Asbestos is the principal carcinogen associated with malignant mesothelioma. Indeed, malignant mesothelioma was rare before the widespread use of asbestos.
In 1960 the first convincing evidence of a link between malignant mesothelioma and both occupational
and incidental asbestos exposure was reported, on the basis of data from South Africa.
There are two principal forms of asbestos: long, thin fibers known as amphiboles, one type of which
is called blue asbestos, and feathery fibers known as chrysotile or white asbestos. Whether only amphibole fibers cause malignant mesothelioma or whether chrysotile fibers can also cause mesothelioma is
still debated.
The association of chrysotile with malignant mesothelioma was once thought to be
due to contamination of chrysotile with the amphibole tremolite; however, current evidence, particularly
from electron microscopical studies, supports the view that chrysotile itself can cause malignant
mesothelioma, although at rates lower than those of mesothelioma caused by amphiboles. Malignant mesotheliomas occur initially on the parietal surface of the pleural mesothelium, rather
than on the visceral surface.Several mechanisms might account for this finding; one possibility is
that the asbestos fibers stick out from the lung surface and cause repeated cycles of scratching, damage,
inflammation, and repair in the adjacent parietal mesothelial-cell layer.Simian virus 40 (SV40), a DNA virus, has been implicated as a cofactor in the causation of malignant
mesothelioma.
This virus, which blocks tumor-suppressor genes, is a potent oncogenic virus
in human and rodent cells; SV40 DNA sequences have been found in brain and bone tumors, lymphomas,
and malignant mesotheliomas, as well as in atypical mesothelial proliferations and superficial
noninvasive lesions of the mesothelium.
There is some evidence that SV40 may have been inadvertently transmitted to humans in injectable poliomyelitis vaccines 35 to 50 years ago. The putative involvement
of SV40 in the pathogenesis of malignant mesothelioma has become a controversial issue,and its role remains unclear and unproved.
In rare cases, malignant mesothelioma is caused by radiation or one of a small number of other factors.
he possibility that carbon nanotubes would show asbestos-like behaviour in the human body was raised ten
years ago with a call for appropriate research1. Exposure to asbestos is known to cause mesothelioma — cancer of the lining of the lungs (pleura) and abdominal cavity (peritoneum). The nanotube and asbestos analogy relies on several points of material similarity: small fibre diameter, long length and chemical stability in physiological environments (biopersistence). There are also differences between these two fibrous materials, such as their chemical composition and surface properties,
so the validity and usefulness of the nanotube and asbestos analogy have been unclear. Two recent studies provide important new insight into the possibility that carbon nanotubes may indeed induce mesothelioma — a disease that is rare in unexposed populations and is thus a sensitive marker for asbestos exposure
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