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Friction Products (e.g. brake linings)
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Friction Products (e.g. brake linings)Automotive mechanics and garage workers constitute a large population of workers potentially exposed to chrysotile derived from brake linings. For example, brake blocks and linings used in Australia have contained only Canadian chrysotile for many years, and the materials are either imported as pre-formed brake blocks and linings, or chrysotile is imported into Australia for subsequent manufacture of these products. It has been estimated that this group of mechanics amounts to at least 900,000 workers in the US, and the figure may be even higher if one adds in all those who have ever worked in the automotive repair industry but then moved into other employments, and those who have retired. For Australia, the number of persons employed as mechanics in 1991 amounted to 85,155 (84,293 males); for 1996, the corresponding figures are 83,647 (82,827 males), out of a total population of 16,852,256 in 1991 (8,363,677 males); for 1996, the total population was 17,892,423 (8,849,224 males). These figures for Australia include all mechanics, including automotive, brake and engine mechanics, together with supervisors and apprentices; the figures for 1996 also include mechanics' assistants (not included for the 1991 figures).21 Taking into account the fact that the Australian population is less than 1/10th that of the United States population, these statistics appear to be roughly comparable. The literature contains anecdotal reports of malignant mesothelioma among automotive and brake mechanics. However, the question that arises is whether these anecdotal reports are explicable as the chance occurrence of spontaneous or background mesotheliomas among a large population of mechanics, or whether this group of workers has sustained other significant exposures to asbestos, including one or more amphiboles. In other words, the question is whether there is a general increase in the incidence of mesothelioma among automotive and brake mechanics with no other exposures to asbestos. Brake repair workers are potentially exposed to asbestos during a number of procedures, which include removal of dust from bakes by air hoses, and a variety of other manipulations that include bevelling, grinding and drilling. Clearance of dust from brakes by use of an air hose can create a cloud of visible dust, and airborne fibre concentrations of 2.0 to 29.4 f/ml have been recorded in the immediate vicinity [225, 226]. Please see also Table 11 in EHC 203 (pp 42-43). In North America, chrysotile has been used almost exclusively in brake linings since the 1940s; chrysotile is also the type of asbestos used in brake linings in Europe (and also Australia). As I mentioned previously, commercial chrysotile (e.g. Canadian chrysotile) contains on average small quantities of contaminant amphiboles in the form of tremolite (usually ≤ 1%). However, the significance of this type of potential exposure among brake repair workers is complicated by a number of factors: During moderate braking of automobiles, temperatures as high as ≥ 500oC can be reached within the brakes, and at this temperature a proportion of the chrysotile undergoes dehydroxylation and recrystallization to form the mineral forsterite, which is not implicated in mesothelioma induction. "Heating of chrysotile at 700°C for an hour converts it to an amorphous, anhydrous magnesium silicate material ... Intensive dry grinding also destroys the structure of chrysotile. Analysis of wear debris from brake linings made with asbestos has shown that virtually all of the chrysotile fibre is converted to amorphous material, in association with the mineral forsterite (a recrystallization product). The conversion is explained by localized temperatures above 1000°C at the point of contact between the brake lining and the drum" ... . [EHC 203, p 14]. Most chrysotile fibres released from brakes comprise short-length fibres < 0.4 µm in length (> 80 per cent of all chrysotile fibres from brakes). However, some fibres > 5 µm in length and even > 10 µm in length appear to survive (e.g. please see Table 11 in EHC 203). The short-length fibres appear to have only questionable or limited carcinogenicity, and this property is thought to reside primarily in fibres > 5 µm in length. In addition, limited fibre burden studies on brake repair workers have shown a low pulmonary asbestos content. "The fibres found in the brake wear debris are predominantly (99%) less than 0.4 µm in length … Rödelsperger et al., (1986) found less than 1% of fibres longer than 5 µm." [EHC 203, p 14] One also needs to remember that assessment of mesothelioma risk among brake repair workers can be confounded by other occupational exposures to asbestos [227]. In a review of changing risk groups for malignant mesothelioma, Huncharek [228] gives the follow account for brake mechanics: "A major problem with epidemiologic studies of this workforce is the difficulty in tracing a large, non-unionized group of workers. Estimation of disease risk has been impeded by lack of quantitative data on exposure levels among individuals with long-term exposure. ... In 1976, investigators at the Mount Sinai School of Medicine studied asbestos exposure among brake repair workers in New York city. Both clinical examinations and fibre counts produced by various operations in brake maintenance workers were analyzed. Samples taken at a distance of 3-5 feet from brake drums during periods of blowing dust showed fibre concentrations of 6.6 to 29.4 fibres/ml, with a mean of 15.9 fibres/ml. In addition, ten samples of brake drum dust were analyzed by phase contrast, optical microscopic examination and transmission electron microscopic examination to determine the percentage of short fibres (i.e., 25-500 angstroms x 760 to 3750 angstroms22). Eighty-three percent of all chrysotile fibres were in this category, and almost 20% of the total mass of 10 samples was chrysotile (determined by electron diffraction). 