Document 6BLwzeKMxMB6Qky8R1b3gba23

cf- 5r*s7>. /2 . - zs? //*?s8 6aj s/s/ir THE OCCURRENCE OF PULMONARY FIBROS PULMONARY AFFECTIONS IN ASBESTOS WORKERS (Conclude!) E. R. A. Meriwether, M.D. E. Id. Medical Inspector cf Factoriu MORBED A.Vi.TOMT Professor M. J. Stewart of Leeds has kindly supplied me with a detailed description (dated March 28, 1929) of postmortem appearances of several cases. From this account the follow ing points are extracted: The irate morbid anatomy of this leiion, aa aean in parsons who have been at work for many years in the duaty atmosphere of aa asbestos fa tory, consists in a widespread pulmonary fibrosis affecting especially the basal region of both lungs. There are ertenai-.densely fibrous adhesions through out tbe pleural sacs, which may, indeed, be come completely obliterated. In particular, the bases of the lungs are firmly adherent to the diaphragm, and betwsan these two structures there is often e thick, homo geneous layer of fibrous tissue, similar in appearance and consistence to yellow fibroeanilsge. As a result of these two proc esses--pulmonary fibrosis and pleural sac obliteration--bronchiectasis develops in the midst of the more grossly diseased tissue and may go on to the formation of multiple abeeeasea with smooth walls. Tbe distribution of the fibroaia, apart from tbe extensive basal lesions, ia rather irregular, the peripheral, subpleural regions of the lungs being more affected than the central areas. In one case in which the pa tient had been away from work for four years (following nineteen years in the mill), the artas of dense fibrosis were extraordinarily sharply circumscribed. Tbe nodular char acter of the lesion which is so striking a feature of silicosis in its earlier stages is not met with in this disease. Tbe totally fibrosed areas of lung show grayish-black mottling, owing to immobili sation of carbon. This tissue is execs.'. vely dense and completely airless; but the:* ia no evidence vl calcification. Other por tions of lung show varying grades of the same process, and even the least affected parts are definitely tougher than normal. In none of the cases was there any evidence of an aetive tuberculoui lesion. Histology The essential lesion is a chronic interstitial fibrosis of tbe lung. In the earlier stages, there ia fibroblastic proliferation ia the alveolar walla, which become increasingly thickened in consequence; and there ia catarrhal desquamation of the alveolar epithe lium. At thia stage of tbe diseasepecu liar golden-yellow "asbestosis bodies" are found in varying, usually con siderable numbers, both in the alveolar spaces and in the thickness of their walls. Detached portions of bodies may often be seen engulfed by alveolar phagocytes, or a large unbroken body may be partially surrounded by these cells. In the more advanced stages of the disease, the fibrosis of the lung ia complete. Alveoli can no longer be made out; but the asbestosis bodies still remain embedded in tbe fibrous tissue. Varying quantities of carbon are also present, chiefly in little masses as though it had been contained with in phagocytes. Chronio bronchitic Voi. t: Ho. I THE JOURNAL OF INDUSTRIAL HYGIENE chaser* are prwvnt ;uid all prudes of bronchiectasis may be aeon, in cases of longstanding, from simply bronchial and bronchiolar dilatation, with walls si ill more or less intact, to large spaces filled with pus in which the original bronchial walls have become converted more or leas completely into fibrous and granulation tissue. Many of the smaller arteries show obliterative end arteritis. Reversionary metamorphosis of alveolar epithelium is frequent, and such alveoli ore often filled with albuminous fluid, in which clumps of "bodies" may be found. Asbestos Dust i*r the Lunus Dr. W. E. Cooke has supplied (under date of June 25, 1929) details of his researches on asbestos dust in the lungs, and on tho constitution of the curious bodies, which have been freely used in the following notes:-- Sections of lung and the results of digestion of the lung with trypsin chow an enormous amount of fine black granular dust, much of which is carbonaceous. In addition, there are two striking features. The first is the almost complete absence of the fine translucent spicules of fiber which make up a great proportion of asbestos dust in factories. The second feature is the presence of large fragments vary ing in length from 10 to 360 microns. They are found in fibrotic and necrotic areas, singly and in groups. The particles are so large--masses of them are seen in some sections--that they must have occluded small bronchi and resulted in fibrosis of the surrounding area. Comparing these large particles in the lung with those found in asbestos dust, cW resemblance in sires, shapes, and colors is apparent. There are the ATn black, blue, brown, and trans lucent fragments. In fact it is easy to take such a single particle from the lung and immediately find its brother in a slide made from the dust. Curious Bodies In addition to the fine granular dust and larger fragments of asbestos, sec tions of the lungs show curious bodies. They are found in alveoli, bronchioles, fibrous and necrotic areas, and in phagocytes in sections of both lungs. If a portion of lung is teased and extracted with water, or digested-with trypsin, or, as Professor Stewart pointed out (192S), if a smear is made from the cut surface of the fresh lung, these bodies are seen in myriads. The larger bodies measure from 20 to 100 microns or more in length, and are of a golden-brown color. They may have single clubbed ends or may appear as elongated dumb-bells; some are filamentous, while others suggest a series of disks. Single coccal and spore-like forms, and aggregations of these, and strepto coccal forms are not uncommon; the color varies in the smaller types from a very pale yellow to s yellowish-brown. The bodies do not stain with any of the aniline dyes, but in chrysotile workers, they give the Prussian blue reaction for iron in varying degrees of intensity. These curious bodies have been found in every autopsy in pulmonary asbes tosis. It would be unprofitable to retrace all the steps which led down many by-lanes during the couree of the work, but I will mention & few per tinent details of interest. Professor Stewart suggested to me J. 1. H. J4M. IttO WV, ' 5: i'r*~:''\ C9b!l3001S 55T/. PULMONARY FIBROSIS IN ASBESTOS WORKERS 241 that a better method than simple extraction of the lungs with water or saline would be to digest portions with trypsin. During this process it was found that if the specific gravity was kept at about 1,070, the black dust and larger fragments of asbestos, as well as the partially digested lung tissue, sank to the bottom of the tube. By decanting the appareutly clear supernatant liquid, and by centrifugaliiing, neutralising, and washing the deposit, the curious bodies could be obtained in a pure state. This was done, and sufficient material for Xray examination was obtained. The bodies were attached to a hair with gum and subjected to a seven-hour exposure by Professor