Malignant mesothelioma is caused by exposure to asbestos. Almost 90 percent of cases of malignant mesothelioma are known to be caused by asbestos exposure; the other 10 percent of cases are individuals who may have been exposed to asbestos but where some uncertainty exists about where the patient’s asbestos exposure may have come from.

Asbestos is found in nature and is actually quite ubiquitous. It is a mineral, found in the rocks of countries all over the world. Because rock tends to erode, asbestos fibers find their way into the air and water naturally. Although this environmental exposure is widespread, it is also usually not a significant health hazard. The body can handle the tiny quantities of asbestos that are present in nature, in much the same way that the body can deal with the background radiation of our planet. It is when a higher dose of asbestos enters the body that health problems begin to be noticeable.

Asbestos fibers are a lot like arsenic or other ingested poison. When they are not able to enter the body, they are harmless; you can sit on top of a mountain of arsenic and never have a single health problem. When asbestos fibers break down, however, they can enter the air and be breathed in. Asbestos has been widely used in many industrial and commercial products and processes, including insulation, cement, pipes, ceiling tiles, and many other building materials. When these materials break down after a fire or a flood, the asbestos fibers, which had been safely bound to the other components of the material, become friable and can enter the air very easily.

Many people have worked directly with asbestos, either in mining it or in using it on job sites. Construction, ship-building, and appliance making are just a few of the jobs where heavy occupational exposure to asbestos has been a possibility. Although most uses of asbestos are now banned, and those jobs still dealing with the substance tend to have stringent safety measures now that the risk posed by asbestos fibers is well-understood, it is still prudent to be aware of the potential for asbestos exposure in occupational and environmental situations.

The causal mechanism for mesothelioma is well-understood for pleural mesothelioma, the most common form. Asbestos fibers enter the lungs and settle at the bottom of the lung due to the effect of gravity. The fibers penetrate the lung tissue and from there migrate into the pleura, the layer of mesothelial cells that surround the lung cavity. Once in the pleura, the asbestos fibers begin irritating the cells. The body releases killer cells called macrophages to attack the foreign body; unfortunately this process ends up accelerating the development of mesothelioma, as the macrophages cannot destroy the asbestos fibers, and the substances they emit in trying to do so damage the mesothelial cells further. Eventually the cellular damage causes a cell to mutate, growing out of control – the beginning of cancer. As the cancer cells replicate, tumors form on the mesothelium, the thin layer of cells that surround various parts of the body. Without a mesothelioma diagnosis and the start of asbestos cancer treatment, the mesothelium thickens and begins to compress the organs that it is meant to protect, and the disease continues to spread.

The same mechanism is thought to operate for the other two main types of mesothelioma, pericardial mesothelioma and peritoneal mesothelioma, but it is not well-understood how the asbestos fibers reach those portions of the mesothelium, as there is no direct connection from the lungs. It is possible that fibers might be swallowed in water or in food to reach the peritoneal mesothelium, which surrounds the stomach and intestines. It is also possible that tiny fibers might reach the bloodstream or lymph system and be distributed around the body via that mechanism.