Tremolite Asbestos - Malignant Mesothelioma Cases
What is Mesothelioma?
"Mesothelioma (malignant)"
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.
Tremolite - Chrysotile
The Asbestos Dilemma
In the cohort of some 11,000 Quebec miners and millers (47,53), 25 cases of mesothelioma were identified from miners in the Thetford Mines region and 8 from the large mine at Asbestos. The proportion of tremolite in the chrysotile was 3 times higher in the former than in the latter region.
The analysis of deaths from mesothelioma in men employed in the Thetford Mines, with matched references, showed that odds ratios for work in the central mines, where the tremolite content was 3 times higher, were significantly elevated for mesothelioma and lung cancer.
By contrast, in the peripheral mines, where the tremolite content was 3 times lower, there was little or no evidence of increased risk. The authors conclude that these long-term studies Ð including data from as early as 1970's Ð show that chrysotile rarely caused mesothelioma and was not a major cause of lung cancer, except at very high levels of exposure.
They attribute the remaining risk to tremolite asbestos, because its biopersitence is much higher than that of chrysotile. However, the Mount Sinai group (54) in their analysis of the lung and mesothelial tissues taken from 151 human malignant mesothelioma cases, found asbestos fibres in almost all the lung tissues as well as in the mesothelial tissue, the most common asbestos types being an admixture of chrysotile and amphiboles, followed by amphiboles alone and chrysotile alone. The most common of asbestos types in the mesothelial tissues were chrysotile alone, followed by chrysotile plus amphibole, and amphibole alone.
They conclude that chrysotile can induce human malignant mesothelioma without the presence of amphiboles, since, in some of the mesothelioma cases, the fibres detected in the lung or mesothelial tissues were exclusively chrysotile fibres. The controversy continues.
Are health effects of asbestos fibres threshold or non-threshold effects?
All asbestos-related diseases are dose-related: the higher the concentration and duration of exposure, the higher the prevalence of the disease and mortality.
However, the form of the dose-response curve at low doses, typical for the exposure of general population, is not known. There are contradictory opinions as to whether the dose-response relationship in the region of low doses is linear or not. It is practically impossible to measure the effects at such low doses either epidemiologically or experimentally.
It is for this reason that mathematical extrapolations ('low-dose extrapolations"), which carry errors of several orders of magnitude, are used in the quantitative risk assessments. I criticized these extrapolations in 1988 (55) and again in 1991 (56) and in 1993 (57).
Recently, in 2001, Berman (58), reported that 'the published doseresponse coefficients for asbestos vary by more than a factor of 500 for lung cancer and more than a factor of 1,000 for mesothelioma".
Extrapolation of the most frequently used linear relationship into the origin of coordinates means that there is no exposure threshold, i.e. that even the lowest exposure to asbestos may carry some risk of disease and death.
Others, however, believe that there is an asbestos fibre exposure threshold for chrysotile below which there will be no pathologic effects (particularly asbestosis or lung carcinoma) or that the effects are so rare that they cannot be epidemiologically detected.
As negative effects cannot be proven in practical risk assessment, the issue remains unresolved. An expert group of the CEC concluded the following in 1977 (27): It is impossible to come to reliable quantitative assessment of the risk of malignancies for the general public. It is possible that there is a level of exposure (perhaps already achieved in the general public) where the risk is negligibly small.
Mesothelioma Information
Mesothelioma treatment options include surgery, chemotherapy, radiation therapy, photodynamic therapy, immunotherapy and gene therapy. Unfortunately, mesothelioma treatments have thus far been unable to combat the fatally incurable cancer of the mesothelium; however, researchers remain optimistic for future successes.