Bragg's method. If the bodies were mineral, the X-ray film would have shown a definite translatable atomic pattern. The film*, however, did not do so, and we were then able to exclude the sugges tion that the bodies were altered asbestos fiber. The result definitely pointed to the greater proportion of their composition being of nonmineral origin. The bulk of the curious bodies is soluble in strong acids and alkalies, and if the solution is observed under dork ground illumination, their bases are seen as extremely fine spicules some of which by transmitted light would probably be invisible. Under a dis secting microscope it is possible par tially to fracture the larger bodies and to show a central fine core. The greater portion of asbestos dust consists of slender translucent fibers. In sections and extracts of the lungs there is a remarkable paucity of these fine spicules. The end-results of diges tion show the fine granular dust and the large black, blue, and brown par ticles and what appear to be pieces of quartz. Relatively few fine spicules are found; but curious bodies of all descriptions ore present in enormous numbers. All these facts lead us to imagine the bodies >o consist of a central nucleus of asbestos spicules, upon which colloidal aggregate of blood proteins plus, possibly, soluble frac tions of asbestos and, in the case of chrysotile workers, an iron salt have been adsorbed and molded by currents in the bronchi and the alveoli. The method of formation would appear to be as follows: The fine spicules of asbestos cause, by mechan ical action on the bronchioles and alveoli, either minute extravasations of whole blood, or serous exudates which envelop them. Solution of any soluble fraction of asbestos takes place. The total amount of asbestos that is soluble must be extremely small, as is proved by the X-ray pattern of the curious bodies, but in the case of chrysotile workers some solution is suggested by the free iron reaction. We must remember, how ever, that the Prussian blue reaction may be due to the iron of the hemo globin. Any surface in contact with a colloidal solution may act as an adsorbent, and in the present case the fine spicules must be considered to do bo. Interaction between the soluble fraction of chrysotile and plasma proteins takes place, syneresis occurs, and, with the loss of water, the adsorp tion is rendered irreversible. The adsorbent is permanently easheathed with stable colloidal aggregates which become molded into the familiar 242 THE JOURNAL OF INDUSTRIAL HYGIENE shapes by alveolar and bronchial cur* rents. Support for this idea is found in the f&ct that micro-organisms adsorb col loidal material in the presence of blood serum and colloidal asbestos. Staphy lococci so treated appear as large round ycllowiah-brown disks; in the process their property of staining with aniline dyes is lost. The organisms coalesce and form masses; and I think it probable that some of the coccol and spore forms are similar organisms. Finally, are the curious bodies diagnostic of pulmonary asbestosis? Asbestos is unique among minerals in being fibrous; and its dust, generated during manufacturing processes, is also unique. As can be imagined from the formation of the curious bodies, there is no reason why, given any silicosis, a fine spicule of mineral should not have colloidal matter deposited around it and become molded into a curious body. But as no other min eral dust is fibrous, this occurrence must be so rare as to be negligible from a diagnostic point of view. The con ditions which, apparently, must obtain for the formation of curious bodies ore the presence of plasma proteins and fine spicules soluble only with diffi culty. These conditions are ideally found in asbestos workers; for this reason I believe that curious bodies, if found in any numbers, are pathog nomonic of pulmonary asbestosis.-- W. E. Cooke and also Roodhouae Gloyne (20) have independently dem onstrated the presence of a mineral core, evidently derived from asbestos fibers, in the curious bodies. The bodies have been found (Stewart and Haddow (21)) in the in tmthoracic lymphatic glands; in material obtained by lung puncture during life, first sug CO gested by S. A. Henry; in the sputa; --I and in smear preparations from the CD cut surface of the lung. In smear CD cn preparations they can be readily demonstrated. JD The importance of these findings cn lies in the fact that the bodies have cn never been found, as yet, in any other human affection. That similar bodies may be found to occur in the lungs of workers exposed to other dusts, is quite possible, although they have not kasn frtl m>l i nr\w warn fn#v present, according to Simoon (6), in the fibrosed lungs of a hematite miner. They appear within a comparatively short period of time after exposure to asbestos dust; at the moment, their presence in the sputa, in the absence of clinical or radiologic evidence of pul monary fibrosis, cannot be taken as in dicating anything more than previous inhalation of asbestos dust. The pres ence of the bodies, however, in the sputa of persons with signs of a diffuse fibrosis, or their presence in numbers on postmortem examination of a fibrosed lung, has evident implications. Professor Stewart, who is continuing his work on this subject, has devised the following method of examining sputum for asbestosis bodies; Hall an ounce or so of sputum i added to in equal quantity of undiluted intiformin. This is gently agitated until the aputum ia completely dissolved, ifter which it ii diluted with 2 or 3 ounces of wster end allowed to stand in a large test tube for three or four hours. The bulk of the super natant fluid having then been decanted, the remainder is centrifuged at a moderate speed for ten to fifteen minutes. The whole of the supernatant fluid is now poured off, and the depoait transferred to an ilbumin- * t. " i.i V: t. J. I. H. Jam*. MO fr ixed ali loop or a hot p burner, water, ||JQ( toaia b< low por firmed Asaru bere, p film, worker one or the fvlr neceea; ware, tione; QjkiioD The eh!ori< demon given ' A Ou ined, to be in gr these 8(011 dust mini cont. men per to b due and cent dise T part foiu nat: and (24 Vol. No. PULMONARY FIBROSIS IN ASBESTOS WORKERS 243 lied slide, by mesne of either s platinum loop or s pipette. After thorough drying on s hot piste sad Sasl fixation orer s bunsea burner, the film is very gently wished in water, dried, snd mounted in Canada balssm. After s little experience the asbes (39.9 per cent.) and of sandstone workers (58.1 per cent.). The precise significance of the lower figure found in this inquiry is not easy to determine. At first sight it seems tosif bodies are readily picked up with the to indicate that asbestos dust is less low power, sad their true nature is then con potent as a cause of pulmonary fibrosis firmed with the 4-iaeh or oil immersion lens. As a rule they are present in eery small num than are the dusts containing free -y?