What are the 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.