Risk Factors for Malignant MesotheliomA

Risk Factors for Malignant Mesotheliom:
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 refers to a family of fibrous minerals made of silicate. It is a naturally occurring mineral that can be found in dust and rocks in certain parts of the United States and other parts of the world.

When chrysotile 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) and are then cleared by being coughed up or swallowed. But the long, thin amphibole 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 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. 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.

Because of its heat and fire resistant properties, asbestos was once 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 has become well known, so its use in the United States has decreased dramatically. Most use stopped after 1989, but it is still used in some products.

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 thought to be much less hazardous 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 drop with time after exposure to asbestos. The risk appears to be lifelong and undiminished.

Zeolites

Zeolites are silicate minerals that are chemically related to asbestos. An example is erionite, which is common in the soil in parts of Turkey. High mesothelioma rates in these areas are believed to be due to exposure to this mineral.

Radiation

There have been a few published reports of mesotheliomas that developed following exposure to high doses of radiation to the chest or abdomen or after injections of thorium dioxide (Thorotrast). This 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 may cause mesothelioma. For example, intentional infection with SV40 of some lab animals, such as hamsters, causes mesotheliomas to develop. Researchers also have noticed that SV40 can cause mouse cells grown in 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 non-cancerous human tissues.

So far, the largest studies addressing 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.