- bers,, perhaps only one or two in a whole silica. Other factors, however--such film. In the ease of some of the older as the measure of exhaust ventilation, workers, however, numerous bodies, up to or other means of reducing the concen one or two per field in certain portions of tration of dust, in the several indus the film, have been found. It is obviously tries; differences in the average length necessary to cleanse thoroughly all glass ware, etc., used in making these examina of employment of the comparable 'M tions; otherwise there is a risk of contami- groups in the samples examined in the aslion of subsequent specimens. respective inquiries; and the relative The films eaa bo treated with hydro chloric acid and ferroeyanide of potash to demonstrate the Prussian blue reaction given by the bodies. numbers employed in the more dusty and the leas dusty processes--will affect the crude incidence rates ex pressed by these percentages. ASBZSTOBXB JlS OCCUPATIONAL From comparison of the amounts of Hazard dust evolved--determined visually and Oat of & total of 374 workers exam by means of dust counts--in dun: / ined, 105, or 28.1 per cent., were found asbestos processes, uncontrolled by to be affected with fibrosis of the lungs, local exhaust ventilation, with obvi in greater or less degree. But when ously defective exhaust ventilation, these figures are corrected bp the exclu and with comparatively good exhaust sion of cases in which previous work in ventilation, there can be no doubt that dusty occupations (e.g., quarrying, coal this preventive measure, which has W- mining) may have been the prime, or & been applied in some degree for more contributory, factor in the develop than seventeen years, has been a posi ment of the fibrosis, ninety-five, or 26.2 tive factor in minimizing the produc per cent., of 363 workers were found tion of fibrosis--probably in the direc to be affected with pulmonary fibrosis tion of lengthening the period before due to the inhalation of asbestos dust, the fibrosis becomes fully developed. and a further twenty-one, or 5.8 per cent., showed precursive signs of this disease. Consideration given to the distribu tion of ttic workers examined both according to length of employment, and according to age, together with the This percentage may well be com incidence rates of fibrosis in each cose, pared with the corresponding figures indicates the outstanding importance found by Middleton (22) in his exami of length of employment (and hence nation of metal grinders (46.9 per cent.) length of exposure to dust), and the and by Sutherland and Bryson (23) negligible effects of age, on the produc (24) in their examinations of potters tion of fibrosis. Thus age groups 30 to Tat II No. tiff t ! !. CO a- --I CD CD co - i.-i i>. I * kf ilk V.O CO cr \i ki to5 *r *., f r : -i ..v* ! P-A i !.j *.w '* 1 ' : ,' i ;; j iHrsf -*f * i*-'*< 1 t *W -m i ; . fc - fe: 9 ir f... . ft*** !f 1 A-r^- ` I !***, .: It i, i Is... ,5 ij "r - i* : N'' Ibtr.-. 1i i :!S'V; ' i .*A> - % THE JOURNAL OF INDUSTRIAL HYGIENE 39 and 50 to 59 have incidence rates of 30 per cent, and 37.9 per cent., respec tively--not a wide difference, since the groups include workers in various processes exposed to different concen trations of dust. Moreover, the aver age length of employment in these two groups, excluding the cases of fibrosis, fibrosis in age group 20 to 29, has been shortened by increased susceptibility at ages under 20. As, however, the average age of this group of eases of fibrosis is 26.7 years, and also because the establishment of fibrosis does not necessitate immediate retirement from work, this is largely discounted. TABLE 5.--DISTRIBUTION OF WORKERS EXAMINED WHO HAD BEEN EMPLOYED FOR FIVE YEARS OR OVER, TOGETHER WITH CASES OF FIBROSIS, ACCORDING TO PROCESS IN WHICH WORKER WAS LONGEST EMPLOYED noexu mo. n4JCMXO C4BZS or rtsxosxs No. Per Cent, of Group arxnaoa lknotx or ttmortourr in tu. Group Less Cases of Fibrosis Cases of Fibrosis 1. Crashing, opening, disintegrating, and mixing............ 2. Hording.................................... 3. Spinning, twisting, doubling, 4. Mattress making.................... 5. Wearing and associated pros- euei..................................... a. Doth wearing................... 28 b. Band wearing.................... 21 e. Cloth and hand and uo- classified wearers............ 14 d. Warpers, beamers, loom tuners, charge hands, and others associated with wearing................. 25 6. Miscellaneous processes and remaining unclassified workers................................ 21 0 3B 16 42.9 41.0 8.9 10.9 9.1 13.2 87 12 13.8 10.1 18.7 15 7 46.7 7.3 13.0 88 40 45.5 10.0 12.7 21 75.0 9.5 H.l 4 19.0 10.9 13.5 4 28.6 7.2 9.5 11 44.0 11.2 10.8 24 11 45.8 8.3 13.8 Total.......................................... 274 95 34.7 9.6 13.5 The effects of length of employ ment ore such that after five years' exposure, the incidence rate of fibrosis mounts rapidly, and after ten years increases almost in geometric progres sion. Table 5 shows the distribution of 274 workers examined, and ot those with fibrosis, according to the process ST00-6li.96 7. r c c c c r. v t ( o b (. t r. ir h u 1. tl S ir P'. Cl in le fe P: tl cc fil Vo No been lility the ea of ause < not from 8.7 3.0 2.7 38 5.5 oyire' jsis are esioa 090 ess . H. 1930 l PULMONARY FIBROSIS EN ASBESTOS WORKERS 245 in which they were longest employed. This was & compromise necessitated by the common customs in the indus try of having several different processes going on in the same room, and of workers transferring from one process to another. Since the intention was to obtain some measure of the relative incidence of fibrosis in workers in the various processes enumerated, and since no case of fibrosis dearly due to asbestos dust was found among eightynine workers examined, who had been employed for less than five years, in clusion of this group of workers would only have introduced a varying source of error. The processes differ, some very materially, in the amount of dust which they cause, and consequently in the amount inhaled by the operators (30). The outstanding point brought out by Table 5 is the relatively low incidence rate of fibrosis in "spinnere" (group 3), as compared with each of the other groups. They ore corrobo rated by counts of the dust partides in samples of air, a number of which have been taken and are discussed under Dust Risk. There is also some indication in these figures that not only are "spinners" leu likely to develop fibrosis, but when they do, it takes longer to develop. Study of the radiograms suggests that in such cases (t.e., in the leu dusty processes) the resulting fibrosis is, for a considerable period, much more linear in its radiologic features, and shows leu mottling. The history and clinical features in these cases, and the com parative dust counts, all contribute to the view that with comparatively low concentrations of dust, the resulting fibroeis is longer in developing and remains longer in the milder, and radiologically linear stage. This view is evidently of consider able practical importance since it sug gests that in such coses, the rate of accumulation of dust in the lung has not greatly exceeded the