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Jeremiah R. Lynch National Center for Urban and Industrial Health Brake Lining Decomposition Products A number of investigators have found asbestos bodies in the lungs of urban residents who were not oc cupationally exposed to asbestos. A relationship between the carinogenic properties of asbestos and the urban excess of lung cancer has-been suggested. The decomposition products of brake linings have been described as a possible source of these fibers. Brake testing laboratory methods were used to test fiber emission from a variety of friction products. Only a very small fraction of the asbestos escaped as free fiber while the remainder was transformed Into some other, nonfibrous, mineral. A significant release of free fiber occurred only under conditions extreme enough to produce brake failure. Table I- Asbestos bodies in human lungs. Location Cape Town (2) Miami (2) Pittsburgh (3) Johannesburg (4) Finland (5) Montreal (6) San Francisco (1) Year 1963 1965 1965 1965 1966 1966 1966 % Positive 26 27 41 39 58 48 42 Table II. Brake lining composition. Ingredient Automobile Truck Asbestos Resins and polymers Oxides and pigments Metals Carbon, graphite, etc. 55 28 9 3 5 100% 33 48 16 2 1 100% Mr. Lynch is Chief of the Lab oratory of Engineering, Occupa tional Health Program, Public Health Service, Department of Health, Education, and Welfare, 1014 Broadway, Cincinnati, Ohio 45202. The occurrence of coated fibers re ferred to as "asbestos bodies" in the lungs of urban populations not oc cupationally exposed to asbestos has been noted by several investigators. The results of several autopsy series, as summarized by Cooper,1 are shown in Table I.1-6 Thompson,7 in suggesting possible modes of exposure that could account for these bodies, stated: The average private motor vehicle wears out three or four sets of brake linings and one or two clutch linings in its lifetime, and commercial and public transport vehicles wear out many more. These linings consist largely of asbestos which is ground to dust as the linings wear, and most wear occurs in built-up areas. This alone in most cities would involve the discharge of many tons of asbestos dust and fibers in the streets each year. Since asbestos has been implicated as a carcinogen8 and lung cancer has a higher incidence9 among urban popula tions, the fate of the asbestos worn from brake linings becomes significant. Brake Lining Composition The average composition of typical ' brake linings of the type tested is shown in Table II. Individual mixes may vary considerably from these averages. Each of these ingredients performs a particular function. The asbestos pro vides strength, heat resistance, and selec tive decomposition under stress. The resins and polymers hold the other in gredients together. Various oxides are added as pigments and to improve the grip of the lining on the drum. Soft metals such as lead and brass improve wear while carbon and graphite serve as friction modifiers. Brake lining quality depends to a large extent on the type of binder used. Linseed oil, which begins to decompose at about 450F, is used for light service brakes. More demanding service re quires brakes made from cashew type resins or oil modified phenolic resins. These ingredients are mixed, either dry or with a solvent added, formed into shape, and cured in an oven. The hardened lining is then cut, ground, and sanded to the precise dimensions of the finished linings. The dust produced by the abrading operations in asbestos friction product factories (Figure 1) contains free asbestos fibers that are similar to those in industries where can cer is known to be in excess.10 The question to be resolved, therefore> is whether the dust produced by the normal wear of brake linings contains this same type of free asbestos fiber. Test Methods In an initial exploratory survey, the dust obtained from inside automobile brake drums removed for brake relining was examined by electron micrograph. Figure 2, which is typical of the series, does not reveal any free fibers. How ever, this finding did not eliminate the possibility that free asbestos fibers were released and that they had escaped into the atmosphere. To examine this hy pothesis a series of experiments was devised to permit sampling decomposi tion products of the lining under simu lated operating conditions. The tests (Table III) were performed with the brake testing machines in the laboratory of a major brake lining manufacturer. Most of the tests were performed on a 824 Journal of the Air Pollution Control Association Reprinted from APCA Journal, Vol. 18, No. IS, December, 1968 HWBUI0002855 Figure 1. Brake lining factory dust. Figure 2. Du$t removed from brake drum. Figure 3. Decomposition product from nor mal wear of brake lining. friction-testing machine which used one-inch-square samples of the test lining. Heating and cooling apparatus permitted control of drum temperature from those encountered with inter mittent brake use to the maximum temperature which occurred only in extremely rapid or "panic" stops from high speed. Other tests were performed on a brake-testing dynamometer which subjected a complete brake assembly to a series of stops and starts from different speeds and at different deceleration rates to simulate actual driving con ditions. In these tests the drum tem peratures varied according to the driv ing condition simulated. During each run at a test condition, a sample was collected on an 0.8 /i pore size membrane filter. Electron micrographs of these filters were examined to determine the presence of free asbestos fibers and to quantify these fibers when more than a few occurred. Results The object of the tests was to deter mine what proportion of the asbestos known