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J*r**r'<ir. .......... Quantitative risk; of mesothelioma and lung cancer 579 Table 5. Possion regression of pleural cancer against cumulative exposure by fibre type Fit/fibre type A# r 95* Cl for r Residual Degrees of deviance freedom P 1. Independent fits 0 1.4 a 0.02 r Overall 0.0057 2. Best common slope 0 0.93 a 0.13 r Overall 0.0047 3. Common slope, amphiboles only 0 0.88 a 0.120 Overall 0.62 1.2 0.72 0.75 0.77 (-0.54, 1,43) (-0.32, 3.5) (0.17. 1.79) (0.27.1.3) (-0.069. 1.62) 0.25 3.68 3.93 0.36 0.49 3.68 4.53 0.39 0.44 0.83 1 0 4 5 2 1 6 7 2 1 2 0.62 0.45 0.56 0.84 0.48 0.72 0.72. * 0.82 0.51 - 0.66- Table 6. Possion regression of peritoneal cancer against cumulative exposure by fibre type Fil/fibre type / 95* Cl for r Residual Degrees of deviance freedom p 1. Independent fits A 0.0022 2.1 (0.93, 2.9) 0.10 1 0.75 0.00018 2.4 (0.41,6.4) 1.4 -1.7 (-22. 0.91) 2.60 4 0.63 Overall 2.70 5 0.75 2. Common slope, amphiboles only 0 0.0022 0.10 2 0.95 2.1 (1.2,2.9) 1 0.76 a 0.0006 0.09 2 0.91 Overall. 0.19 e mates of the peritoneal and pleural slopes is con strained to be 2.4, the best fit pleural and peritoneal slopes are : 0.86 (95%CI 0.51-1.15) and 2.1 (95%CI 1.2-3.6). If the ratio of slopes is constrained to be 3.2. the estimated values are r=0.67 (95%CI 0.400.90) and r=*2.I (95%CI 1.3-2.9). Support for a convex (r<l) increase of pleural mesothelioma risk with exposure can be found in the detailed dose-specific analyses of the Wittenoom mesotheliomas by Berry (1991). Most of these cases (62 of 72) were pleural. Figure R plots the constant terms in the four exposure categories of Berry's analysis against their mean cumulative exposure. The slope is very close to 0.5. In addition, Goggon et at. (1995), concluded from a comparison of the ranking of occupations by mortality from pleural and perito neal cancers and from asbestosis that "a more plaus ible explanation (of the different rankings) is that the >sure response relations for mesothelioma and .x(osis are non-linear, with the risk of pleural mesothelioma rising relatively more steeply at low exposures, but less steeply at high exposures". A non-linear relationship between exposure and the rales .of pleural and peritoneal mesothelioma means that the percent excess mortality per f/ml.yr (#M) will Fig. 8. Scaling constant ia the four exposure groups of Berry (1991) analysis of the Wittenoom crocidolite cohort, plotted against the mean cumulative exposure in each group. The plot ted line is proportional to the square root of cumulative exposure. not provide a consistent summary of the effect for mesothelioma at the two sites considered individu ally. Each additional unit of exposure will add--pro gressively--less risk for pleural tumours, and more for peritoneal tumours. The point at which the absol-