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588 J. T. Hodgson and A. Oamton arguments based on the assumed equivalence of the fibre types to cause lung cancer. They also show un analysis of the mesothelioma mortality of a small sub set of the US insulators study which shows that the pattern of deaths over time implies that members of this cohort were exposed to a pleural carcinogen before 1935. Since, reportedly, amosite was first used from around 1935, and prior to this date only chrysotile was used, some of these deaths must have been due to chrysolite. The authors do not mention the possible role of crocidolite, but if we accept that no amphibole fibre was used before 1935 by US insu lation workers, these observations do show that some of the cases in this cohort must have been caused by exposure to chrysolite prior to 1935. These exposures will often have been heavy. In fact the contemporary US trade journal `Asbestos' (published monthly from July 1919) makes it clear that both amosite and cro cidolite were used in the US through the 1920s, though probably in limited quantities, since the US industry seems to have been resistant to their use on technical grounds, and chrysolite was the fibre of choice for most applications. Slayner er al.'i review of the issues (1996), sets out similar arguments, and also points to the evidence that all three commercial fibre types have produced a similar level of lung tumours in animal inhalation experiments. This is the most problematic evidence to reconcile with the human evidence that amphibole fibres are substantially more potent lung carcinogens. However, the time periods needed to induce cancer in humans (yr) and in rats (months) are very different, and it is at least plausible that all fibres are equally potent in rats because none of them ore materially cleared from the rat lung over the months needed to. initiate a rat lung tumour. By contrast, in humans chrysolite (cleared in months) might have less effect than the amphibole fibres (cleared in yean). A detailed elaboration of this argument has recently been published by Beny (1999). It may also be rel evant that the animal experiments were made with exposure concentrations massively in excess of those represented m the human data. The differences in the human data summarised in this review seem reason ably clear (certainly in respect of mesothelioma), and are based on a range of independent data. In the end, if a choice has to be made between animal and human evidence as a basis for assessing human risk, adequate human data must be given priority. Many of the arguments presented against the `amphibole hypothesis' in connection with mesotheli oma are variants on the basic theme that it is simply unbelievable that such a small component of the exposure could be responsible for the observed risk. If it is true that the mesothelioma risk is proportional to a less than unit power of exposure, then these argu ments are correct in their basic perception that a dis proportionate effect of the amphibole component was required to explain the data. Low levels of amphibole do have a disproportionate effect. REFERENCES Acheson. E. 0.. Gardner. M. J.. Pippard. E. C and Grime. L. P. (1982) Mortality of two groups of women who manufac tured gas masks from chrysolite and crocidolite asbestos: a 40-year follow-up. British Journal of Industrial Medicine 39. 344-348. Acheson. E. D.. Gardiner, M. I.. Winter. P. D. and Bennett. C. (1984) Cancer in a factory using amosite asbestos. International Journal of Epidemiology 13. 3-10. Albin. M.. Jacobson, K.. Attawcll. R,, Johannson. L. and Wellindcr. H. (I990u) Mortality and cancer morbidity ia cohorts of asbestos cement workers and referents. British Journal of Industrial Medicine 47. -602-610. Albin, M., Johansson. L.. Pooley, F. 0., Jakobsson. K., Allow ed. R. and Mitha, R. (1990b) Mineral fibres, fibrosis and asbestos products in the lungs from deceased asbestos cement worker*. British Journal of Industrial Medicine 47. 747-774. Armstrong. B. K.. de Klerk, N. H.. Musk. A. W. and Hobbs. ' M. S. T. (1989) Cancer mortality in relation to measures of occupational exposure to crocidolite at Wittenoom Gorge in Western Australia. British Journal of Industrial Medicine 46. 329-536. Berry G. (1991) Prediction of mesothelioma.rPung cancer, and asbestosis in former Wittenoom asbestosworkers. British Journal of Industrial Medicine. 48. 793-802. Berry. G. (1995) Environmental mesothelioma incidence, time since exposure to asbestos- and level of exposure. Environmetrics 6, 221-228. Berry. G. (1999) Models for mesothelioma incidence following exposure to fibres in terms of timing and duration of exposure and the biopenistenee of the fibres. Inhalation Toxicology 11. 101--120. Betty. G. and Newhouse, M. L. (1983) Mortality of workers manufacturing friction materials using asbestos. British Journal of Industrial Medicine 40, 1-7. Beny. G., Newhouse. M. L. and Antonis. P. (1985) Combined effect of asbestos and smoking on mortality from lung cancer and mesothelioma in factory workers. British Journal of Industrial Medicine 42, 12-18. Boutin. C- Dumortier, F- Rey. F, Violin. R. and DeVuyst. P. (1996) Black spots concentrate oncogenic asbestos fibres in the parietal pleura-thoroscopic and mineralogies! study. American Journal ofRespiratory and Critical Care Medicine 153. 444-449. Coggon, D., Inskip. R, Winter. P. and Panned, B. (1995) Dif ferences in occupational mortality from pleural cancer, per itoneal -cancer and asbestosis. Occupational and Environ mental Medicine 52, 775-777. Cullen. M. R. (1998) Chrysotile asbestos: enough is enough. Lancet 351. 1377-1378. de Klerk. N. H,, Musk. W. A.. Armstrong. B. K. and Hobbs. M. S. T. (1991) Smoking, exposure to crocidolite, and Use incidence of lung cancer and asbestosis. British Journal of Industrial Medicine 48. 4(2-417. de Klerk, N. R, Musk, A. W. and Armstrong, B. K. (1992) The epidemiology of asbestos and mesothelioma. In Malignant Mesothelioma, eds D. W. Henderson, K. B, Shilkirn S. L Longlois and D. Whitaker, pp. 223-250. Hemisphere Pub- - lishing Corp, New York. de IGetk, N. R, Musk, A. W,, Armstrong, B. K. and Hobbs. M. S. T. (1994) Diseases in miners and millers of crocidolite from Wittenoom, Western Australia: a further follow-up to December 1986. Annals of Occupational Hygiene 38(supp I). 647-655. Dement. J. M. (1991) Carcinogenicity of chrysotile asbestos: