Causes of Mesothelioma - Mesothelioma Compensation
Chrysotile Asbestos - Environmental Health Criteria 203
INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY (IPCS)
Organisation Mondiale de la Santé - 1998
Studies reviewed are restricted to those that were considered by the Task Group to be of clear relevance to characterizing the risks associated with exposure to chrysotile. Limitations of particle-to-fibre count conversions on which the exposure estimates in the following studies are based are presented in Chapter 2.
7.1 Occupational exposure
7.1.1 Pneumoconiosis and other non-malignant respiratory effects
The non-malignant lung diseases resulting from exposure to asbestos fibres comprise a somewhat complex mixture of clinical and pathological syndromes not readily definable for epidemiological study. Traditionally, the prime concern has been asbestosis, generally implying a disease associated with diffuse interstitial pulmonary fibrosis accompanied by varying degrees of pleural involvement. More recently, as severe asbestosis has become less frequent clinically, attention has been directed primarily to syndromes reflecting fibrosis of the small and large airways rather than of the lung parenchyma. As a cause of death, the pneumoconioses have never been reliably recorded on death certificates. In investigations of mortality, therefore, all chronic non-malignant respiratory diseases are generally considered as one group. Additionally, mortality studies are generally not sufficient to detect clinically significant morbidity. Equally, in studies of morbidity, the etiological or diagnostic specificity of the usual methods of assessment, i.e. chest radiography, physiological testing and symptom questionnaire, is limited.
Early studies in both the United Kingdom and USA demonstrated an extremely high prevalence of asbestosis among textile workers exposed only to chrysotile at very high dust levels (Dreeson et al., 1938).
Extensive morbidity surveys of chrysotile workers were initiated in the Quebec chrysotile mines and mills in the 1960s (McDonald et al., 1974). These studies included the use by six readers of the then newly developed UICC/Cincinnati (later ILO) radiographic classification of nearly 7000 films, examinations by questionnaire and lung function tests of over 1000 current employees, and detailed assessments of cumulative dust exposure for each man. In the initial survey, there was a fairly systematic relationship between exposure and these measures of response. The authors concluded that exposure to 70-140 mpcm (2-4 mpcf) for a working life of 50 years was associated with a 1% risk of acquiring clinically significant disease. Based on additional study of radiological changes in 515 men aged 60-69 years (average 64.6 years) who had been employed for at least 20 years (average 42.3 years) at Thetford Mines, the dustier of the two Quebec mining regions, dose-response relationships for small opacities were essentially linear (Liddell et al., 1982). However, any increase in prevalence in small opacities (>1/0 or >2/1) above the level of the intercepts (which were high) only became apparent at an accumulated exposure at age 45 of 1200 f/ml-years, equivalent to an average concentration of about 30 f/ml (Liddell et al., 1982). In contrast to small opacities, pleural thickening was not related to cumulative exposure, although it was more common in men with long service.
Becklake et al. (1979) reported a second study in Quebec of 86 men whose last chest film was taken within 12 months of leaving employment in 1960-1961, and who were examined again in 1972. In 66 men who had been employed for at least two years, there was evidence of an increase in small irregular parenchymal opacities in 8 men (12%) but in none of the 20 men with shorter employment. Increase of pleural thickening was seen in a further 13 (20%) of the 66 men and 4 (20%) of the 20 men.
A dose-related reduction in vital capacity (p= 0.023) and expiratory volume (p<0.001) was observed with increasing cumulative exposure (i.e. 3 8 f/ml-years) to chrysotile asbestos in miners and millers (stratified random sample of 111 men) in Zimbabwe, exposed for more than 10 years. The relationship between cumulative exposure and radiographic parenchymal category demonstrated a steep increase with each change in category (p<0.00001). Individual estimates of cumulative exposure based on company records of employment history and fibre concentrations (measured and estimated) ranged from 1.1 to 654 fibres/ml-years. Controls were a subset of miners (n=66) with no prior respiratory illness, who were lifelong non-smokers with normal chest X-ray and minimal cumulative exposure to chrysotile asbestos (<8 fibres/ml-years) (Cullen et al., 1991).