rate of elim ination, the changes still being centered is the main lymphatic system. If this hypothesis is accurate, it should follow that in order to prevent the full development of the disease, among asbestos workers, within an average working lifetime, it is neces sary to reduce the concentration of dust in the air of the workrooms to a figure somewhat below that pertaining to spinning at the present time. Fortunately, also, the spinning group is the largest individual group in this section of the industry--e.g., out of over 400 workers employed by one firm, this group accounts for more than one-third. Although all workers examined who have been included as spinners have been engaged in spinning, or in simi lar processes for convenience termed "spinning," for a longer time than on any other process, only a few have been employed solely on spinning, and in workrooms where no other, and more dusty processes, were being carried on. These two factors, prior work in more dusty processes and present work in proximity to more dusty proc esses---especially the latter--have had, it is believed, some effect in raising the incidence rate of fibrosis for this group. Dust Risk In an effort to obtain additional data on the influence of concentration of dust in workrooms, a number of sam ples of air were taken and the dust con- Val. 13 No. % OO --I o o C'' UD cr> CO > L-; $ fcJ:. LVwSr-- t3 te> :! 246 THE JOURNAL OF INDUSTRIAL HYGIENE TABLE #.--SUMMARY OF DETERMINATION OF DOST CONTENT OP ATMOSPHERE OF WORKROOMS AND OTHER PLACES I 1m SUtfU KO. PBOClSS AND SOUYCX OP BAHTLX LOCJX EXSA0ST yxntxla- TlOtf tO PJtOCfcaB Oh NOT OTKX2 PROCsuu IN BJJktB soon NO. OP PXJtCXNT- ra AOS TICtXB AND PCI c.c. OKDBA 1. 1/2 2. 1/5 3. 1/3 4. 1/7 5. i/a a. vs 7. 1/1 8. 2/U 8. 2/1 10. 2/14 11. 2/10 12. 2/S 13. 2/7 14. 2/9 15. 3/1 18. 3/4 17. 3/3 18. 3/5 18. 3/2 20. 4/1 21. 4/1 22. 4/3 23. 4/4 24. 4/5 25. 4/54 26. 4/6 27. 4/7 28. 4/8 29. 4/10 30. 4/11 31. 4/12 32. 4/13 33. 6/1 34. 6/2 33. 6/3 36. 6/9 37. 6/6 Carding; passageway between cards Spinning Cloth wearing, dry Opening Sieving; shoveling fiber on to sieve Plaiting 065ee OSee Cloth weaving Spinning Mattress malting; filling Mattreaa making; beating, dvnp Mattreaa making; beating, dry Mattreaa making; alter filling 2 mattreaaee Band weaving, wet Opening Carding; between eards Carding; feeding r.ard Office Mattreaa making; filling Maltreat making; filling banket Emptying nettling chamber Same Inaulating sections; leveling fiber Carding; side of dirty card Carding; end of dean card Carding; feeding card Cloth weaving, very slightly damp Cloth weaving, damp Braiding Spinning Office Band weaving, dry Cloth weaving, dry Band weaving, dry Opcaing Carding; feeding card Yes No No res No No .. .. No No Yea Yea Yea Yes No Yes Yes Yes No No No No No Yes Yes Yes No No No No No Yes Yes Yes Yes Yea 2,139 81.3 95.1 Yes 1,456 Yea 1,756 Yea 2,384 Yea 2,829 95.9 84.1 80.9 3.0 100.0 96.7 93.7 97.6 So 1,687 99.0 99.0 . 901 98.8 100.0 ... 1,480 98.7 100.0 Yas 1,768 88.9 98.6 Yea 1.073 96.2 100.0 No 985 92.5 98.1 No 970 85.9 95.8 No 1,629 86.9 96.6 No 1,204 96.6 99.4 No 808 96.6 100.0 No 2,313 77.7 96.6 Yea 1.528 82.5 97.4 Ye. 1,321 82.9 96.4 1,049 96.3 100.0 Yes 1,858 60.1 90.1 Yea 1,390 .... ..... Yes 3,401 Yes 4,711 Yes 1.S38 .... .... -- No 1,402 No 1,092 No 1,689 Vet 3,033 .... .... .... Yea 758 Yea 506 Yes 954 , . . 689 Yea 2,202 No 885 No 701 Yea M3 Yea 562 .... .... lh. Jvh. 109 C/3 --H CD CD cr> **UD CP> vo ><' rs, 'U." /.TiS? 39>7> c* - xa DU 5.1 '0.0 *.7 <a.7 >7. a 9.0 0.0 0.0 *8.8 o.o *a.i *5.8 0.8 *9.4 O0 0.8 *7.4 *8.4 0.0 *0.1 .*U. r . .'4Miaj n1 1 : .< i * .->'l -t-1 .ij l. L R. m. t PULMONARY FIBROSIS IN ASBESTOS WORKERS 247 TABLE 6.--CmUtnutd bartle MO. process and source or SAxru LOCAL EXHAUST VENTILA TION TO OTHER PROC ESSES IN BANE process OR NOT ROOK no. or PERCENT PERCENT PAR AGE AGE TICLES 2M AND 7.581 AND PER C.C. UNDER UNDER 38. 6/4 39. 8/5 40. 8/7 41. 8/8 43. 7/2 42.7/1 44. 7/3 a 7/4 48. .7/5 47.7/6 48. 7/7 40. 8/2 50. 8/3 SL 8/5 Cuding; center of room Carding; grinding rollers Spinning Office Cloth weaving, dry Opening; aboveling fiber into box Opening Band weaving, wet Spifl&iflf Carding; between cards Office Cloth weaving, dry Band weaving, dry Csrdinc Yee Yes No Yes Yes Ye* No No Yes No No Yes Yee 620 Ye 540 Yes 620 ... 666 Yet 4,688 Yes 4,880 ...... .... ...... .... ...... .... .... ........ Yes 6,324 Yss 5,353 Yes 6,044 Yes 4,607 . . 3,413 .... .... ..... .... .... ..... Yes 1,2S7 88.6 94.9 Yee 1,233 78.2 93.7 Yes 1,195 67.2 94.9 tent was determined by means at Owens' jet apparatus. Here also the undetermined effect of dust from neighboring processes must be noted. Nevertheless some information of prac tical value was obtained. Reference to Table 6 shows that the counts ranged from 506 to 6,324 particles per cubic centimeter in the air of workrooms, and from 6G6 to 3,413 per cubic centimeter in the air of offices associated with the factories. In some cases there was a smaller count in a workroom than in an office of the same factory; this was due either to there being much fewer smoke particles in the air of the workroom, or to adher ence of the smoke particlee to the inorganic dust in the workroom. The character of the dust in the two cases is, however, entirely different. In the air of the office almost all the particles ore black, or brownish, and amorphous, evidently smoke; in the air of the workroom, the majority of the particles are, just os clearly, crystalline particles of asbestos, to gether with, frequently, some cotton fibers and numbers of black and brown ish particles, some adherent to asbestos spicules and some free. Most of the latter are iron containing and are derived from the asbestos, and there are relatively few smoke particles. Iso lated golden-brown scales or plaques, also derived from the asbestos, are scattered about the field. The asbestos fiber first fragments longitudinally, and apparently this process can go on indefinitely, since there is no ultimate fiber comparable to a vegetable fiber such as cotton. The dust collected in the Owens appa ratus contains numbers of these very fine fibers--some about 0.5 micron in transverse diameter, many less--which have fragmented transversely. Thus spicules of very diverse length--up to vl u No. < r* 1 CO --I o CO cr> co --i CO '' / ! -A 4 ... w 248 THE JOURNAL OF INDUSTRIAL HYGIENE about 80 microns--but about 0.5 micron broad, are found in numbers on the sample, together with the black, brownish, and yellow particles already mentioned. The great majority of the spicules are found broken down into particles of the order of 2 microns and less, many being about 0.5 micron and more or leas rounded. The proportion of elongated to rounded particles seems to vary in the different processes; for example, a greater proportion of elon gated particles is found in the air near dry cloth weaving than near a spinning frame. Weaving damp reduces the proportion of elongated particles. This variety of microscopic spicules, particles, and frequently some cotton fibers, found in the samples of dust col lected