'Throughout the examination by electron microscopy, attention was given to the morphology of the fibres. A majority of fibres showed little alteration in the typical chrysotile fibre'. In an additional report from Mount Sinai, Rohl et al. analyzed residual dust recovered from brake linings and made direct measurements of the free asbestos fiber content of 'workroom air' in areas in which brake lining maintenance and brake shoe installation occurred. Airborne asbestos dust concentrations were similar to those cited by Lorimer et al. (i.e., mean airborne fiber concentrations during compressed air blowing of brake drums ranged from 2.6 fibers/ml at a distance of 10-20 feet to 16.0 fibers/ml at 3-5 feet). Samples of brake drum dust showed that the proportion of chrysotile in this material averaged 3% to 6% (both as free fibre and as particulates in pulverized binder). Regarding the health effects resulting from the exposures described, Langer and McCaughey published ... a case of mesothelioma in a brake repair worker ... a 55-year-old man who had worked in the used car, tire and car repair business since the age of 19 years. He reported routinely servicing automobiles, including the replacement of brake linings. No other source of asbestos exposure was found. Analysis of lung tissue showed the presence of chrysotile fibres (no amphibole was found) confirmed by electron diffraction techniques. Ten percent of the fibers found were longer than 10 microns.23 The authors point out that 'Controversy over the potential of chrysotile to cause mesothelioma has continued despite evidence from asbestos textile fabricators, thought to have used only chrysotile, from workers making brake pads, from chrysotile miners and millers, and from animal studies'. They also state 'The risk of malignant asbestos disease among these workers seems to be low but mortality data have yet to be thoroughly evaluated.' The most recent report of mesothelioma in a brake mechanic reviews a pleural mesothelioma occurring in a 47-year-old male automobile mechanic whose only known exposure to asbestos was from clutch and brake repair work during an 11-year period. ... Another case of mesothelioma in a brake repair worker was recently published. In this report, a 56‑year-old male elevator mechanic ... reported working as an elevator mechanic for 30 years. He reported exposure primarily from elevator brake linings that he routinely cut, fitted, and removed during elevator installation and maintenance. Several recent studies from Scandinavia on this topic also deserve mention. Hansen, from the Institute of Community Medicine in Denmark, completed a historical cohort study examining the mortality of car mechanics from ischaemic heart disease and malignant neoplasms. The study cohort was identified using records of a nation-wide census carried out in Denmark in November 1970. Comparison was made with another cohort of skilled male workers who were not exposed to asbestos or 'petrochemical substances'. Of 583 observed deaths, one case of pleural mesothelioma was found. Likewise, Jarvholm and Brisman, in a 1998 report, used the Swedish death register and the census of 1960 to study the occurrence of asbestos-associated tumors in car mechanics. One hundred and eighty-seven deaths attributable to cancer were observed, whereas 154 were expected. Thirty-nine were caused by lung cancer, whereas only 23 were expected. Again, one death from pleural mesothelioma was found. ... It has been estimated that 20,000 deaths from asbestos-related cancer will occur during the next 40 years among automotive maintenance workers in the United States. With the many difficulties faced by epidemiologists studying this workforce, it is unclear how accurate this estimate will prove to be. Clearly, what is needed is better information on duration and intensity of exposure to respirable asbestos fibers in this occupational group. Additional study is needed to accurately determine the incidence of mesothelioma among members of this workforce." [pp 2704-2705]. The situation is further complicated by other reports on garage mechanics and workers involved in friction products manufacture [229, 230]. These studies have been reviewed briefly by Wong [231]: three found no increase in the RR for mesothelioma among garage mechanics (RR = 0.9, 0.65 and 1.0 respectively). An analysis of > 13,000 workers at a UK friction products factory showed no detectable excess mortality due to lung cancer or other cancers; 13 mesotheliomas were found but 11 had known exposure to crocidolite [229, 232]. McDonald et al. [230] identified excess mortality from lung cancer among friction products workers, but there were no mesotheliomas: "A study by McDonald et al. (1984) investigated mortality due to lung cancer, mesothelioma and asbestosis in three US factories manufacturing friction products and packings. The cohort comprised 3641 men employed between 1938-1958. During the 1930s exposures for most processes were 1-5 mpcf (millions of particles per cubic foot) and > 10 mpcf during dry mould mixing. By the 1960s most exposures were < 0.5 mpcf. A significant excess of deaths (reference was to mortality rates for Connecticut) due to respiratory cancer was observed however this was not related to duration of employment. No cases of mesothelioma were reported. There was limited evidence of an increase in risk of lung cancer with increasing exposure. However the SMR for lung cancer was noted in workers with less than one year of service. A study by Finkelstein, (1989) investigated mortality rates among 1657 employees at two Ontario factories manufacturing chrysotile friction materials. The study population consisted of workers employed for at least 12 months after 1 January 1950. The study showed a significant increase in mortality from laryngeal cancer and lung cancer. No increase in mortality was noted from gastrointestinal cancer or from non-malignant respiratory disease. One or two deaths may have been due to pleural mesothelioma. Case-control analysis demonstrated a lack of association between the risk of death from laryngeal or lung cancer and the duration of employment or employment in departments where chrysotile had been used. The author also noted that cigarette smoking is a risk factor for laryngeal cancer and lung cancer, and therefore, increased risk may be in part attributable to differences in smoking habits." [NICNAS 99, p 65]. Similarly, Woitowitz and Rödelsperger [227, 233] found that: "There is no evidence that car mechanics are exposed to an increased risk of mesothelioma even if they do brake repairs, but asbestos exposure in other employment is an important confounding factor, so that if there is a mesothelioma risk for car mechanics but it was small, it would not be detectable." Nonetheless, the 1999 Report for the Australian Mesothelioma Register24 (AMR 99) records 58 mesotheliomas among brake mechanics with no other exposures to asbestos, during the almost 13‑year period between 01 January 1986 and 31 October 1999 (total cases with a stated history of asbestos exposure = 2585). Mechanics who frequently or consistently work on brake linings and brake blocks represent only a sub-fraction of the total workforce of mechanics in Australia. If one takes the 1996 census figure of 82,827 for male mechanic25, this amounts to 58 mesotheliomas in 1,062,946 person-years (≡ 54.6 mesotheliomas per million person-years). If one rounds off the workforce to 100,000 male mechanics, the figure becomes 45 mesotheliomas per million person-years. If one then doubles the workforce population to take into account retirees and other workers who moved on to other occupations (although a figure of 200,000 is almost certainly an overestimate because it would include all mechanics, whereas brake mechanics constitute a smaller sub-class), the mesothelioma rate becomes 22.6 per million person-years — well under the rate of 337 mesotheliomas per million person-years for the Quebec chrysotile miners and millers but still substantially above the upper limit of the estimated background rate of 1-2 mesotheliomas per million person-years (about 10-fold). One might suspect that mesotheliomas in brake mechanics will cluster in those involved in the grinding, bevelling and other operations on new brake blocks and brake linings (i.e. brake materials unaltered by heat). Using an earlier set of data for Australia, NICNAS 99 came to a similar conclusion: "Out of 2119 mesothelioma cases registered (with a response to history) for the period 1986-1995, 46 cases were listed for the category 'brake lining - manufacture/repair', 40 of which were recorded in car mechanics, of which 37 were exposed to asbestos in this occupation only ... Overall the numbers indicate a slight increase of around 1-2 cases per year, which is roughly proportional to the growth rate of all mesothelioma cases in Australia" ... [p 66]. It is apparent that these considerations apply to occupational circumstances. Evidence indicates that the general population is exposed to only very low levels of asbestos derived from the braking of passing automobiles, and that most of these fibres represent short-length fibres and heat-altered chrysotile. NICNAS 99 has this to say on the subject: "It is claimed that the amount of asbestos found in the dust arising from braking is rarely more than 1% of the wear product (Asbestos Information Committee, 1975). It is not known what quantity of chrysotile is imported in brake linings and other friction materials, but ABS [Australian Bureau of Statistics] data indicates in excess of 750,000 articles (brake linings, pads and clutch facings) being imported in 1997 containing asbestos and therefore possibly containing chrysotile. Assuming each unit weighs 200 g and contains 50% chrysotile, this equates to around 150 tonnes of chrysotile per annum. Assuming a further 1000 tonnes of chrysotile present in friction products manufactured in Australia, it is estimated that (assuming a worst case scenario of 1% release per annum, i.e. all products are completely worn in one year), around 11.5 tonnes of chrysotile will be released per annum countrywide or 32 kg per day spread all around the country. It is acknowledged that this figure may be an overestimate, as studies have shown that some of the chrysotile is degraded to magnesium silicates and forsterite ... In addition, some of the debris will be retained in the brake system and removed and disposed of under controlled conditions." [p 78]. Directory: english -> tratop e -> dispu e dispu e -> Annex g list of exhibits submitted by the parties dispu e -> World Trade Organization dispu e -> World Trade Organization Download 1.28 Mb. Share with your friends:
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