A number of other studies of radiographic and functional changes have been conducted in occupational populations exposed primarily to chrysotile, in some cases during mining and milling operations (Rubino et al., 1979a; McDermott et al., 1982; Viallat et al., 1983; Cordier et al., 1984; Enarson et al., 1988), asbestos-cement (Weill et al., 1979; Jones et al., 1989) and asbestos textiles (Berry et al., 1979; Becklake et al., 1980). Results were generally comparable to those already described, the presence of small opacities increasing with cumulative exposure (although with some variability in the shape and steepness of these trends) and pleural changes primarily related to time since initial exposure. As demonstrated in several of these studies, e.g., Becklake et al., 1979; Rubino et al., 1979a; Berry et al., 1979; Viallat et al., 1983, and as well recognized clinically, X-ray changes can develop among workers after exposure ceases, in some cases many years later.
Studies that correlate disease prevalence or symptoms with cumulative exposure can underestimate disease risk due to progression of disease after employment ceases. Although workers were exposed to both chrysotile and crocidolite (the latter being approximately 5% of all asbestos used), results for 379 men employed at least 10 years in the Rochdale asbestos textile plant are informative in this regard (Berry et al., 1979). Exposure estimated from work histories ranged from an average of 2.9 to 14.5 f/ml. Overall, small opacities (>1/0) were recorded in 88/379 (23%) of chest radiographs, with evidence of a gradient seriously confounded by date of first employment and transfer of subjects with suspected asbestosis to less dusty conditions. On the basis of data on incidence, the authors drew conclusions on exposure-response between cumulative exposure and prevalence or incidence of crepitations, possible asbestosis and certified asbestosis - all three depending on clinical opinion and judgement. The authors concluded that possible asbestosis occurs in no more than 1% of men after 40 years of exposure to concentrations between 0.3 and 1.1 f/ml.
Mortality studies of Quebec miners and millers by McDonald et al. (1994) have shown exposure-response relationships for pneumoconiosis-related mortality. Crude rates of 0.23 cases per 1000 man-years were observed for those with cumulative exposures less than 3530 mpcm-years (100 mpcf-years) and a rate of 2.7 cases per 1000 man-years was reported for those with more than 10 590 mpcm-years (> 300 mpcf-years). Dement et al. (1994) also reported mortality due to non-malignant respiratory diseases among chrysotile textile workers. An SMR of 1.88 was observed for those with cumulative exposures less than 2.7 f/ml-years and rose rapidly to 12.78 with cumulative exposures greater than 110 f/ml-years. It was noted that cases of pneumoconioses recorded on death certificates are often verified by pathological diagnosis.
Chest X-ray changes among textile and friction product workers in China were reported by Huang (1990). A total of 824 workers employed for at least 3 years in a chrysotile products factory from the start-up of the factory in 1958 until 1980, with follow-through to September 1982, were studied. Chest X-ray changes compatible with asbestosis were assessed using the Chinese standard system for interpretation of X-rays. Cases were defined as Grade I asbestosis (approximately equivalent to ILO >1/1). Overall, 277 workers were diagnosed with asbestosis during the follow-up period, corresponding to a period prevalence of 31%. Exposure-response analysis, based on gravimetric data converted to fibre counts, predicted a 1% prevalence of Grade I asbestosis at a cumulative exposure of 22 f/ml-years.
Mesothelioma Information
Malignant mesothelioma is a rare type of cancer that affects the serous membrane lining three of the body's largest cavities. Therefore there are three distinct types of mesothelioma:
- Pleural mesothelioma
- Peritoneal mesothelioma
- Pericardial mesothelioma
Pleural mesothelioma is the most common type of the disease accounting for an approximate 75% of all documented cases. Because of its close proximity to the lungs, pleural mesothelioma is often misdiagnosed as a type of lung disease such as asbestos lung cancer or viral pneumonia.
Causes of mesothelioma have been limited to asbestos exposure, making asbestos product manufacturers liable for injuries incurred. Mesothelioma compensation is expected to exceed $200 billion when all is said and done.
If you or a loved one has been diagnosed with malignant mesothelioma or another type of asbestos-related disease, you may be eligible for compensation. Contact a mesothelioma attorney or asbestos lawyer to get more information about filing your asbestos lawsuit.
Mesothelioma cancer research has been unable to produce an effective treatment modality for malignant mesothelioma. Clinical research at facilities such as the University of California’s Keck School of Medicine is essential in development of new cancer treatment modalities.