in the Owens apparatus, results in a tendencyto clumping on the cover slip, making counting, especially of the higher concentrations, a matter of extreme difficulty. Occasionally, also, part of the slot of the apparatus choked up with dust. The effect of the clump ing is to reduce the counts of all except the low concentrations. Furthermore, this apparatus is much influenced by local air eddies and cur rents. It takes a grab sample, which is accurate enough os an indicator of the local conditions at the moment of sampling, but does not give a picture of the average state of the atmosphere during -a period of time, as does a gravimetric apparatus. Moreover, with high concentrations of dust, accurate counting becomes impossible. The apparatus has, however, many advantages, including those of con venience, quickness in operution, and direct examination of the nature of the dust. By serial observations, too, temporary and local large increases in the dust content of the atmospheresuch as that produced by shoveling asbestos fiber into a sack--can be detected and estimated. This is of great importance since short, but repeated, exposure to massive concen trations of dust is agreed to be very harmful. Counts Nos. 5, 22, 23. and 43 in Table 6 ore examples. Much useful work can be done by means of serial dust counts in various dusty processes in the some factory, and by comparison of the results. The counts set out in Table 6 are derived from eight different factories, and it is a matter of interest to con sider whether counts from different factories con be directly compared with one another. Strictly they cannot, siuce the content of smoke particles varies in each group. The difficulty might be obviated by counting only the particles uerivtiJ from asbestos. The method has been applied success fully to counts of crystalline silica dusts; bui it is not so easily applicable to counts of asbestos dust, which con tains numbers of black and brownish iron containing particles, many of which are not quickly distinguishable from smoke and tarry particles. To classify the processes accurately on the basis of amount of dust evolved, it is necessary to take serial samples at intervals corresponding to the details and complexity of the processes. With some straightforward processes, such as spinning, few samples will be necessary; but in others, such as mat tress making, mixing, grading and opening--especially with old-fashioned plant--in which the evolution of dust rises and falls rapidly in a compara tively short space of time, many more observations are necessary throughout J. l.H. Juu. IU4 i- ST00G497I 1 \ f V f e t a *i 3 V . '( a - - rf e h { e 7 X c cJ d st :-3 1,*e it n. M pulmonary fibrosis in asbestos workers 2i9 & shift, to determine the average con* centration of dust. Repeated controls are also required. Comparative data show that the dustiest processes ore opening (with old-fashioned teasers), sieving (with no local exhaust ventilation), and shoveling, or otherwise handling asbes tos fiber. The heaviest counts of all were foundin this group, in fillingsacks, by hand, with disintegrated fiber in a settling chamber. The visual cloud of dust was intense, and irritating enough to provoke immediate coughing on the part of the observer. The clumping of the dust particles of the sample on the cover slip waa very marked, with the result that the total dust count arrived at was undoubtedly too low. With counts of asbestos particles of this order, however, the precise count is of little practical importance, since the dust concentration is far beyond the safe limit Next in order is cloth weaving, dry and without the application of local exhaust This is undoubtedly a dusty process, and although local exhaust reduces the count at the breathing level of the weaver, much dust still escapes into the workroom. Further counts are necessary to estimate this. Weaving doth wet, not merely damp, reduces the dust count to a remarkable degree. The figure for band (i.e., narrow) weaving, wet, is not accurate since it was raised by dust from a neighboring dry doth loom; still the improvement due to wet methods is noticeable. Ono asbestos weaver, who waa formerly a cotton weaver, and who has to wear glasses, stated that whereas he used to dean his glasses about three times a day when cotton weaving, he has to dean them about five times a day when asbestos tape weaving dry, and only once a day when weaving asbestos wet. Next in order comes mattress making, without any precautions such as ex haust ventilation, or damping floors, doth, and tables. Application of ex haust ventilation and damping reduce the count considerably, but it is be lieved that the figures are too low, since counts for some subsidiary processes such os buttoning, sewing, and cutting out are not available. The figures for spinning, plaiting, and braiding are believed to be rather too high, owing to contamination of dust from neighboring and more dusty processes, and further investigation is required here. A number of the samples taken with the Owens apparatus were examined to determine the size of the partides, and the percentages of partides 2 microns and under, and 7.5 microns and under, were ascertained. The 2micron standard was retained for purposes of comparison, and because of its accepted importance in silicosis. The 7.S-micron figure was also adopted because of the pronounced acicular character of much of the asbestos dust, and because, representing the diameter of the human red blood corpuscle, it is a standard easily distinguishable, and may conceivably represent the limit size of particles which can be con veniently engulfed by phagocytic cells, which are somewhat larger. Among twenty counts from various processes (Table 8), in two, between 60 and 70 per cent, of the particles were of sizes 2 microns and under; in two, 70 to 80 per cent.; in ten, 80 to 90 per cent.; and in six, 90 per cent, and upward. When the counts were analyzed ae- Vat. II Na. I m y7"-*ij2a2- " eJSM 4. :l: -`Si ill V. t i' -T /,* . 