Although asbestos is the primary cause of mesothelioma, a risk factor is anything that increases a person's chance of getting a
disease such as mesothelioma or asbestosis. Different cancers have different risk factors. For
example, unprotected exposure to strong sunlight is a risk factor for skin
cancer and smoking is a risk factor for lung cancer as well as other types of
cancer. Scientists have found several risk factors that make a person more
likely to develop mesothelioma.
Asbestos: The main risk factor for developing mesothelioma is exposure
to asbestos. Asbestos refers to a family of magnesium-silicate mineral fibers.
In the past, asbestos was used widely for insulation because it does not conduct
heat well and it is resistant to melting or burning. As the link between
asbestos and mesothelioma has become well known, the use of this material has
decreased. However, up to 8 million Americans may already have been exposed to
asbestos.
According to the United States Environmental Protection Agency, as many as
733,000 schools and public buildings in the country today contain asbestos
insulation. As many as 10% to 15% of schools in the United States may contain
asbestos insulation. People who may be at risk for occupational asbestos
exposure include some miners, factory workers, insulation manufacturers,
railroad workers, ship builders, gas mask manufacturers, and construction
workers, particularly those involved with installing insulation. Several studies
have shown that family members of people exposed to asbestos at work have an
increased risk of developing mesothelioma, because asbestos fibers are carried
home on the clothes of the workers.
There are two main forms of asbestos -- serpentine and amphiboles. Serpentine
fibers are curly and pliable. Chrysotile is the only type of serpentine fiber
and it is the most widely used form of asbestos. Amphiboles are thin, rod-like
fibers of which there are 5 main types-crocidolite, amosite, anthrophylite,
tremolite, and actinolyte. Amphiboles (particularly crocidolite) are considered
to be the most carcinogenic (cancer-causing). However, even the more commonly
used chrysotile fibers have been associated with malignant (cancerous)
mesothelioma and should be considered dangerous as well.
It may be that asbestos causes cancer by physically irritating cells rather
than by a chemical effect. When fibers are inhaled, most are cleared in the
nose, throat, trachea (windpipe), or bronchi (large breathing tubes of the
lungs). Fibers are cleared by sticking to mucus inside the air passages and
being coughed up or swallowed. The long, thin, fibers are less readily cleared,
and they may reach the ends of the small airways and penetrate into the pleural
lining of the lung and chest wall. These fibers may then directly injure
mesothelial cells of the pleura, and eventually cause mesothelioma.
Asbestos fibers can also damage cells of the lung and result in asbestosis
(formation of scar tissue in the lung), and/or lung cancer. The risk of lung
cancer among people exposed to asbestos is increased by 7 times, compared with
the general population. Indeed, asbestosis, mesothelioma, and lung cancer are
the three most frequent causes of death and disease among people with heavy
asbestos exposure. Peritoneal mesothelioma, which forms in the abdomen, may
result from coughing up and swallowing inhaled asbestos fibers. Cancers of the
larynx, pancreas, esophagus, colon, and kidney have also been linked to asbestos
exposure, but the increased risk is not as great as with lung cancer.
The risk of developing a mesothelioma is related to how much asbestos a
person was exposed to and how long this exposure lasted. People exposed at an
early age, for a long period of time, and at higher levels are most likely to
develop this cancer. Mesothelioma takes a long time to develop. The time between
exposure to asbestos and diagnosis of mesothelioma is usually between 20 and 40
years.
Although the risk of developing mesothelioma rises with the amount of
asbestos exposure, it is clear that genetic factors also play a role in
determining who develops the disease. This explains why not all persons exposed
to high levels of asbestos dust develop mesothelioma.
MALIGNANT MESOTHELIOMA CAUSES
Radiation: There have been a few published reports of pleural and
peritoneal mesothelioma that developed following exposure to thorium dioxide
(Thorotrast). This material was used in the past by doctors for certain x-ray
tests. Because Thorotrast was found to cause cancers, it has not been used for
many years.
Zeolite: This is a silicate mineral, chemically related to asbestos,
common in the soil of the Anatoli region of Turkey. A few cases of mesothelioma
have been described in this region and may have been caused by this mineral.
Simian Virus 40 (SV40): This virus has recently been identified by
researchers in human mesothelioma cells, and has been shown to induce
mesothelioma in the animal model. Polio vaccines administered as a primary
prevention measure during 1955 - 1961 have been shown to be contaminated with
SV40. However the implications of these facts are not totally understood and
further research will be needed to clarify the link between malignant
mesothelioma and a viral etiology.
Tobacco: Although tobacco smoking has not been associated with the
development of mesothelioma, the combination of smoking and asbestos exposure
greatly increases the risk of lung cancer. Asbestos workers who also smoke have
a lung cancer risk 50 to 90 times greater than that of the general population.
More asbestos workers die of lung cancer than of mesothelioma.