to be present in the fining ap peared as free fiber in the decomposi tion product. No attempt was made at a mass balance between the material worn from the lining and the decom position product since it was not pos sible to collect all of the decomposition product. However, the percentages obtained in the fraction of decomposi tion product examined are applicable to the total material worn from the brake and may be compared to the percentage of asbestos originally present. In all but a few tests the automobile drum brake finings showed less than 1% free fiber in the decomposition product as compared to about 50% in the lining. An electron micrograph of the decomposition product obtained from a typical test in this group is shown in Figure 3. In those tests where a sig nificant mass of ffee fiber was released (Figure 4), the temperature was in an extremely high range for the fining in question as evidenced by rapid drop in the coefficient of friction. Had these finings been subjected to like conditions . in a vehicle, the brakes would have failed. Similar results were obtained in the bus and truck drum brake tests and in the clutchtest. The experimental model disk brake tested did release some (>5%) free fibers, but no conclusion can be drawn from a single sample. Discussion Except in all but the most extreme driving conditions, only a very small fraction of the 30 to 50% asbestos pres ent in a brake lining escapes into the atmosphere as free fiber. The question remains: What happened to the as bestos? A prevalent theory of brake operation holds that wear occurs not by abrasion of the lining by the drum but by the production of minute areas of intense heat at the points of contact between the drum and the fining. De composition, not only of the binder but of the asbestos as well, occurs at these locations since the breakdown tem perature of asbestos, about 900F, is exceeded. Decomposition products will include not free asbestos fibers, but a different mineral resulting from thermal metamorphosis of asbestos. December 1968 Volume 18, No. 12 825 HWBUI0002856 Table III. Test results by electron micrograph. Test Product Brand Method Presence No. of Conditions of Free % Free Samples of Test-F Fibers Fiber* .1 Automobile A Friction drum brakes 6 300 - 800 Few <1 2. Automobile B Friction drum brakes 6 250-800 None 0 3. Automobile C drum brakes Friction 5 300-700 Few <1 4. Automobile C drum brakes Friction 1 700-900 Numerous ~10 5. Automobile D Friction drum French brakes 5 300 - 800 Few <1 6. Automobile E drum brakes Friction 5 300-700 Few <1 7. Automobile F Friction drum German brakes 5 300-800 Few <1 8. Automobile G Friction drum brakes 2 100 - 500 Few <1 9. Automobile G Friction drum brakes 1 600 - 700 Numerous ~15 10. Automobile H Friction drum brakes 2 100 - 600 Few <1 11. Automobile J clutch Dynamometer 1 Normal None driving 0 12. Automobile K Dynamometer 1 Normal Few disk driving brake <5 13. Bus drum L Dynamometer 1 City None brake driving 0 14. Bus drum M Friction brake 2 450 - 550 None 0 15. Truck drum F Friction brake (light) 10 300 - 800 Few <1 * Weight estimated from fiber volume. Some independent experiments done at a brake lining research laboratory11 have shown that all of . the magnesium present in the chrysotile asbestos orig inally in the lining can be accounted for in the decomposition products. How ever, the characteristic X-ray diffrac tion pattern of chrysotile asbestos had vanished. Thus, it is apparent that under conditions in which asbestos is. worn from the lining of a brake, the asbestos is destroyed. Conclusion Only a very small proportion of the asbestos worn from brake linings is released as free fiber; the remainder is converted into some other mineral as a result of the extreme temperatures gen erated at small spots on the lining sur face. Thus, although urban air con tains a few free fibers as a result of brake lining wear, they represent a very small 826 proportion of the total asbestos used in manufacture of brakes. Many sources of respirable fibers not associated with asbestos products have been identi fied,12 and free fibers from brake lining wear seem to be an inconsequential health factor in urban air pollution. References 1. Cooper, W. C., "Asbestos as a hazard to health," Arch, of Env. Health. IS: 285 (1967). 2. Thomson, J. G., Kaschiala, R. O. C., and McDonald, R. R., "Asbestosis as a modem urban hazard," S. Afr. Med. J. 37: 77 (1963). 3.- Cauna, D., Totten, R. S., and Gross, P., "Asbestos bodies in human lungs at autopsy," JAMA, 192 : 37 (1965). 4. Webster, I., in discussion of Thomp son, S. F., "Physiological effect of DaO in mammals," Ann. N. Y. Acad. Sci., 84 : 736 (1960). 5. Meurman, L., "Asbestos bodies and pleural plaques in a Finish series of autopsy cases," Acta Path Microbiol. Scand Suppl, 181: 107 (1966). 6. Aryilrel, L. and Thurlbeck, W., "The incidence of asbestos bodies in the lungs of randon autopsies in Montreal, Canada. Med. Assoc. J., 95: 1179 (1966). 7. Thomson, J. G., "Asbestos and the ur ban dweller," Ann. N. Y. Acad. Sci., 132:196 (1965). 8. Selikoff, I. J., Churg, J., and Ham mond, E. C., "Asbestos exposure and neoplasia," JAMA, 188 : 22 (1964). 9. Buell, P. and Dunn, J. E., "Relative impact of smoking and air pollution on lung cancer," Arch, of Env. Health, 15:291 (1967). 10. Enterline, P. E. and Kendrick, M. A., "Asbestos dust exposures at various levels and mortality," Arch. Env. HeaUh, 15:181 (1967). 11. Sinclair, D., "Study of wear dust from brake lining," Johns-Manville Res and Eng Ctr., personal communication (1967). 12. Cralley, L. J., Keenan, R. G., Lynch, J. R., and Lairihart, W. S., "Source and identification of respirable fibers," Amer. Ind. Hyg. Assoc. J., 29: 129 (1968). Journal of the Air Pollution Control Association HWBUI0002857