250 THE JOURNAL OF INDUSTRIAL HYGIENE cording to the 7.5-mieron standard, all twenty showed 90 per cent, or more of the particles to be of this size or less. With regard to the effect on the lungs of different varieties of asbestos, no evidence was found to indicate that any one of its varieties is more potent in producing fibrosis than the others, other factors, such as concentration of dust, being equoL There is now no doubt, however, that both chrysotile and crocidolite will produce fibrosis; while the third, the comparatively newly discovered amosite, resembles crocidolite so closely in its chemical constitution, and in the characteristics of the dust, that there con be no reasonable doubt, also, with respect to it. Length of Dust Exposure No case of diffuse fibrosis clearly due to asbestos was discovered with under 5 years' employment. Three cases were found with 3, 3i, and 4} years' work, respectively, who showed clinical signs of fibrosis. Definite confirmatory radiologic evidence was obtained in the first, definite but slighter radiologic changes in the sec ond, and indefinite suggestive changes in the third. The previous occupation in the first case was fiax weaving for 18 years; in the second, coal hewing for 16 years; and in the third, work in an iron foundry for 1 year. All these occupations are dusty; and in the last two there may be some exposure to free silica. Also increased silica con tent has been found in the lungs, after incineration, of fiax dressers. For these reasons, these three eases could not be certainly ascribed to asbestos dust, although it was, probably, at least a contributory factor. Amoog the cases of fibrosis found, thirty-six had been employed in asbestos for periods ranging froru 5 to 10 years; two of these cases, one employed for 9 years and the other for 7 years, show that a considerable degree of fibrosis may occur with icii thau 10 years' exposure. F. W. Simson (6) examined the lungs of a guinea-pig which had been experimentally dusted by Mavrogordato for two hours a day on each of fifty days between February and April, 1925, and which died from other causes in December, 1927. He states that histologic sections showed a slight generalixed fibrosis. This observer also examined portions of the lungs of two native asbestos mill workers; one had been employed for twelve months and had died from a miliary tuberculosis, and the other, employed for two years, had apparently never recovered from an attack of lobar pneumonia a year before death. In commenting on the amount of fibrosis found, he states that "a comporison between the human cases and the experimental animal showed that the fibrosis was more rapid and extensive in the human coses than in the experimental animal," and, again, that "the amount of fibroBis in two of the human coses .... was quite definite, and, if due to the presence of asbestos dust, the initial rate of production was rapid when compared with present-day non-infective silicosis on the Rond." Experiments by Professor Beattie, in 1912, demonstrated that the lungs of guinea-pigs exposed to asbestos dust for forty-three and sixty-seven hours showed "definite cellular proliferation, though not very extensive, and this is 1.1. H. it***. ia ce i pf fix i as t ch ? w. i pr P* tn Sc i --1 to CD a <0 re< f* 0~'> de 1 du ? "4 na ^ in{ mi ir siL r ho t css V. t ex< rV, "li mi / ---` . 1 ter rhi alt i TCI : the 1 '- or. ter. act are per j1 bee Ta! pul cau K\'v. VoL v Ho. * W`r T iirty-six rtos for yean; ed for 9 a, show fibrosis ) years* , r: S y-; _-! h'v heluoea P^ dato for ; fty day* i, 1925, auae* in. ry: that bi- ?ht gen- . y; ver also --V 5 of two one ntha and erculosio, to yean, red from a a year ,- fv .- y j* ."7 -7..: nonnt of "a oora- cases and wed that .nd extenm in the i. again, in two of vas quite reaeace of te of pronired with ilicosis on >r Beattie, the lungs xatos dust .ven hours iliferation, and this is PULMONARY FIBROSIS IN ASBESTOS WORKERS 251 certainly a preliminary stage in the production of fibrosis." These pathologic and experimental findings suggest that the inhalation of asbestos dust rapidly causes some changes in the lungs. But no evidence was obtained that asbestos dust can produce an acute type of fibrosis com parable to that formerly noted in South Africa after repeated exposure to massive concentration of free silica, a few cases of which have occurred recently in Great Britain, causing death in from two to four yean. The amount of disablement pro duced by the development of pulmo nary fibrosis in these workers is surpris ingly slight for a number of yean, even more so than is generally the case in silicosis. The nature of the work, however, in the majority of the proc esses docs not involve much physical exertion--in fact, in this respect it is "light work." The affected person may, and often does, continue at work --with occasional intermissions, lat terly, due to exacerbations of bron chitis--until the condition is advanced, although he sullen increasing incon venience from shortness of breath on the slightest exertion. Usually these cases cease work a year or more before death, but sometimes a terminal bronchopneumonia, or other acute infection, commences while they are still at work, and there is no long period of invalidism. Fatalities Particulars of eight deaths have been collected, and are summarized in Table 7. In six of these, advanced pulmonary fibrosis wss the primary cause of death. In the remaining two cases, pulmonary tuberculosis was a contributory factor. In six of the eight cases the diagnosis was verified by postmortem examination; in the other two cases, confirmatory radiographic evidence was obtained. Information has recently been ob tained in regard to three other cases, in which death occurred in 1929. The details of one have been published by Wood and Page (25). Postmortem examinations have been mode in all three cases, and in all, the presence of pulmonary asbestosis, without tuber culosis, was verified. Iu one case, the pulmonary fibrosis was the primary cause of death; in one, information as to the extent of the fibrosis lias not yet been obtained; and in the third case, a lobar pneumonia was superimposed on the fibrosis. It is not suggested that these few fatalities, in which the cause of death has been verified by strict inquiry, are any criterion of the true effect of the disease on the mortality rates of asbestos workers. Others are known to have occurred in which the exist ence of the asbestos fibrosis has been determined in life, but no postmortem examination has been possible. Difficulties in Diagnosis Many factors have contributed to impede both the recognition of indi vidual cases of this disease, and the establishment of asbestos dust as the determining cause. These factors are consequent, partly on the nature of the disease, partly on the nature of the dust, and partly on the industry itself, its rapid growth and internal condi tions. Difficulties in diagnosing the disease, its points of resemblance, in its latest stages, to fibroid tuberculosis, and the liability of those affected to be PULMONARY FIBROSIS IN ASBESTOS WORKERS 253 carried off by some intercurreat dis asbestos only for comparatively short ease--which alone is notedon the death periods, and of the processes considered certificate--ore oil of importance. so far, the less dusty spinning group is A concrete example encountered at the same time the largest. Thus during the inquiry will illustrate one of only now the existence of a health risk these points. J. C., Case No. 7 in in the industry is beginning to be Table 7, was k:.own by his own doctor recognized. and by the Chi-t Tuberculosis Officer of the town to 1- affected with advanced fibrosis of the lungs. Four or five months before his death it was thought Does Asbestos Dost Predis pose to Other Puiaionabt Diseases? that a change of air would be of The important question as to advantage to him. Through the gen whether asbestos workers show an in erosity of the firm with which he had creased liability to other diseases of been employed, he was transferred the lungs, such as pneumonia, pleurisy, from