What are the risk factors for malignant mesothelioma?


What are the risk factors for malignant
mesothelioma?
A risk factor is anything that affects your chance of getting a disease such as cancer.
Different cancers have different risk factors. Some risk factors, like smoking, can be
changed. Others, like a person’s age or family history, can't be changed.
But risk factors don’t tell us everything. Having a known risk factor, or even several risk
factors, does not mean that you will get the disease. And some people who get the disease
may have few or no known risk factors.
Researchers have found some factors that increase a person's risk of mesothelioma.
Asbestos
The main risk factor for developing mesothelioma is exposure to asbestos. In fact, most
cases of mesothelioma have been linked to asbestos exposure in the workplace.
Asbestos is a group of minerals that occur naturally as bundles of fibers. These fibers,
found in soil and rocks in many parts of the world, are made of silicon, oxygen, and other
elements.
There are 2 main forms of asbestos:
• Serpentine asbestos fibers are curly. The most common asbestos in industrial use,
known as chrysotile, or white asbestos, has curly fibers.
• Amphibole asbestos fibers are straight and needle-like. There are several types of
amphibole fibers, including amosite, crocidolite, tremolite, actinolite, and
anthophyllite.
Amphibole fibers (particularly crocidolite) are considered to be more likely to cause
cancer, but even the more commonly used chrysotile fibers are linked with
mesotheliomas. When asbestos fibers in the air are inhaled, they tend to stick to mucus in the throat,
trachea (windpipe), or bronchi (large breathing tubes of the lungs). Chrysotile fibers tend
to be cleared from the lungs by being coughed up or swallowed. But the long, thin
amphibole fibers are harder to clear, and they may stay in the lungs, traveling to the ends
of the small airways and penetrating into the pleural lining of the lung and chest wall.
These fibers may then injure mesothelial cells of the pleura, and eventually cause
mesothelioma.
Asbestos fibers can also damage cells of the lung and result in asbestosis (scar tissue in
the lung) and/or lung cancer. Indeed, asbestosis, mesothelioma, and lung cancer are the 3
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.
Many people are exposed to very low levels of naturally occurring asbestos in outdoor air
in dust that comes from rocks and soil containing asbestos. This is more likely to happen
in areas where rocks have higher asbestos content. In some areas, asbestos may be found
in the water supply as well as in the air.
Because of its heat and fire resistant properties, asbestos has been used in many products
such as insulation, floor tiles, door gaskets, soundproofing, roofing, patching compounds,
fireproof gloves, ironing board covers, and brake pads. The link between asbestos and
mesothelioma is well known, so its use in the United States has gone down dramatically.
Most use stopped after 1989, but it is still used in some products.
Still, millions of Americans may already have been exposed to asbestos. People at risk
for asbestos exposure in the workplace include some miners, factory workers, insulation
manufacturers and installers, railroad and automotive workers, ship builders, gas mask
manufacturers, and construction workers. Family members of people exposed to asbestos
at work can also have an increased risk of developing mesothelioma because asbestos
fibers can be carried home on the clothes of the workers. The rate of mesothelioma in
men appears to be dropping, probably because there is now much less direct exposure to
asbestos in the workplace.
Asbestos was also used in the insulation of many older homes, as well as commercial and
public buildings around the country, including some schools. Because these particles are
contained within the building materials, they are not likely to be found in the air in large
numbers. The risk of exposure is likely to be very low unless the particles are somehow
escaping into the air, such as when building materials begin to decompose over time, or
during remodeling or removal.
The risk of developing 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 more likely to develop this cancer. Mesotheliomas
take a long time to develop. The time between first exposure to asbestos and diagnosis of
mesothelioma is usually between 20 and 50 years. Unfortunately, the risk of
mesothelioma does not go down over time after the exposure to asbestos stops. The risk
appears to be lifelong. For more detailed information on asbestos, see our document, Asbestos.
Zeolites
Zeolites are minerals that are chemically related to asbestos. An example is erionite,
which is common in the rocks and soil in parts of Turkey. High mesothelioma rates in
these areas are believed to be caused by exposure to this mineral.
Radiation
There have been a few published reports of mesotheliomas that developed after people
were exposed to high doses of radiation to the chest or abdomen as treatment for another
cancer. Although the risk of mesothelioma is increased in patients who have been treated
with radiation, this cancer still only occurs rarely in these patients.
There have also been reports linking mesothelioma to injections of thorium dioxide
(Thorotrast). This radioactive material was used by doctors for certain x-ray tests until the
1950s. Thorotrast was found to cause cancers, so it has not been used for many years.
SV40 virus
Some studies have raised the possibility that infection with simian virus 40 (SV40) might
increase the risk of developing mesothelioma. Some injectable polio vaccines given
between 1955 and 1963 were contaminated with SV40. As many as 30 million people in
the United States may have been exposed to the virus.
Some lab studies have suggested that SV40 infection might cause mesothelioma. For
example, infecting some lab animals like hamsters with SV40 causes mesotheliomas to
develop. Researchers also have noticed that SV40 can cause mouse cells grown in lab
dishes to become cancerous, and that asbestos increases the cancer-causing effect of
SV40 on these cells. Other researchers have found SV40 DNA in some biopsy specimens
of human mesotheliomas. But fragments of SV40 DNA can also be found in some noncancerous human tissues.
So far, the largest studies looking at this issue in humans have not found an increased risk
for mesothelioma or other cancers among people who received the contaminated vaccines
as children. But the peak age range for diagnosis of mesothelioma is 50 to 70 years. Some
researchers have pointed out that this issue may remain unresolved until more of the
people accidentally exposed to SV40 between 1955 and 1963 reach that age range.
Most experts have concluded that at this time we still don't know whether SV40 is
responsible for some mesotheliomas. Research into this important topic is still under way.
Age
The risk of mesothelioma increases with age. It is rare in people under age 45. About 2
out of 3 people with mesothelioma are older than 65.  Gender
The disease is much more common in men than in women. This is probably because men
have been more likely to work in jobs with heavy exposure to asbestos.

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