Arguments presented by third parties
Spontaneous or background mesothelioma: does it exist?
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Spontaneous or background mesothelioma: does it exist?The rare occurrence of mesothelioma in childhood and even as a congenital malignancy supports the existence of a background of spontaneous mesotheliomas unrelated to asbestos (in addition, mesothelioma has been reported in fish (trout) [114], where inhalation of airborne asbestos fibres cannot be invoked). However, in epidemiological studies on adult populations, it is virtually impossible to separate spontaneous mesotheliomas from those that are arguably attributable to environmental exposure to asbestos [70, 115]. The incidence of mesothelioma in women is sometimes used as an index of the background or spontaneous rate: the crude incidence rate for women in Western Australia is about 2.6 per million person-years at age ≥ 15 years [115]. The incidence rate in other populations is listed in Table 2 (following page). In the answer to the WTO Panel's questions to Canada (Question 9, see Annex II), the following statement is made: "Recent analyses of Canadian data on mesothelioma in Canada, British Columbia and Quebec all agree that the incidence rate of mesothelioma has been stable among women of all age groups since 1984. The rates are 70% higher in Quebec than in the rest of Canada, presumably as a result of more frequent and more intense exposure in the workplace." The statistics for Australia differ on this point (Table 2): mathematical modelling of Western Australian data suggests that the incidence rate in women has risen about two-fold from the 1970s until the 1980s, which might be explicable by increased general environmental exposure to asbestos, plus some occupational exposures among women [70, 115] (please see also AMR 99 — i.e. the graph for the Age Specific-Incidence Rates of Malignant Mesothelioma in Australia Women, 1986-1995, especially for ages 50-64 and 65-79). This increased incidence among women presumably reflects direct or indirect occupational exposure, domestic exposure or environmental exposure [115]; in this respect, it is worth emphasizing that domestic (household contact) exposure to asbestos — e.g. among wives laundering the dust-laden workclothes of an asbestos-exposed husband — is not necessarily low-level exposure, and analysis of the asbestos fibre content of the lungs in a small number of such patients indicates that this type of exposure can approach occupational levels [116]. table 2: incidence of mortality of mesothelioma in various countries and areas over time, 1960s to 1994 (per million inhabitants per year)
The often-cited background or spontaneous rate of mesothelioma of 1-2 per million person-years [10, 117], has in part also been derived from backward extrapolation of the incidence rates in men, to the point where the estimated incidence rates for men and women diverged from each other (i.e. linear extrapolation to the point where the sex ratio = 1:1) [117] . Hillerdal [20] suggests that this incidence probably represents a high estimate and comments in the following terms: "... there seems to be a small spontaneous basal or background incidence of the tumour [mesothelioma] ... However, it is of course possible that some of these background cases might in fact be due to occupational, domestic, or even environmental exposure, unknown to (or forgotten by) the patients themselves. ... There are authors who claim that the presumed background levels must be very low, and retrospective searches for the tumour in the medical literature reveal no convincing cases of mesothelioma before 1946, although such negative evidence is of questionable value.15 McDonald and McDonald, in a recent review, estimated the background level to be 1-2/million per year; they came to this figure by extrapolating backwards from epidemiological studies from various countries. ... It is nevertheless possible that there is a background level of mesothelioma, — that is, that the tumour can occur even in the complete absence of asbestos (or erionite) fibres. However, the data reviewed here indicate that if so, this background level must be very low — probably much < 1 case/million people/year. This figure comes from studies of industrialized countries, where background exposure to asbestos is unavoidable. What the true figure is can only be guessed ... ". [p 507]. De Klerk [115] and Comin et al. [70] have commented that in the absence of specific exposure to asbestos, the final estimated rate for both men and women in Australia is 2.6 per million person-years — higher than the equivalent figure of 1.6 for Los Angeles [115]. This difference may lend some support to the proposition that general environmental exposure to asbestos may have produced an increase in the mesothelioma rate in Western Australia [115]. However, it is difficult or impossible in general to draw firm conclusions from differences between different studies, because of variation in the accuracy of diagnosis and differences in the ways that data are collected. In response to questions from the WTO Panel (Question 9, see Annex II) the Canadian document also observes that: "The incidence of [mesothelioma] among men levelled off after 1984 in British Columbia ... and seems to have levelled off in Quebec after 1990 ... Finally, analysis of Canadian rates between 1973 and 1992 ... estimates that the risk is four times greater for men born before 1940 than for men born between 1951 and 1955. Those analyses therefore suggest that the incidence of mesothelioma has levelled off in Canada, is declining in British Colombia, and has levelled off in Quebec ..." . In response to these observations, I emphasize the following: (i) the incidence of mesothelioma among Australian males shows little evidence of levelling off, and has continued to rise until 1994-1995 and thereafter (please see Table 2 and the 1998 and 1999 Reports for the Australian Mesothelioma Register); (ii) from the recent report by Peto et al. [24], it is also evident that the incidence of mesothelioma in Western Europe continues to rise, with particular emphasis on males born between 1945 and 1950 who used asbestos-containing products in the 1960s and the 1970s (and the early 1980s). 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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