the town to a farm in the country, and pulmonary tuberculosis--espec where he died. The death certificate, ially the laucf--requires further in given locally, stated that pulmonary vestigation. tuberculosis was the cause of death. A history of pleurisy was given by Is another case chronic bronchitis was ten workers, in eight since commencing given as the cause of death. work in asbestos; in addition, one In the less advanced stages of the worker had & slight pleural effusion at disease, the symptoms are so unob the time of examination, and one trusive that the worker rarely consults showed signs of old pleurisy, but no his doctor. In the later stages, bron history was obtainable. Of the two chitis, or the supervention of acute with attacks prior to work in asbestos, bronchopneumonia, pulmonary tuber one had normal lungs, and the other culosis, or other acute infection, all so showed signs of a bronchiectasis. Of much more common, masks the under the eight giving histories of pleurisy, lying fibrosis; and the terminal condi- three showed signs of fibrosis, four ' tion is noted as the cause of death. showed signs of the old pleurisy only In the second place, the silicate dusts (one being due to a gunshot wound --of which asbestos dust is one--repre during the War), and the eighth senting silica in the combined form as showed signs of on old inactive tuber opposed to free silica, have naturally culous lesion of the right upper lobe. been overshadowed by the silica dusts, A history of pneumonia was ob since they do not produce the picture tained in sixteen workers--in ten prior of silicosis and have not been shown to to commencing work in asbestos, and be associated with an increased mor in six since. Of these six, two showed tality from pulmonary tuberculosis. signs of a thickened pleura only, one Moreover, although the asbestos in bowed merely enlarged lung roots, dustry has very rapidly expanded, it is two had diffuse fibrosis, and in one the still comparatively small, and scattered lungs were normal. Of the remaining over the country- A large proportion ten, one hud a thickened pleura, two of the workers have been employed in bronchiectasis, two diffuse fibrosis, one 254 THE JOURNAL OF INDUSTRIAL HYGIENE commencing fibrosis, and in four the lungs were normal. With regard to evidence of bron chitis, and bronchial and pulmonary catarrh, fifty-eight of the 363 workc'-(16 per cent.) showed signs of on<other of these conditions, all bcu.,. quite mild. Of these, thirty-one also showed signs of diffuse fibrosis, repre senting 32.6 per cent, of the fibrotic group, and eight were in the prefibrotic stage (38.1 per cent, of that group). The fact that the clinical examinations were conducted during the wanner months of the year, no doubt operated to reduce the incidence of these com plaints. Under the circumstances, the preponderance among the cases of fibrosis is the more noteworthy, as in dicating persistent irritation resulting from the dust. Of other diseases, eight gave fc:stories of rheumatic lev, -, and of these, two had definite valvulur disease of the heart. In addition, valvular disease of the heart was noted in one worker, but no history of rheumatic fever was ob tained. With regard to pulmonary tubercu losis, so definitely an added risk in sili cosis, this disease group has been drawn up to include all workers pre senting signs indicating a post or pres ent pulmonary tuberculous infection, but excluding cases of old inactive hilar tuberculosis. Under this heading are included not only workers with definite clinical signs of active tuberculous lung infections, or of old inactive lesions, equally definite, but also those workers showing signs of old apical col lapse, usually attributed to a post tuberculous infection. Signs of the latter are not uncom monly found in quite healthy persons, and are of no importance unless it is shown that the initiation of asbestos dust is associated with pulmonary tuberculosis to an increased degree, when such cases, as indicating a group ;>f workers with a latent infection, would assume a greater significance. No close association is apparent, how ever, since only thirty-seven, or 9.9 per cent, of the 374 examined, showed evidence of this disease, excluding cases of old inactive hilar tuberculosis, even when those showing the minor changes mentioned above are included. In only four was there a family history of pulmonary tuberculosis; and three out of four active eases belonged to this group. Thirty-three cases were inac tive, of which twenty-one presented no evidence of dust fibrosis, and twelve presented evidence. Out of the 374 persons examined, fourteen, or 3.7 per cent., gave a family history of tuberculosis. Evi dence of active pulmonary tubercu losis was present in three; in two of these three the source of infection appears to have been a near relative. Thus, no outstanding susceptibility to pulmonary tuberculosis was dis closed, either among asbestos workers as a class, or amon^ those with fibrosis,considering the frequency of old healed apical lesions among the general population. We have, however, by no means dis posed of the question. The superven tion of a tuberculous infection on a lung already the subject of fibrosis pro duces an increase in symptoms, pre viously unnoticed or disregarded, and causes the worker to seek his doctor's advice. He is then appropriately advised to give up his dusty employ ment, migrates from the industry, and it is estoa <naty gree, :roup . tion, once, howr 9.9 med"-* case* even ages In ryof : out this inacd no elve ned, ea Evi- tcu- o of rtion e. ility diskeis osis,.iled eral disvenIQ & propreand .or's tely loyand L H. IM I '.Cj PULMONARY FIBROSIS IN ASBESTOS WORKERS 255 may or may not accept sanatorium treatment. Thus there tends to be a drift of such cases from the fibrosis producing industry. During the course of the inquiry, information was obtained of a number of persona, previously employed in asbestos, who were either at home or in sonotoriums, suffering from chest com* plaints. Where an examination of workers is confined to those at work, a certain number of advanced cases of fibrosis, and a certain number of cases of pulmonary tuberculosis, with or without an asbestos fibrosis, in persons who have either given up work, or who are off work temporarily, will be missed. It is necessary, therefore, to leave this question of increased susceptibility to pulmonary tuberculosis in abeyance, pending further investigation. With respect toany increased suscep tibility to the supervention of other lung diseases in workers with an asbestos fibrosis, the data obtained were insuffi cient to lead to any conclusion, except as to bronchial catarrh. There are in dications, however, that when pneu monia or bronchopneumonia super venes on a Sbrotic lung, the prognosis is grave. Asscstosis and Silicosis Contrasted In view of the relationship between the asbestos fibrosis and silicosis, since they are both varieties of fibrosis of the lungs caused by the inhalation of dusts, it seems desirable to summarize shortly the main points of similarity aad dissimilarity between the two dis eases, so fur as has been ascertained. 