The main risk factor for pleural mesothelioma is exposure to asbestos. In fact, most cases of pleural mesothelioma have been linked to asbestos exposure, usually from high levels of exposure in the workplace. Asbestos is a group of minerals that occur naturally as bundles of tiny fibers. These fibers are found in soil and rocks in many parts of the world. When asbestos fibers in the air are inhaled, they can get into the lungs. Fibers that stay in the lungs can travel to the ends of the small airways and enter the pleural lining ofthe lung and chest wall.
These fibers can then injure the 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. Peritoneal mesothelioma, which forms in the abdomen, can 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 can be found in the water supply as well as in the air. In the past, asbestos was used in many products because of its heat and fire-resistant properties. The link between asbestos and mesothelioma is now well known, so its use in the United States has gone down dramatically. Most use stopped several decades ago, 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, plumbers, and construction workers. Family members of people exposed to asbestos at work can also be exposed because the workers can carry home asbestos fibers on their clothes. Asbestos was also used to insulate 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 somehow escape into the air, such as when building materials begin to decompose over time, or during remodeling or removal. The risk of developing mesothelioma is loosely related to how much asbestos a person is exposed to and how long this lasts. People exposed at an early age, for a long time, and at higher levels are more likely to develop this cancer. Still, most people exposed to asbestos, even in large amounts, do not get mesothelioma. Other factors, such as a person’s genes, may make them more likely to develop mesothelioma when exposed to asbestos.
Mesotheliomas related to asbestos exposure 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.
Cancers, including mesotheliomas, occur when cells in the body suffer damage to their DNA. DNA is the chemical in each of our cells that makes up our genes– the instructions for how our cells function. We usually look like our parents because they are the source of our DNA. But DNA affects more than how we look. Some genes control when cells in the body grow, divide into new cells, and die at the right time.
Changes in these genes may cause cells to grow out of control, which can lead to cancer. Asbestos exposure is the main cause of pleural mesothelioma. When asbestos fibers are breathed in, they travel to the ends of small air passages and reach the pleura, wherethey can cause inflammation and scarring. This may damage cells’ DNA and cause changes that result in uncontrolled cell growth. If swallowed, these fibers can reach theabdominal lining, where they can have a role in causing peritoneal mesothelioma.
But most people exposed to asbestos, even in large amounts, do not get mesothelioma. Other factors, such as a person’s genes, may make them more likely to develop mesothelioma when exposed to asbestos. For example, researchers have found that some people who seem to be at high risk have changes in BAP1, a gene that normally helps keep cell growth under control. Other genes are probably important as well. Radiation treatments for other cancers have been linked to mesothelioma in some studies.
Radiation can damage the cells’ DNA, leading to out-of-control cell growth. It is still not known if infection with the SV40 virus increases the risk of mesothelioma, or exactly how it might do so. In lab studies, researchers have found that the virus can affect certain genes that have been linked with cancer, but further researchin this area is needed.