For many years, dust concentrations of 200 f/cc and higher could be found in certain asbestos industry sectors, particularly those involving direct contact with friable, sprayed-on applications. Elevated exposures have resulted in a number of asbestos-related diseases among workers. In some cases, the families of workers who were exposed to high fibre levels generated by contaminated clothing being introduced into the home also contracted asbestos-related illness. The primary asbestos-related diseases are:
Asbestosis:
The scarring of the lung tissue which can impede normal respiratory function and possibly result in death due to heart failure, respiratory infections or lung cancer - Asbestosis is neither malignant nor necessarily fatal, but it can lead to respiratory impairment and to a higher risk for the development of lung cancer and cardio-respiratory complications. The fibrotic changes (scarring) are irreversible. While the manifestation of symptoms depends on the type of exposure, the average time span from first exposure to first symptoms is 17 years.
Lung cancer:
A malignant, invasive growth or tumour in the lung - Unlike asbestosis, lung cancer is not specifically related to asbestos. The most important cause of lung cancer is undoubtedly smoking, responsible for at least 85% of known lung cancer cases in the general public. Lung cancer related to asbestos exposure usually involves a delay of 20 to 40 years between the first exposure and the onset of cancer. Thus asbestos related lung cancers being diagnosed today are primarily related to high level exposures in the 1960s and 70s.
Mesothelioma:
A malignant tumour of the lining of the chest or abdominal cavities. Generally, cases of mesothelioma are rapidly fatal. Mesothelioma is rarely known to develop less than 15 years after exposure.Most begin to occur 20 years after exposure, and some even develop more than 50 years after exposure.
For a time, mesothelioma was thought to be exclusively related to exposure to asbestos fibre, but more recent studies indicate that up to 30% of cases may have occurred in the absence of any known asbestos exposure. Other possible causes have been reported such as exposure to a fibrous zeolite (erionite), ionizing radiations and biogenic silica. With regard to asbestos-related mesothelioma, it is generally agreed that it is strongly associated with exposure to amphiboles (crocidolite and amosite). Relatively brief but intense exposures to amphiboles have been related to the increased risk of mesothelioma occurance many years later.
The tumor is comparatively rare in chrysotile-only exposed populations and has been found only under conditions of long and heavy exposure.
A more detailed analysis of the nature of mesothelioma risks has been prepared by The Asbestos Institute in a special report titled Understanding Mesothelioma.
Pleural plaques:
Asbestos exposure can also produce scarring, both localized and diffuse, of the lining of the pleura lining the chest cavity, giving rise to thickening or plaques. Plaques are raised areas of tissue which may calcify or harden. In general, these limited pleural changes and plaques are not associated with clinical and functional abnormalities. They are simply markers of asbestos exposure.
Other cancers
Cancers at other sites (gastro intestinal and laryngeal cancers) have been reported but the causal relationship with chrysotile asbestos exposure is disputed. A meta-analysis of the relation between asbestos exposure and colorectal cancer mortality has been published recently (Homa et al, 1994), using published reports of 20 asbestos-exposed cohorts, in which the authors conclude that chrysotile asbestos is not associated with colorectal cancer. Considerable evidence shows that ingested fibres are readily excreted and do not cause disease. Asbestos fibres thus are a health hazard only when they are air-borne and inhaled.
There is no substantial evidence either for asbestos-related cancers at other sites. The risks of ingested asbestos are reviewed in greater detail in Module 6.
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