1. Inhalation of asbestos dust, under favorable conditions of concentration of dust and length of exposure, will produce a serious and even fatal degree of pulmonary fibrosis. 2. The type of fibrosis produced, however, exhibits a number of diverg ences from that caused by the inhala tion of free silica dust, and the outcome of these variations is not yet apparent. 3. The divergences noted so far are exhibited in the radiologic picture, and on postmortem examination of the affected lungs, both in the gross morbid anatomy and in microscopic sections. 4. In the latter case the appearances of the asbestos fibrosis seem to be sufficiently distinctive not to permit of confusion between it and silicosis. 5. The radiologic appearances of the asbestos type of fibrosis, even in the more developed stage, ore more deli cate, softer, and more diffuse than the silicotic fibrosis. 6. It follows, therefore, that &n opinion os to the degree and intensity of an asbestos fibrosis, based on a com parison of the radiologic changes with those shown in standard silicosis films, will be an underestimate. 7. In the absence of a full medical and industrial history, possibilities of confusion of the two types of fibrosis will arise in the interpretation of radio grams. 8. There is evidence that other in organic dusts, containing no free silica, may be productive of this fine type of fibrosis in varying degree. Suuourt 1. There is a definito risk of the development of a diffuse pulmonary fibrosis in persons exposed to the in halation of asbestos dust. 2. This risk varies directly as the length of employment (and hence VoJ. u No. I OLG'iSOOlS 256 THE JOURNAL OF INDUSTRIAL HYCTUXE a.' ' f- length of exposure to dust), and is unaffected by the age of the worker. 3. There is a considerable difference between the least dusty and the most dusty processes investigated, end there is a corresponding variation in the rela tive risk of the development of fibrosis in the workers in these processes. 4. With continued exposure to high concentrations of dust, the disease may be fully developed in seven to nine years, and may cause death after about thirteen years' exposure, exceptionally in a shorter period. 5. Fibrosis as it occurs in asbestos workers differs-considerably from the disease as it occurs in workers exposed to free crystalline silica dust; it differs in the radiologic picture, and also on postmortem examination of the affected lungs. 6. The more dusty processes are those involving the preparation and manipulation of the raw asbestos (other than crushing), dry cloth weav ing, and mattress making. 7. The less dusty processes are spin ning and similar processes, and proc esses, other than dry weaving, in volving the manipulation of the spun yam. 8. Further investigation is required to determine whether there is any in creased susceptibility to the superven tion of pulmonary tuberculosis asso ciated with the development of the asbestos fibrosis. Much generous assistance has been re i ceived from many sources during the course of this inquiry, and for this acknowledgment i is gratefully tendered. In addition to those already referred to, the writer is beholden especially to Dr. S. A. Henry, who did much to facilitate the inves O tigation, particularly by his detailvi reports on various relevant matters, and. by much cr>! organ [ration and liaison work; to many others of his colleagues in the different dis to. tricts, to s number of directors and officials --i of various firms, foremen, and employees; to Dr. W. E. Cooke of Wigan and Professor VO M. J. Stewart of Leeds for pathologic mate rial and for much information as to the progress and results of their own researches into the gross anatomic and the histologic features of the asbestos fibrosis, and the constitution and location of the curious bodies; to Dr. MacGregor, Medical Officer of Health for Glasgow, for various clinical rc: and radiotogie /acuities, and for his ever present readiness to lend the aid of his Department, aod especially to Dr. H. E. Seiler of his stall; to Dr. J. C. Robertson [or the loan of a radiogram, together with detailed clinical notes of one patient and other valuable information; to Dr. W. H. Bateman for the loan of a radiogram, for some clinical histories, for obtaining and examining several specimens of sputum, and for other aid; and to Dr. J. Rennie, Chief Tuberculosis Officer at idieffield, for a series of radiograms showing various stages of silicosis. BIBLIOGRAPHY 1. Departmental Committee on Compen sation for Industrial Diseases. Min utes of Evidence, Appendices and In dex, 1907. Cd. 3496, p. 127; Report, 1907. Cd. 3493, p. 14. 2. Cooxz, W. E.: Fibrosis of the Lungs Due to the Inhalation of Asbestos Dust. Brit. Med. Jour., 1924, l, 147. 3. Cooxc, W. E.: Pulmonary Asbestosis. Ibid., 1927, i, 1024. 4. McDoHiUJ, S.: Histology of Pulmo nary Asbestosis. Ibid., p. 102S. 5. Srttta, H. E.: A Case of Pneumoconio sis. Result of the Inhalation oi Asbestos Dust. Ibid., 1023,1, 982. 8. Sites on, F. W.: Pulmonary Asbestosis in South Africa. Ibid., 1023, /, 333. 7. Royal Commission on Metalliferous Mines aod Quarries. Second Report. 1914. Cd. 7478, p. 148. i. t. a. JuO. 100 V 8. 3 8. C 10. I II. T 12. G 13. A 14. P: 16. G 16. At 17. D< 18. Si 19. W 20. G Voi. i: No. 50he reno ittt . 10, 'FtM ieh ny li*- *; -*or ita th* hc tio lh oui leeearl 'ertua E. for nth and E. for &ad aad Uia/ riea > Of no* aio* o( MIS SS5. rous ort, IB. . IMO e PULMONARY FIBROSIS IN ASBESTOS WORKERS 257 8. 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L.: Report on the Grinding of Metal* and Cleaning of Castings with Special Reference to the Eflecta of Duat In halation upon the Workers. London, H. M. Stationery Office, 1923. 23. SunrenLANn, C. L., and Brtsqx, S.: Report on the Ioeideocs of Silicosis in the Pottery Industry. London, K. M. Stationsry Office, 1926. 24. Sutherland, C. L., and Brtson, S.: Report on the Occurrence of Silicoii.! among Sandstone Workers. London, H. M. Stationery Office, 1929. 25. Wood, W. B., and Pass, D. S.: A Case of Pulmonary Ashestotie. Tubercle, 1929, 10, 457. For the geologic and the manufacturing and industrial aspects of asbestos, see: 26. CmsZL, F.: Cbrysotile-Asbestos, its Occurrence, Exploitation, Milling, and Uses. Second edition. Ottawa, Govt. Printing Bureau, 1910. 27. Asbestos, its Sources, Extraction, Prep aration, Manufacture and Uses in Industry and Engineering. Berlin, Becker and Haag, 1928. 28. Taoara, E.: A Dictionary of Applied Chemistry. London, Longmans, Green Jt Co., 1912. 29. Aibeifot, s monthly trade journal pub lished in Philadelphia, Pa. For description of processes, and further discussion on the incidence rate of Sbrosis, and effect of work in different procesees and also preventive measures, tee: 30. Mexewktme.i. E. R. A., and Price, C. W.: Report on Effects of Asbestos Dust on the Lungs end Dust Suppres sion in the Ajbvstos Industry. Lon don, H. M. Stationery Office, 1030. VL 11 K*. 0 0 6 (t9 U 0 iS m- ST006498I