Chrysotile and Amphiboles -
Amphiboles and Mesothelioma
Mesothelioma Cancer Diagnosis
Diagnosis of Mesothelioma
Our articles will give you a better understanding of the treatments available for anyone suffering from Mesothelioma.
Mesothelioma is a rare form of cancer. It affects the linings of the cavities around the lungs, stomach, and heart. It is caused by inhaling asbestos fibers, but the cancer usually does not appear until 10 to 40 years after a person first inhales asbestos.
The latest information about Mesothelioma diseases, their diagnoses, causes, treatments and the medical research currently underway to prevent and someday cure Mesothelioma. We want to help those potentially exposed to Mesothelioma to understand which materials contain Mesothelioma causing cancers and how exposures occur.
Inconsistency in findings of animal and human research
Past animal studies comparing chrysotile and amphiboles have failed to confirm epidemiological findings of the stronger association between amphiboles and mesothelioma. A review of experimental protocols used demonstrates why.
Up until recently, most studies published in the field of animal experimentation, following administration of different asbestos fibre types by inhalation or injections, have not identified significant differences in the pathogenic potential of the various asbestos fibre types. The reported effects were not consistent with epidemiological observations indicating that amphibole fibres are markedly more potent than chrysotile in inducing asbestosis, lung cancer and mesothelioma.
However, in many of these studies, the comparisons of effects (fibrogenicity and tumor yield) has been based on gravimetric measures: that is, using mass as the means defining the dose of the tested minerals.
The use of fibre mass rather than fibre number in animal studies has resulted in an overstatement of the health effects of chrysotile compared to other fibres. It is widely known that similar masses of chrysotile and amphiboles or other mineral fibres can vary significantly in fibre number. For example, for fibres longer than 8 microns, the number of fibres per mg of chrysotile may be up to 100 times higher than that for crocidolite (Palekar et al., 1988).
In fact, attempts to transform gravimetric doses into fibre numbers have indicated that fibre for fibre, chrysotile is less pathogenic than the amphibole varieties. More recently, studies using both fibre mass and fibre number as units of dose have confirmed that amphiboles are more pathogenic than chrysotile. The in vitro models of Yegles et al. (1993) and of Heintz et al. (1993) and the inhalation experiments of McConnell et al. (1994) all support this finding.
Another important factor in the apparent inconsistency between human and animal data is that inadequate measures have been applied to control for the fibre dimensions of the different substances being tested. Fibre length and diameter are now recognized as extremely important determinants of the fibrogenic and carcinogenic potential of a given substance.
Heintz NH, Janssen YM and Mossman BT. (1993) Persistent induction of c-fos and c-jun expression by asbestos. Proc. Natl Acad. Sci. 90: 3299-3303.
McConnell EE, Chevalier HJ, Hesterberg TW, Hadly JG and Mast RW. (1994) in ILSI Monograph "Toxic and carcinogenic effects of solid particles in the respiratory tract", eds. DL Dungworth, JL Mauderly and G. Oberrster, ILSI Press, pp. 461-467.
Palekar LD, Most BM & Coffin DL. (1988) Environ. Res., 46:142-152.
Yegles M, St-Etienne L, Renier L, Janson X and Jauran MC. (1993) Induction of metaphase and anaphase abnormalities by asbestos fibers in rat pleural mesothelial cells in vitro. Amer. J. Resp. Cell. Mol. Biol.9: 186-191.
Chrysotile, tremolite and mesothelioma
Although it has been demonstrated that there is a very weak association between chrysotile exposure and mesothelioma, the presence of occasional fibrous tremolite, an amphibole mineral, in some chrysotile ore body has been cited as a potential risk factor amongst chrysotile workers. The available evidence, however, shows that mesotheliomas in chrysotile mining populations are extremely rare relative to rates in amphibole-exposed populations. In fact, less than 40 mesothelioma cases over several decades have been reported amongst chrysotile miners and millers (McDonald et al. 1993).
In their analysis of the implications of the 37 mesothelioma cases identified up until 1992 in the 11,000 person cohort, McDonald & McDonald (1995) found that they were concentrated in workers from specific areas of the mines. Further post-mortem lung tissue analysis showed that workers in these areas had tremolite lung content four times higher than those workers in other areas of the mines studied, suggesting that the rare cases of mesothelioma among chrysotile miners are mainly, if not wholly, due to tremolite exposure.
The authors note that it should be kept in mind that these mesothelioma cases occurred as a result of long, heavy exposures 20 to 70 years ago. They conclude: "The geological distribution of tremolite within the Quebec chrysotile ore body may well vary in time and place and, at present levels of environmental controls, any mesothelioma risk from exposure (...) would be far below the limits of epidemiological detection."
The previous review by Dr. Andrew Churg, a pathologist at the University of British Colombia in Canada, supports this conclusion. Churg (1988) writes,
"whether tremolite or chrysotile be the critical agent, these observations suggest that chrysotile ore, in both crude and processed forms, does cause mesothelioma in man, but that it is an extremely weak carcinogen and that in today's terms, the doses required are extremely high. As a practical matter, the data indicate that chrysotile will not produce mesotheliomas in those exposed to any current or recently regulated number of fibers..."
Churg, A. (1988) Chrysotile, Tremolite, and Malignant Mesothelioma in Man. Chest 93: 621-628.
McDonald, JC and McDonald, AD (1995) Chrysotile, Tremolite, and Mesothelioma. Science 267: 776-777.