What are the risk factors for
malignant mesothelioma?
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.
