Risk Factors for Malignant Mesothelioma
A risk factor is anything that increases a person's chance of getting
a disease such as cancer. 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) mesotheliomas 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. Mesotheliomas take 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.
Radiation: There have been a few published reports of pleural and peritoneal
mesotheliomas 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 mesotheliomas, 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.
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