinsohn, H.C, and Bosch S.A., (1979). Asbestos: A Study of Dose-Response Relationship in an Asbestos Textile Factory, British Journal of Industrie! Medicine, 36*. 9S-U2. _ 3. Peto, J. (197S). The Hygiene Standard for Chrysotile Asbestos. The Lancet, 464-439. 4. Peto, J., Doll, R., Howard, S.V., JCinlen, L.J.* end Lewinsohn, H.C. (1977). A Mortality Study among Workers in an English Asbestos Factory. British Journal of Industrial Medicine, 34, 169-173, 5. Health 4 Safety Commission. Final Report of the Advisory Committee on Asbestos October, 1979 K.M.S.O. 6. Beckett, S.T., Hey, ft.JC., Hirst, R., Hunt, R.D., Jervis, J.L., and Rickards, A.L., (1976). A Comparison of Airborne Asbestos Fibre Couneing with and without an Eyepleee Graticule. Annals of Occupational Hygiene, 191[ 69-76, V i f. <. IX